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Document and Entity Information - shares	3 Months Ended
Document and Entity Information - shares	Mar. 31, 2020	May 01, 2020
Cover [Abstract]
Document Type	10-Q
Amendment Flag	false
Document Period End Date	Mar. 31, 2020
Document Fiscal Year Focus	2020
Document Fiscal Period Focus	Q1
Trading Symbol	SLDB
Entity Registrant Name	Solid Biosciences Inc.
Entity Current Reporting Status	Yes
Entity Central Index Key	0001707502
Current Fiscal Year End Date	--12-31
Entity Filer Category	Accelerated Filer
Entity Shell Company	false
Entity Small Business	true
Entity Emerging Growth Company	true
Entity Ex Transition Period	true
Entity Common Stock, Shares Outstanding		46,070,724
Entity File Number	001-38360
Entity Tax Identification Number	90-0943402
Entity Address, Address Line One	141 Portland Street, Fifth Floor
Entity Address, City or Town	Cambridge
Entity Address, State or Province	MA
Entity Address, Postal Zip Code	02139
City Area Code	617
Local Phone Number	337-4680
Entity Interactive Data Current	Yes
Title of 12(b) Security	Common Stock $0.001 par value per share
Security Exchange Name	NASDAQ
Entity Incorporation, State or Country Code	DE
Document Quarterly Report	true
Document Transition Report	false

Condensed Consolidated Balance Sheets - USD ($) $ in Thousands	Mar. 31, 2020	Dec. 31, 2019
Current assets:
Cash and cash equivalents	$ 53,187	$ 76,043
Available-for-sale-securities	400	7,481
Prepaid expenses and other current assets	4,316	2,778
Total current assets	57,903	86,302
Property and equipment, net	10,866	11,645
Operating lease, right-of-use assets	4,655	4,988
Other non-current assets	209	209
Restricted cash	327	327
Total assets	73,960	103,471
Current liabilities:
Accounts payable	2,383	7,124
Accrued expenses	8,507	9,178
Operating lease liabilities	1,799	1,736
Finance lease liabilities	191	186
Other current liabilities		52
Total current liabilities	12,880	18,276
Operating lease liabilities, excluding current portion	3,941	4,414
Finance lease liabilities, excluding current portion	683	733
Total liabilities	17,504	23,423
Commitments and contingencies (Note 11)
Preferred stock, $0.001 par value; 10,000,000 shares authorized at March 31, 2020 and December 31, 2019; no shares issued and outstanding at March 31, 2020 and December 31, 2019
Common stock, $0.001 par value; 300,000,000 shares authorized at March 31, 2020 and December 31, 2019; 46,070,724 shares issued and outstanding at March 31, 2020 and 45,987,571 shares issued and outstanding at December 31, 2019; 2,295,699 pre-funded warrants outstanding at March 31, 2020 and December 31, 2019	48	48
Additional paid-in capital	399,382	396,278
Accumulated other comprehensive (loss) gain	(1)	1
Accumulated deficit	(342,973)	(316,279)
Total stockholders’ equity	56,456	80,048
Total liabilities and stockholders’ equity	$ 73,960	$ 103,471

Condensed Consolidated Balance Sheets (Parenthetical) - $ / shares	Mar. 31, 2020	Dec. 31, 2019
Statement Of Financial Position [Abstract]
Preferred stock, par value	$ 0.001	$ 0.001
Preferred stock, shares authorized	10,000,000	10,000,000
Preferred stock, shares issued	0	0
Preferred stock, shares outstanding	0	0
Common stock, par value	$ 0.001	$ 0.001
Common stock, shares authorized	300,000,000	300,000,000
Common stock, shares issued	46,070,724	45,987,571
Common stock, shares outstanding	46,070,724	45,987,571
Pre-funded warrants outstanding	2,295,699	2,295,699

Condensed Consolidated Statements of Operations - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
Income Statement [Abstract]
Revenue	$ 0	$ 0
Operating expenses:
Research and development	19,665	23,269
General and administrative	5,250	7,033
Restructuring charges	1,944
Total operating expenses	26,859	30,302
Loss from operations	(26,859)	(30,302)
Other income (expense):
Interest income	164	508
Other income	1	212
Total other income (expense), net	165	720
Net loss	$ (26,694)	$ (29,582)
Net loss per share attributable to common stockholders, basic and diluted	$ (0.56)	$ (0.85)
Weighted average shares of common stock outstanding, basic and diluted	48,059,279	34,776,488

Condensed Consolidated Statements of Comprehensive Loss - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
Statement Of Income And Comprehensive Income [Abstract]
Net loss	$ (26,694)	$ (29,582)
Other comprehensive loss:
Unrealized (loss) gain on available-for-sale securities	(2)	12
Comprehensive loss	$ (26,696)	$ (29,570)

- Definition

The consolidated profit or loss for the period, net of income taxes, including the portion attributable to the noncontrolling interest.

+ References

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-Publisher FASB
-Name Accounting Standards Codification
-Topic 220
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-Section 45
-Paragraph 1A
-Subparagraph (a)
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Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 810
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-Section 50
-Paragraph 1A
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Reference 3: http://www.xbrl.org/2003/role/disclosureRef
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-URI http://asc.fasb.org/extlink&oid=118930883&loc=SL7669625-108580

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Reference 7: http://www.xbrl.org/2003/role/disclosureRef
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Reference 8: http://www.xbrl.org/2003/role/disclosureRef
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Reference 9: http://www.xbrl.org/2003/role/disclosureRef
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-Name Accounting Standards Codification
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-Paragraph 32
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-URI http://asc.fasb.org/extlink&oid=115929826&loc=d3e8933-108599

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-Name Accounting Standards Codification
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-URI http://asc.fasb.org/extlink&oid=116874947&loc=SL4591552-111686

+ Details

Condensed Consolidated Statements of Changes in Stockholders' Equity - USD ($) $ in Thousands	Total	Common Stock [Member]	Additional Paid In Capital [Member]	Accumulated Other Comprehensive Income (Loss) [Member]	Accumulated Deficit [Member]
Beginning balance at Dec. 31, 2018	$ 125,183	$ 35	$ 324,209	$ (5)	$ (199,056)
Beginning balance at Dec. 31, 2018		35,432,460
Equity-based compensation	3,500		3,500
Net loss	(29,582)				(29,582)
Forfeiture of restricted stock awards units		(17,546)
Unrealized (loss) gain on available-for-sale securities	12			12
Ending balance at Mar. 31, 2019	99,113	$ 35	327,709	7	(228,638)
Ending balance, units at Mar. 31, 2019		35,414,914
Beginning balance at Dec. 31, 2019	80,048	$ 48	396,278	1	(316,279)
Beginning balance at Dec. 31, 2019		48,283,270
Equity-based compensation	3,104		3,104
Net loss	(26,694)				(26,694)
Vesting of restricted stock units		121,975
Forfeiture of restricted stock awards units		(38,822)
Unrealized (loss) gain on available-for-sale securities	(2)			(2)
Ending balance at Mar. 31, 2020	$ 56,456	$ 48	$ 399,382	$ (1)	$ (342,973)
Ending balance, units at Mar. 31, 2020		48,366,423

- Definition

The portion of profit or loss for the period, net of income taxes, which is attributable to the parent.

+ References + Details

Condensed Consolidated Statements of Cash Flows - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
Cash flows from operating activities:
Net loss	$ (26,694)	$ (29,582)
Adjustments to reconcile net loss to net cash used in operating activities:
Amortization of (discount)/premium on available-for-sale securities	(21)	(148)
Equity-based compensation expense	3,104	3,500
Depreciation expense	1,026	596
Changes in operating assets and liabilities:
Prepaid expenses and other current and non-current assets	(1,205)	(707)
Accounts payable	(4,309)	1,666
Accrued expenses and other current and non-current liabilities	(1,175)	(1,252)
Net cash used in operating activities	(29,274)	(25,927)
Cash flows from investing activities:
Purchases of property and equipment	(681)	(1,953)
Proceeds from sale and maturities of available-for-sale securities	7,500	10,924
Purchases of marketable investments	(401)	(15,416)
Net cash provided by (used in) investing activities	6,418	(6,445)
Cash flows from financing activities:
Net decrease in cash, cash equivalents and restricted cash	(22,856)	(32,372)
Cash, cash equivalents, and restricted cash at beginning of period	76,370	86,693
Cash, cash equivalents, and restricted cash at end of period	53,514	54,321
Supplemental disclosure of non-cash investing and financing activities:
Operating lease liabilities arising from obtaining right-of-use asset		1,629
Property and equipment included in accounts payable and accruals	$ 55	$ 1,043

Nature of the Business and Basis of Presentation

3 Months Ended

Mar. 31, 2020

Accounting Policies [Abstract]

Nature of the Business and Basis of Presentation

1. Nature of the Business and Basis of Presentation

Nature of Business

Solid Biosciences Inc. was organized in March 2013 under the name SOLID Ventures Management, LLC. In October 2013, the company changed its name to Solid Ventures, LLC and in June 2015, the company changed its name to Solid Biosciences, LLC. The company operated as a Delaware limited liability company under the name Solid Biosciences, LLC until immediately prior to the effectiveness of its registration statement on Form S-1 on January 25, 2018, at which time it completed a statutory corporate conversion into a Delaware corporation (the “Corporate Conversion”) and changed its name to Solid Biosciences Inc. (the “Company”). In addition, entities formed solely for the purpose of holding membership interests in the Company’s limited liability company were merged with and into the Company. As a result of the Corporate Conversion, all of the Series 1 and 2 Senior Preferred, Junior Preferred Units, Series A, B, C and D Common Units of Solid Biosciences, LLC converted into shares of common stock of Solid Biosciences Inc. on a one for 0.8485 basis and all of the unit holders of Solid Biosciences, LLC became holders of common stock of Solid Biosciences Inc.

The Company’s mission is to cure Duchenne muscular dystrophy (“Duchenne”), a genetic muscle-wasting disease predominantly affecting boys. It is caused by mutations in the dystrophin gene, which result in the absence or near-absence of dystrophin protein. Dystrophin protein works to strengthen muscle fibers and protect them from daily wear and tear. Without functioning dystrophin and certain associated proteins, muscles suffer excessive damage from normal daily activities and are unable to regenerate, leading to the build-up of fibrotic, or scar, and fat tissue. The Company’s lead product candidate, SGT-001, is a gene transfer candidate under investigation for its ability to drive functional dystrophin protein expression in patients’ muscles and improve the course of the disease. SGT-001 has been granted Rare Pediatric Disease Designation and Fast Track in the United States and Orphan Drug Designations in both the United States and European Union. The Company filed an Investigational New Drug application (“IND”) in September 2017 and initiated a Phase I/II clinical trial for SGT-001 in the United States during the fourth quarter of 2017, which is called IGNITE DMD. In November 2019, IGNITE DMD was placed on clinical hold by the U.S. Food and Drug Administration (“FDA”). In April 2020, the Company submitted a response to the FDA that included changes to the clinical protocol designed to enhance patient safety, as well as information related to improvements to its manufacturing process. The FDA has responded by maintaining the clinical hold and requesting further data and analyses relating to this manufacturing process. The Company is in the process of generating these data and expects to submit this information to the FDA before the end of the third quarter of 2020.

The Company is subject to risks and uncertainties common to early-stage companies in the biotechnology industry, including, but not limited to, development by competitors of new technological innovations, dependence on licenses, protection of proprietary technology, dependence on key personnel, compliance with government regulations and the need to obtain additional financing to fund operations. Product candidates currently under development will require significant additional research and development efforts, including extensive pre-clinical studies and clinical trials and regulatory approval, prior to commercialization. These efforts require significant amounts of additional capital, adequate personnel infrastructure and extensive compliance and reporting capabilities.

The Company’s product candidates are in development. There can be no assurance that the Company’s research and development will be successfully completed, that adequate protection for the Company’s intellectual property will be obtained, that any products developed will obtain necessary government regulatory approval or that any approved products will be commercially viable. Even if the Company’s product development efforts are successful, it is uncertain when, if ever, the Company will generate significant revenue from product sales. The Company operates in an environment of rapid change in technology and substantial competition from, among others, other pharmaceutical and biotechnology companies. In addition, the Company is dependent upon the services of its employees, partners and consultants.

Liquidity

The accompanying condensed consolidated financial statements have been prepared on a basis that assumes the Company will continue as a going concern and which contemplates the realization of assets and satisfaction of liabilities and commitments in the ordinary course of business. Through March 31, 2020, the Company has funded its operations primarily with the proceeds from the sale of redeemable preferred units and member units as well as the sale of common stock and prefunded warrants to purchase shares of its common stock in private placements and the sale of common stock in its initial public offering.

In accordance with Accounting Standards Codification (“ASC”) 205-40, Going Concern, the Company has evaluated whether there are conditions and events, considered in the aggregate, that raise substantial doubt about the Company’s ability to continue as a going concern within one year after the date the financial statements are issued. As of March 31, 2020, the Company had an accumulated deficit of $342,973. During the three months ended March 31, 2020, the Company incurred a net loss of $26,694 and used $29,274 of cash in operations. The Company expects to continue to generate operating losses in the foreseeable future. Based upon its current operating plan, the Company expects that its cash, cash equivalents and available-for-sale securities of $53,587 as of March 31, 2020 will be sufficient to fund its operating expenses and capital requirements into 2021.

In accordance with the requirements of ASC 205-40, the Company determined that there is substantial doubt about the Company’s ability to continue as a going concern within twelve months of the issuance date of these financial statements. The financial statements do not include any adjustments that might result from the outcome of this uncertainty. Although the Company has been successful in raising capital in the past, there is no assurance that it will be successful in obtaining such additional financing on terms acceptable to the Company, if at all, nor is it considered probable under the accounting standards. As such, under the requirements of ASC 205-40, management may not consider the potential for future capital raises or management plans to reduce costs that are not considered probable in its assessment of the Company’s ability to meet its obligations for the next twelve months. If the Company is unable to obtain funding, the Company would be forced to delay, reduce or eliminate some or all of its research and development programs, pre-clinical and clinical testing or commercialization efforts, which could adversely affect its business prospects.

The accompanying condensed consolidated financial statements have been prepared in accordance with accounting principles generally accepted in the United States of America (“GAAP”). The accompanying condensed consolidated financial statements include the accounts of Solid Biosciences Inc. and its wholly owned or controlled subsidiaries. All intercompany accounts and transactions have been eliminated.

In the opinion of management, the Company’s accompanying unaudited condensed consolidated financial statements include all adjustments, consisting of normal recurring accruals, necessary for a fair statement of the Company’s financial statements for interim periods in accordance with GAAP. The information included in this quarterly report on Form 10-Q should be read in conjunction with the Company’s consolidated financial statements and the accompanying notes included in the Company’s Annual Report on Form 10-K for the year ended December 31, 2019. The year-end condensed consolidated balance sheet data presented for comparative purposes was derived from the Company’s audited financial statements but does not include all disclosures required by GAAP. The results of operations for the three months ended March 31, 2020 are not necessarily indicative of the operating results for the full year or for any other subsequent interim period.

Summary of Significant Accounting Policies

3 Months Ended

Mar. 31, 2020

Accounting Policies [Abstract]

Summary of Significant Accounting Policies

2. Summary of Significant Accounting Policies

The Company’s accounting policies are described in the “Notes to Consolidated Financial Statements” in its 2019 Form 10-K and updated, as necessary, in this report.

Use of Estimates

The preparation of the Company’s condensed consolidated financial statements in conformity with GAAP requires management to make estimates, judgments and assumptions that affect the reported amounts of assets and liabilities, the disclosure of contingent assets and liabilities at the date of the condensed consolidated financial statements and the reported amounts of expenses during the reporting periods. Significant estimates and assumptions reflected in these condensed consolidated financial statements include, but are not limited to, the recognition of research and development expenses and equity-based compensation. Estimates are periodically reviewed in light of changes in circumstances, facts and experience. Changes in estimates are recorded in the period in which they become known. The full extent to which the COVID-19 pandemic will directly or indirectly impact the Company’s business, results of operations and financial condition, including clinical trials and employee-related amounts, will depend on future developments that are highly uncertain, including new information that may emerge concerning COVID-19 and the actions taken to contain it or treat its impact. The Company has made estimates of the impact of COVID-19 within its financial statements and there may be changes to those estimates in future periods. Actual results could differ from the Company’s estimates.

Cash Equivalents

The Company considers all short-term, highly liquid investments with original maturities of 90 days or less at acquisition date to be cash equivalents.

Restricted Cash

The Company held restricted cash of $327 in separate restricted bank accounts as security deposits for leases of the Company’s facilities as of March 31, 2020 and December 31, 2019. The Company has included restricted cash of $327 as a non-current asset as of March 31, 2020 and December 31, 2019. A reconciliation of the amounts of cash and cash equivalents and restricted cash from the cash flow statement to the balance sheet is as follows:

	March 31, 2020		December 31, 2019
Cash and cash equivalents as presented on balance sheet	$	53,187	$	76,043
Restricted cash, non-current, as presented on balance sheet		327		327
Cash and cash equivalents and restricted cash as presented on cash flow statement	$	53,514	$	76,370

Segment Data

The Company manages its operations as a single segment for the purposes of assessing performance and making operating decisions. The Company’s singular focus is on developing treatments through gene therapy and other means for patients with Duchenne. All of the Company’s tangible assets are held in the United States.

Recently Adopted Accounting Pronouncements

In August 2018, the Financial Accounting Standards Board issued ASU No. 2018-13, Fair Value Measurement (Topic 820): Disclosure Framework - Changes to the Disclosure Requirements for Fair Value Measurement. This new standard modifies certain disclosure requirements on fair value measurements. This new standard became effective for the Company on January 1, 2020. The adoption of this new standard did not have a material impact on the Company’s disclosures.

Recently Issued Accounting Pronouncements

In June 2016, the Financial Accounting Standards Board (“FASB”) issued ASU No. 2016-13, Financial Instruments—Credit Losses (Topic 326): Measurement of Credit Losses on Financial Instruments (‘‘ASU 2016-13’’), which requires the measurement and recognition of expected credit losses for financial assets held at amortized cost. ASU 2016-13 replaces the existing incurred loss impairment model with an expected loss model. It also eliminates the concept of other-than-temporary impairment and requires credit losses related to available-for-sale debt securities to be recorded through an allowance for credit losses rather than as a reduction in the amortized cost basis of the securities. These changes may result in earlier recognition of credit losses. In November 2018, the FASB issued ASU No. 2018-19, Codification Improvements to Topic 326, Financial Instruments—Credit Losses, which narrowed the scope and changed the effective date for non-public entities for ASU 2016-13. The FASB subsequently issued supplemental guidance within ASU No. 2019-05, Financial Instruments—Credit Losses (Topic 326): Targeted Transition Relief (‘‘ASU 2019-05’’). ASU 2019-05 provides an option to irrevocably elect the fair value option for certain financial assets previously measured at amortized cost basis. For public entities that are Securities and Exchange Commission filers, excluding entities eligible to be smaller reporting companies, ASU 2016-13 is effective for annual periods beginning after December 15, 2019, including interim periods within those fiscal years. For all other entities, ASU 2016-13 is effective for annual periods beginning after December 15, 2022, including interim periods within those fiscal years. Early adoption is permitted. This standard will be effective for the Company on January 1, 2023. The Company is currently evaluating the potential impact that this standard may have on its financial statements and related disclosures.

Fair Value of Financial Assets and Liabilities

3 Months Ended

Mar. 31, 2020

Fair Value Disclosures [Abstract]

Fair Value of Financial Assets and Liabilities

3. Fair Value of Financial Assets and Liabilities

The following tables present information about the Company’s assets and liabilities that are measured at fair value on a recurring basis and indicate the level of the fair value hierarchy utilized to determine such fair values:

	Fair Value Measurements as of March 31, 2020 Using:
	Level 1		Level 2		Level 3		Total
Assets:
Cash equivalents	$	—	$	14,053	$	—	$	14,053
Available-for-sale securities		—		400		—		400
	$	—	$	14,453	$	—	$	14,453

	Fair Value Measurements as of December 31, 2019 Using:
	Level 1		Level 2		Level 3		Total
Assets:
Cash equivalents	$	—	$	50,037	$	—	$	50,037
Available-for-sale securities		—		7,481		—		7,481
	$	—	$	57,518	$	—	$	57,518

As of March 31, 2020 and December 31, 2019, the fair values of the Company’s available-for-sale debt securities were determined using Level 2 inputs. At March 31, 2020, the Company’s portfolio of available-for-sale securities consisted of corporate bond securities. At December 31, 2019, the Company’s portfolio of available-for-sale securities consisted of corporate bond securities and commercial paper. During the three months ended March 31, 2020 and the year ended December 31, 2019, there were no transfers between Level 1, Level 2 and Level 3.

The fair value of the Company’s cash, restricted cash, accounts payable, and accrued expenses and other current liabilities approximate their carrying value due to their short-term maturities.

Available-for-Sale Securities

3 Months Ended

Mar. 31, 2020

Investments Debt And Equity Securities [Abstract]

Available-for-Sale Securities

4. Available-for-Sale Securities

As of March 31, 2020 and December 31, 2019, the fair value of available-for-sale securities by type of security was as follows:

	March 31, 2020
	Amortized Cost		Gross Unrealized Gain		Gross Unrealized Loss			Fair Value
Investments:
Corporate bond securities	$	401	$	—	$	(1	)	$	400
	$	401	$	—	$	(1	)	$	400

	December 31, 2019
	Amortized Cost		Gross Unrealized Gain		Gross Unrealized Loss		Fair Value
Investments:
Corporate bond securities		1,502		1		—		1,503
Commercial paper		5,978		—		—		5,978
	$	7,480	$	1	$	—	$	7,481

The estimated fair value and amortized cost of the Company’s available-for-sale securities by contractual maturity are summarized as follows:

	March 31, 2020				December 31, 2019
	Amortized Cost		Fair Value		Amortized Cost		Fair Value
Due in one year or less	$	401	$	400	$	7,480	$	7,481
Total available-for-sale securities	$	401	$	400	$	7,480	$	7,481

The weighted average maturity of the Company’s available-for-sale securities as of March 31, 2020 and December 31, 2019 was approximately 0.2 years and 0.2 years, respectively.

Property and Equipment

3 Months Ended

Mar. 31, 2020

Property Plant And Equipment [Abstract]

Property and Equipment

5. Property and Equipment

Property and equipment consists of the following:

	March 31, 2020		December 31, 2019
Furniture and fixtures	$	203	$	203
Laboratory equipment		9,529		9,425
Leasehold improvements		4,686		4,686
Computer equipment		436		428
Computer software		541		372
Construction in process		1,288		1,322
		16,683		16,436
Less accumulated depreciation		5,817		4,791
	$	10,866	$	11,645

Depreciation expense was $1,026 and $596 for the three months ended March 31, 2020 and 2019, respectively.

Prepaid Expenses and Other Current Assets

3 Months Ended

Mar. 31, 2020

Deferred Costs Capitalized Prepaid And Other Assets Disclosure [Abstract]

Prepaid Expenses and Other Current Assets

6. Prepaid Expenses and Other Current Assets

Prepaid expenses and other current assets consist of the following:

	March 31, 2020		December 31, 2019
Prepaid research and development expenses	$	1,036	$	1,290
Prepaid expenses and other assets		3,280		1,488
	$	4,316	$	2,778

Accrued Expenses and Other Current Liabilities

3 Months Ended

Mar. 31, 2020

Payables And Accruals [Abstract]

Accrued Expenses and Other Current Liabilities

7. Accrued Expenses and Other Current Liabilities

Accrued expenses and other current liabilities consist of the following:

	March 31, 2020		December 31, 2019
Accrued research and development	$	4,427	$	3,742
Accrued compensation		2,459		3,583
Accrued other		1,621		1,853
	$	8,507	$	9,178

Stockholders' Equity

3 Months Ended

Mar. 31, 2020

Equity [Abstract]

Stockholders' Equity

8. Stockholders’ Equity

On July 30, 2019, the Company issued and sold in a private placement (i) 10,607,525 shares of its common stock at a price per share of $4.65 and (ii) 2,295,699 pre-funded warrants to purchase shares of its common stock at a price per warrant of $4.64. Each pre-funded warrant is exercisable for one share of common stock at an exercise price of $0.01 and the pre-funded warrants have no expiration date. The Company received gross proceeds from the private placement of $59,977, before deducting offering costs of $2,079.

No warrants were exercised during the three months ended March 31, 2020 and no warrants were outstanding during the three months ended March 31, 2019.

Equity-Based Compensation

3 Months Ended

Mar. 31, 2020

Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]

Equity-Based Compensation

9. Equity-Based Compensation

In connection with the closing of the Company’s initial public offering, the board of directors and stockholders approved the 2018 Omnibus Incentive Plan, which provides for the reservation of 5,001,000 shares of common stock for equity awards. During the three months ended March 31, 2020 and March 31, 2019, the Company granted options for the purchase of 816,116 and 1,543,421 shares of common stock, respectively. During the three months ended March 31, 2020, the Company granted 957,655 restricted stock units.

The Company recorded equity-based compensation expense related to all of its share-based awards to employees and non-employees in the following captions within its condensed consolidated statements of operations:

	Three Months Ended March 31,
	2020		2019
Research and development	$	1,239	$	1,819
General and administrative		1,014		1,681
Restructuring		851		—
Total	$	3,104	$	3,500

Income Taxes

3 Months Ended

Mar. 31, 2020

Income Tax Disclosure [Abstract]

Income Taxes

10. Income Taxes

During the three months ended March 31, 2020 and 2019, the Company recorded no income tax benefits for the net operating losses incurred or for the research and development tax credits and orphan drug credits generated in each year due to its uncertainty of realizing a benefit from those items. The Company has provided a valuation allowance for the full amounts of its net deferred tax assets because, at March 31, 2020 and December 31, 2019, it was more likely than not that any future benefit from deductible temporary differences and net operating loss and tax credit carryforwards would not be realized.

As of March 31, 2020, and December 31, 2019, the Company had not recorded any amounts for unrecognized tax benefits. The Company files income tax returns as prescribed by the tax laws of the jurisdictions in which it operates. In the normal course of business, the Company is subject to examination by federal and state jurisdictions, where applicable. There are currently no pending income tax examinations. The Company’s C-Corporation tax years beginning with the year ended December 31, 2018 are open under statute. Any tax credit or net operating loss carryforward can be adjusted in future periods after the respective year of generation’s statute of limitation has closed.

Commitments and Contingencies

3 Months Ended

Mar. 31, 2020

Commitments And Contingencies Disclosure [Abstract]

Commitments and Contingencies

11. Commitments and Contingencies

Legal Proceedings

The Company may periodically become subject to legal proceedings and claims arising in connection with ongoing business activities, including claims or disputes related to patents that have been issued or that are pending in the field of research on which the Company is focused. The Company is not aware of any material claims as of March 31, 2020.

Net Loss per Share

3 Months Ended

Mar. 31, 2020

Earnings Per Share [Abstract]

Net Loss per Share

12. Net Loss per Share

Basic and diluted net loss per share attributable to common stockholders were calculated as follows:

The numerator for basic and diluted net loss per share attributable to common stockholders is as follows:

	Three Months Ended March 31,
	2020			2019
Net loss attributable to common stockholders	$	(26,694	)	$	(29,582	)

The denominator is as follows:

Three Months Ended March 31,

2020

2019

Weighted average shares of common stock outstanding,

basic and diluted

48,059,279

34,776,488

Net loss per share attributable to common stockholders, basic and diluted is as follows:

	Three Months Ended March 31,
	2020			2019
Net loss per share attributable to common stockholders	$	(0.56	)	$	(0.85	)

The following potential common stock equivalents, presented based on amounts outstanding at each period end, were excluded from the calculation of diluted net loss per share attributable to common stockholders for the periods indicated because including them would have had an anti-dilutive effect for the three months ended March 31:

	2020		2019
Options to purchase shares of common stock		3,099,758		2,681,071
Unvested shares of common stock		219,284		599,426
Unvested restricted stock units		1,011,955		—
		4,330,997		3,280,497

Restructuring

3 Months Ended

Mar. 31, 2020

Restructuring And Related Activities [Abstract]

Restructuring

13. Restructuring

In January 2020, the Company’s board of directors approved a restructuring plan to reduce operating costs and better align the Company’s workforce with the needs of its business following the Company’s November 2019 announcement that the SGT-001 IGNITE DMD trial was placed on clinical hold by the U.S. Food and Drug Administration.

Under the restructuring plan, the Company made changes to its management team and reduced headcount by approximately 30 percent. Affected employees were eligible to receive severance payments and outplacement services in connection with the restructuring plan. In the three months ended March 31, 2020, the Company recorded aggregate restructuring charges of $1,944 related to severance payments and other employee-related costs. The Company does not expect to incur any additional significant costs associated with this restructuring. During the three months ended March 31, 2020, $963 of the estimated restructuring charges were paid. The Company expects the remaining accrued restructuring costs of $981 will be paid in the next 12 months.

The following table shows the total amount expected to be incurred and the liability related to the 2020 restructuring as of March 31, 2020:

	One-Time Employee Termination Benefits
Accrued restructuring costs beginning balance	$	-
Restructuring charges incurred during the period		1,944
Amounts paid during the period		(963	)
Accrued restructuring costs as of March 31, 2020	$	981

Summary of Significant Accounting Policies (Policies)

3 Months Ended

Mar. 31, 2020

Accounting Policies [Abstract]

Use of Estimates

Cash Equivalents

The Company considers all short-term, highly liquid investments with original maturities of 90 days or less at acquisition date to be cash equivalents.

Restricted Cash

	March 31, 2020		December 31, 2019
Cash and cash equivalents as presented on balance sheet	$	53,187	$	76,043
Restricted cash, non-current, as presented on balance sheet		327		327
Cash and cash equivalents and restricted cash as presented on cash flow statement	$	53,514	$	76,370

Segment Data

Recently Adopted Accounting Pronouncements

Recently Issued Accounting Pronouncements

Summary of Significant Accounting Policies (Tables)

3 Months Ended

Mar. 31, 2020

Accounting Policies [Abstract]

Reconciliation of Amounts From Cash Flow Statement to Balance Sheet

A reconciliation of the amounts of cash and cash equivalents and restricted cash from the cash flow statement to the balance sheet is as follows:

	March 31, 2020		December 31, 2019
Cash and cash equivalents as presented on balance sheet	$	53,187	$	76,043
Restricted cash, non-current, as presented on balance sheet		327		327
Cash and cash equivalents and restricted cash as presented on cash flow statement	$	53,514	$	76,370

Fair Value of Financial Assets and Liabilities (Tables)

3 Months Ended

Mar. 31, 2020

Fair Value Disclosures [Abstract]

Summary of Assets and Liabilities Measured at Fair Value on Recurring Basis and Indicate Level of Fair Value Hierarchy Utilized

	Fair Value Measurements as of March 31, 2020 Using:
	Level 1		Level 2		Level 3		Total
Assets:
Cash equivalents	$	—	$	14,053	$	—	$	14,053
Available-for-sale securities		—		400		—		400
	$	—	$	14,453	$	—	$	14,453

	Fair Value Measurements as of December 31, 2019 Using:
	Level 1		Level 2		Level 3		Total
Assets:
Cash equivalents	$	—	$	50,037	$	—	$	50,037
Available-for-sale securities		—		7,481		—		7,481
	$	—	$	57,518	$	—	$	57,518

Available-for-Sale Securities (Tables)

3 Months Ended

Mar. 31, 2020

Investments Debt And Equity Securities [Abstract]

Schedule of Fair Value of Available-for-Sale Securities by Type of Security

As of March 31, 2020 and December 31, 2019, the fair value of available-for-sale securities by type of security was as follows:

	March 31, 2020
	Amortized Cost		Gross Unrealized Gain		Gross Unrealized Loss			Fair Value
Investments:
Corporate bond securities	$	401	$	—	$	(1	)	$	400
	$	401	$	—	$	(1	)	$	400

	December 31, 2019
	Amortized Cost		Gross Unrealized Gain		Gross Unrealized Loss		Fair Value
Investments:
Corporate bond securities		1,502		1		—		1,503
Commercial paper		5,978		—		—		5,978
	$	7,480	$	1	$	—	$	7,481

Summary Estimated Fair Value and Amortized Cost of Available-for-Sale Securities by Contractual Maturity

The estimated fair value and amortized cost of the Company’s available-for-sale securities by contractual maturity are summarized as follows:

	March 31, 2020				December 31, 2019
	Amortized Cost		Fair Value		Amortized Cost		Fair Value
Due in one year or less	$	401	$	400	$	7,480	$	7,481
Total available-for-sale securities	$	401	$	400	$	7,480	$	7,481

Property and Equipment (Tables)

3 Months Ended

Mar. 31, 2020

Property Plant And Equipment [Abstract]

Summary of Property and Equipment

Property and equipment consists of the following:

	March 31, 2020		December 31, 2019
Furniture and fixtures	$	203	$	203
Laboratory equipment		9,529		9,425
Leasehold improvements		4,686		4,686
Computer equipment		436		428
Computer software		541		372
Construction in process		1,288		1,322
		16,683		16,436
Less accumulated depreciation		5,817		4,791
	$	10,866	$	11,645

Prepaid Expenses and Other Current Assets (Tables)

3 Months Ended

Mar. 31, 2020

Deferred Costs Capitalized Prepaid And Other Assets Disclosure [Abstract]

Schedule of Prepaid Expenses and Other Current Assets

Prepaid expenses and other current assets consist of the following:

	March 31, 2020		December 31, 2019
Prepaid research and development expenses	$	1,036	$	1,290
Prepaid expenses and other assets		3,280		1,488
	$	4,316	$	2,778

Accrued Expenses and Other Current Liabilities (Tables)

3 Months Ended

Mar. 31, 2020

Payables And Accruals [Abstract]

Summary of Accrued Expenses and Other Current Liabilities

Accrued expenses and other current liabilities consist of the following:

	March 31, 2020		December 31, 2019
Accrued research and development	$	4,427	$	3,742
Accrued compensation		2,459		3,583
Accrued other		1,621		1,853
	$	8,507	$	9,178

Equity-Based Compensation (Tables)

3 Months Ended

Mar. 31, 2020

Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]

Schedule of Equity-based Compensation Expense

	Three Months Ended March 31,
	2020		2019
Research and development	$	1,239	$	1,819
General and administrative		1,014		1,681
Restructuring		851		—
Total	$	3,104	$	3,500

Net Loss per Share (Tables)

3 Months Ended

Mar. 31, 2020

Earnings Per Share [Abstract]

Schedule of Basic and Diluted Net Loss per Share Attributable to Common Stockholders

The numerator for basic and diluted net loss per share attributable to common stockholders is as follows:

	Three Months Ended March 31,
	2020			2019
Net loss attributable to common stockholders	$	(26,694	)	$	(29,582	)

The denominator is as follows:

Three Months Ended March 31,

2020

2019

Weighted average shares of common stock outstanding,

basic and diluted

48,059,279

34,776,488

Net loss per share attributable to common stockholders, basic and diluted is as follows:

	Three Months Ended March 31,
	2020			2019
Net loss per share attributable to common stockholders	$	(0.56	)	$	(0.85	)

Schedule of Antidilutive Securities Excluded from Calculation of Diluted Net Loss per Share Attributable to Common Stockholders

	2020		2019
Options to purchase shares of common stock		3,099,758		2,681,071
Unvested shares of common stock		219,284		599,426
Unvested restricted stock units		1,011,955		—
		4,330,997		3,280,497

Restructuring (Tables)

3 Months Ended

Mar. 31, 2020

Restructuring And Related Activities [Abstract]

Schedule of Total Amount Expected to be Incurred and Liability Related to Restructuring

The following table shows the total amount expected to be incurred and the liability related to the 2020 restructuring as of March 31, 2020:

	One-Time Employee Termination Benefits
Accrued restructuring costs beginning balance	$	-
Restructuring charges incurred during the period		1,944
Amounts paid during the period		(963	)
Accrued restructuring costs as of March 31, 2020	$	981

Nature of the Business and Basis of Presentation - Additional Information (Detail) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019	Dec. 31, 2019	Jan. 25, 2018
Nature Of Business And Basis Of Presentation [Line Items]
Net loss	$ 26,694	$ 29,582
Accumulated deficit	342,973		$ 316,279
Net cash used in operating activities	29,274	$ 25,927
Cash, cash equivalents and available-for-sale securities	$ 53,587
Corporate Conversion [Member]
Nature Of Business And Basis Of Presentation [Line Items]
Common stock, conversion ratio				84.85%

- Definition

The consolidated profit or loss for the period, net of income taxes, including the portion attributable to the noncontrolling interest.

+ References

+ Details

Summary of Significant Accounting Policies - Additional Information (Detail) - USD ($) $ in Thousands	Mar. 31, 2020	Dec. 31, 2019
Accounting Policies [Abstract]
Restricted cash deposit	$ 327	$ 327
Restricted cash noncurrent	$ 327	$ 327

Summary of Significant Accounting Policies - Reconciliation of Amounts From Cash Flow Statement to Balance Sheet (Detail) - USD ($) $ in Thousands	Mar. 31, 2020	Dec. 31, 2019	Mar. 31, 2019	Dec. 31, 2018
Reconciliation Of Beginning And End Of Period Amounts From Cash Flow Statement To Balance Sheet [Abstract]
Cash and cash equivalents as presented on balance sheet	$ 53,187	$ 76,043
Restricted cash, non-current, as presented on balance sheet	327	327
Cash and cash equivalents and restricted cash as presented on cash flow statement	$ 53,514	$ 76,370	$ 54,321	$ 86,693

Fair Value of Financial Assets and Liabilities - Summary of Assets and Liabilities Measured at Fair Value on Recurring Basis and Indicate Level of Fair Value Hierarchy Utilized (Detail) - USD ($) $ in Thousands	Mar. 31, 2020	Dec. 31, 2019
Assets:
Cash equivalents	$ 14,053	$ 50,037
Available-for-sale securities	400	7,481
Assets Fair Value Disclosure	14,453	57,518
Level 2 [Member]
Assets:
Cash equivalents	14,053	50,037
Available-for-sale securities	400	7,481
Assets Fair Value Disclosure	$ 14,453	$ 57,518

Fair Value of Financial Assets and Liabilities - Additional Information (Detail) - USD ($)	3 Months Ended	12 Months Ended
	Mar. 31, 2020	Dec. 31, 2019
Fair Value Disclosures [Abstract]
Fair value, assets, level 1 to level 2 transfers, amount	$ 0	$ 0
Fair value, assets, level 2 to level 1 transfers, amount	0	0
Fair value, assets, transfers into level 3, amount	0	0
Fair value, assets, transfers out of level 3, amount	$ 0	$ 0

Available-for-Sale Securities - Schedule of Fair Value of Available-for-Sale Securities by Type of Security (Detail) - USD ($) $ in Thousands	Mar. 31, 2020	Dec. 31, 2019
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	$ 401	$ 7,480
Gross Unrealized Gain		1
Gross Unrealized Loss	(1)
Fair Value	400	7,481
Corporate Bond Securities [Member]
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	401	1,502
Gross Unrealized Gain		1
Gross Unrealized Loss	(1)
Fair Value	$ 400	1,503
Commercial Paper [Member]
Debt Securities, Available-for-sale [Line Items]
Amortized Cost		5,978
Fair Value		$ 5,978

Available-for-Sale Securities - Summary Estimated Fair Value and Amortized Cost of Available-for-Sale Securities by Contractual Maturity (Detail) - USD ($) $ in Thousands	Mar. 31, 2020	Dec. 31, 2019
Investments Debt And Equity Securities [Abstract]
Due in one year or less, amortized cost	$ 401	$ 7,480
Amortized Cost	401	7,480
Due in one year or less, fair value	400	7,481
Total available-for-sale securities, fair value	$ 400	$ 7,481

Available-for-Sale Securities - Additional Information (Detail)	3 Months Ended	12 Months Ended
	Mar. 31, 2020	Dec. 31, 2019
Investments Debt And Equity Securities [Abstract]
Available-for-sale securities weighted average maturity period	2 months 12 days	2 months 12 days

Property and Equipment - Summary of Property and Equipment (Detail) - USD ($) $ in Thousands	Mar. 31, 2020	Dec. 31, 2019
Property, Plant and Equipment [Line Items]
Property and equipment, gross	$ 16,683	$ 16,436
Less accumulated depreciation	5,817	4,791
Property and equipment, net	10,866	11,645
Furniture and Fixtures [Member]
Property, Plant and Equipment [Line Items]
Property and equipment, gross	203	203
Laboratory Equipment [Member]
Property, Plant and Equipment [Line Items]
Property and equipment, gross	9,529	9,425
Leasehold Improvements [Member]
Property, Plant and Equipment [Line Items]
Property and equipment, gross	4,686	4,686
Computer Equipment [Member]
Property, Plant and Equipment [Line Items]
Property and equipment, gross	436	428
Computer Software [Member]
Property, Plant and Equipment [Line Items]
Property and equipment, gross	541	372
Construction in Progress [Member]
Property, Plant and Equipment [Line Items]
Property and equipment, gross	$ 1,288	$ 1,322

Property and Equipment - Additional Information (Detail) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
Property Plant And Equipment [Abstract]
Depreciation expense	$ 1,026	$ 596

Prepaid Expenses and Other Current Assets - Schedule of Prepaid Expenses and Other Current Assets (Detail) - USD ($) $ in Thousands	Mar. 31, 2020	Dec. 31, 2019
Deferred Costs Capitalized Prepaid And Other Assets Disclosure [Abstract]
Prepaid research and development expenses	$ 1,036	$ 1,290
Prepaid expenses and other assets	3,280	1,488
Prepaid expenses and other current assets	$ 4,316	$ 2,778

Accrued Expenses and Other Current Liabilities - Summary of Accrued Expenses and Other Current Liabilities (Detail) - USD ($) $ in Thousands	Mar. 31, 2020	Dec. 31, 2019
Accrued Expenses And Other Current Liabilities [Line Items]
Accrued expenses and other current liabilities	$ 8,507	$ 9,178
Accrued Research and Development [Member]
Accrued Expenses And Other Current Liabilities [Line Items]
Accrued expenses and other current liabilities	4,427	3,742
Accrued Compensation [Member]
Accrued Expenses And Other Current Liabilities [Line Items]
Accrued expenses and other current liabilities	2,459	3,583
Accrued Other [Member]
Accrued Expenses And Other Current Liabilities [Line Items]
Accrued expenses and other current liabilities	$ 1,621	$ 1,853

Stockholders' Equity - Additional Information (Detail) - USD ($) $ / shares in Units, $ in Thousands		3 Months Ended
	Jul. 30, 2019	Mar. 31, 2020	Dec. 31, 2019	Mar. 31, 2019
Class Of Stock [Line Items]
Gross proceeds from private placement	$ 59,977
Warrants outstanding		2,295,699	2,295,699	0
Private Placement [Member]
Class Of Stock [Line Items]
Offering costs	$ 2,079
Common Stock [Member] \| Private Placement [Member]
Class Of Stock [Line Items]
Stock issued and sold	10,607,525
Stock price per share	$ 4.65
Pre-Funded Warrants [Member]
Class Of Stock [Line Items]
Warrants exercised		0
Pre-Funded Warrants [Member] \| Private Placement [Member]
Class Of Stock [Line Items]
Warrants to purchase common stock	2,295,699
Warrants price per share	$ 4.64
Warrants exercisable for common stock	1
Exercise price of warrant	$ 0.01

Equity-Based Compensation - Additional Information (Detail) - 2018 Omnibus Incentive Plan [Member] - shares	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of shares authorized for common stock for equity awards	5,001,000
Common stock purchased	816,116	1,543,421
Restricted Stock Units [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Restricted stock units granted	957,655

Equity-Based Compensation - Schedule of Equity-based Compensation Expense (Detail) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items]
Stock-based compensation expense	$ 3,104	$ 3,500
Research and Development [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items]
Stock-based compensation expense	1,239	1,819
General and Administrative [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items]
Stock-based compensation expense	1,014	$ 1,681
Restructuring [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items]
Stock-based compensation expense	$ 851

Income Taxes - Additional Information (Detail) - USD ($)	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019	Dec. 31, 2019
Income Tax Disclosure [Abstract]
Income tax expense (benefit)	$ 0	$ 0
Unrecognized tax benefits	$ 0		$ 0

Net Loss per Share - Schedule of Basic and Diluted Net Loss per Share Attributable to Common Stockholders (Detail) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
Earnings Per Share Basic [Line Items]
Net loss attributable to common stockholders	$ (26,694)	$ (29,582)
Weighted average shares of common stock outstanding, basic and diluted	48,059,279	34,776,488
Net loss per share attributable to common stockholders	$ (0.56)	$ (0.85)
Common Stock [Member]
Earnings Per Share Basic [Line Items]
Weighted average shares of common stock outstanding, basic and diluted	48,059,279	34,776,488

Net Loss per Share - Schedule of Antidilutive Securities Excluded from Calculation of Diluted Net Loss per Share Attributable to Common Stockholders (Detail) - shares	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Common stock equivalents excluded from calculation of diluted net loss per share	4,330,997	3,280,497
Options to Purchase Shares of Common Stock [Member]
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Common stock equivalents excluded from calculation of diluted net loss per share	3,099,758	2,681,071
Unvested Restricted Stock Units [Member]
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Common stock equivalents excluded from calculation of diluted net loss per share	1,011,955
Unvested Shares of Common Stock [Member]
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Common stock equivalents excluded from calculation of diluted net loss per share	219,284	599,426

Restructuring- Additional Information (Detail) $ in Thousands	3 Months Ended
	Mar. 31, 2020 USD ($)
Restructuring Costs [Abstract]
Number of headcount reduced, percentage	30.00%
Restructuring charges related to severance payment and other employee-related costs	$ 1,944
Restructuring charges were paid	963
Restructuring costs	$ 981

Restructuring - Schedule of Total Amount Expected to be Incurred and Liability Related to Restructuring (Detail) $ in Thousands	3 Months Ended
	Mar. 31, 2020 USD ($)
Restructuring Costs [Line Items]
Restructuring charges incurred during the period	$ 1,944
Amounts paid during the period	(963)
Accrued restructuring costs as of March 31, 2020	981
One-Time Employee Termination Benefits [Member]
Restructuring Costs [Line Items]
Restructuring charges incurred during the period	1,944
Amounts paid during the period	(963)
Accrued restructuring costs as of March 31, 2020	$ 981