Summary of Significant Accounting Policies - Additional Information (Detail) - USD ($)		3 Months Ended						9 Months Ended
	Mar. 17, 2020	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Sep. 30, 2020	Sep. 30, 2019	Feb. 28, 2020	Dec. 31, 2019
Summary Of Significant Accounting Policies [Line Items]
Net loss		$ (2,316,000)	$ (81,687,000)	$ (184,123,000)	$ (36,351,000)	$ (36,301,000)	$ (29,369,000)	$ (268,124,000)	$ (102,021,000)
Net cash used in operating activities								(104,486,000)	$ (167,255,000)
Working capital		171,349,000						171,349,000			$ 166,600,000
Proceeds from issuance of registered offering, net of issuance costs	$ 85,289,000							$ 85,465,000
Proceeds from registered equity offering, gross	$ 90,439,000	$ 46,153,000
Warrants issued	19,000,000
Warrants outstanding		19,000,000						19,000,000
Maximum allowed aggregate gross proceeds from share issuance under ATM program								$ 20,000,000
Warrant exercise price		$ 5.95						$ 5.95
Maximum amount of share issuance allowed per quarter								$ 6,000,000
Sale price		$ 5.95						$ 5.95
Cash and cash equivalents		$ 155,205,000						$ 155,205,000			$ 96,791,000
Antidilutive securities excluded from computation of earnings per share								13,141,298	8,043,920
Convertible Senior Notes Due 2023 [Member]
Summary Of Significant Accounting Policies [Line Items]
Debt instrument interest rate		5.00%						5.00%
Senior Facility [Member]
Summary Of Significant Accounting Policies [Line Items]
Maximum aggregate principal amount										$ 59,600,000
Aggregate principal amount borrowed										49,700,000
Additional draw provided		$ 9,900,000						$ 9,900,000		$ 9,900,000