Net Loss Per Share - Computation of Company's Basic and Diluted Net Loss Per Share (Detail) - USD ($) $ / shares in Units, $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Earnings Per Share [Abstract]
Net loss	$ (72,176)	$ (50,636)	$ (44,995)
Weighted-average common shares used in computing net loss per share, basic and diluted	58,457,868	40,068,307	17,432,434
Net loss per share, basic and diluted	$ (1.23)	$ (1.26)	$ (2.58)