Selected Quarterly Financial Data (Detail) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Quarterly Financial Information Disclosure [Abstract]
Total revenues	$ 10,570	$ 10,575	$ 8,301	$ 5,773	$ 5,318	$ 1,461	$ 1,254	$ 165	$ 35,219	$ 8,198	$ 327
Net loss	(36,756)	(33,459)	(42,713)	(39,672)	$ (39,969)	$ (40,528)	$ (39,444)	$ (38,958)	(152,600)	(158,899)	(95,154)
Net loss allocable to common stockholders	$ (36,756)	$ (36,726)	$ (42,713)	$ (39,672)					$ (155,867)	$ (158,899)	$ (95,154)
Net loss per share allocable to common stockholders - basic and diluted	$ (0.44)	$ (0.49)	$ (0.66)	$ (0.62)	$ (0.64)	$ (0.65)	$ (0.63)	$ (0.63)	$ (2.16)	$ (2.55)	$ (1.81)
Weighted average shares used to compute basic and diluted net loss per share allocable to common stockholders	83,868	75,106	65,088	63,778	62,694	62,650	62,346	61,744	72,024	62,362	52,613