Net Loss per Share - Schedule of Basic and Diluted Net Loss Per Share Attributable to Common Stockholders (Detail) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended						9 Months Ended
	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Sep. 30, 2019	Sep. 30, 2018
Numerator:
Net loss	$ (38,232)	$ (36,487)	$ (41,538)	$ (43,640)	$ (43,875)	$ (41,569)	$ (116,257)	$ (129,084)
Net loss:	$ (38,232)	$ (36,487)	$ (41,538)	$ (43,640)	$ (43,875)	$ (41,569)	$ (116,257)	$ (129,084)
Denominator:
Weighted average common shares outstanding, basic and diluted	38,125			37,818			38,043	37,712
Net loss per share, basic and diluted	$ (1.00)			$ (1.15)			$ (3.06)	$ (3.42)