Business and Basis of Presentation - Additional Information (Detail) - USD ($)		1 Months Ended	3 Months Ended												12 Months Ended
	Oct. 20, 2016	Oct. 31, 2016	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Oct. 31, 2017
Organization Consolidation And Presentation Of Financial Statements Disclosure [Line Items]
Net loss			$ 64,078,000	$ 77,180,000	$ 77,832,000	$ 72,865,000	$ (72,661,000)	$ (65,781,000)	$ (66,597,000)	$ (70,972,000)	$ (61,719,000)	$ (60,417,000)	$ (64,694,000)	$ (52,454,000)	$ (291,955,000)	$ (276,011,000)	$ (239,284,000)
Net cash used in operating activities															(172,470,000)	(141,690,000)	(154,468,000)
Cash and cash equivalents and marketable securities			$ 81,700,000												$ 81,700,000
Net proceeds from issuance of common stock																$ 161,854,000	$ 205,133,000
Silicon Valley Bank and Oxford finance | Term Loan
Organization Consolidation And Presentation Of Financial Statements Disclosure [Line Items]
Term loan, maximum borrowing capacity																		$ 100,000,000
Underwritten Public Offering
Organization Consolidation And Presentation Of Financial Statements Disclosure [Line Items]
Net proceeds from issuance of common stock	$ 161,900,000	$ 161,900,000