Background and Organization - Additional Information (Detail) - USD ($) $ / shares in Units, $ in Thousands							12 Months Ended
	Jan. 07, 2021	Jan. 04, 2021	Dec. 06, 2019	Dec. 03, 2019	Aug. 17, 2018	Aug. 14, 2018	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Organization Consolidation And Presentation Of Financial Statements [Abstract]
Net loss							$ (107,908)	$ (120,199)	$ (76,396)
Net cash used in operations							(63,474)	(79,567)	$ (122,194)
Cash and cash equivalents							$ 156,720	$ 226,783
Underwriters | Common Stock
Organization And Nature Of Operations [Line Items]
Underwriting agreement date				Dec. 03, 2019		Aug. 14, 2018
Number of shares issued in transaction				41,550,000		16,216,217
Sale of stock, price				$ 3.13		$ 9.25
Aggregate proceeds from issuance of common stock			$ 138,400		$ 161,900
Underwriters | Common Stock | Subsequent Event
Organization And Nature Of Operations [Line Items]
Underwriting agreement date		Jan. 04, 2021
Number of shares issued in transaction		11,856,500
Sale of stock, price		$ 4.85
Aggregate proceeds from issuance of common stock	$ 53,500