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								12 Months Ended
	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, 2017	Dec. 31, 2016	Dec. 31, 2015
Numerator:
Net loss	$ (50,534)	$ (34,188)	$ (28,880)	$ (23,879)	$ (23,112)	$ (17,782)	$ (14,185)	$ (16,815)	$ (137,481)	$ (71,894)	$ (46,315)
Net loss:	$ (50,534)	$ (34,188)	$ (28,880)	$ (23,879)	$ (23,112)	$ (17,782)	$ (14,185)	$ (16,815)	$ (137,481)	$ (71,894)	$ (46,315)
Denominator:
Weighted average common shares outstanding, basic and diluted	36,644	31,931	31,826	31,704	29,347	27,524	22,666	21,570	33,027	25,297	21,497
Net loss per share, basic and diluted	$ (1.38)	$ (1.07)	$ (0.91)	$ (0.75)	$ (0.79)	$ (0.65)	$ (0.63)	$ (0.78)	$ (4.16)	$ (2.84)	$ (2.15)