Net Loss Per Share - Computation of Basic and Diluted Net Loss per Share (Detail) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended		12 Months Ended
	Mar. 31, 2019	Mar. 31, 2018	Dec. 31, 2018
Earnings Per Share [Abstract]
Net loss, basic and diluted	$ (44,076)	$ (17,713)	$ (49,200)
Weighted average shares used to compute basic and diluted net loss per share	30,363,623	29,836,277
Net loss per share, basic and diluted	$ (1.45)	$ (0.59)