Significant Accounting Policies - Calculation of Net Loss and Number of Shares Used to Compute Basic and Diluted Net Loss per Share (Detail) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Earnings Per Share [Abstract]
Net loss—basic and diluted	$ (24,431)	$ (24,095)
Weighted average shares outstanding—basic and diluted	328,712	285,207
Net loss per share—basic and diluted	$ (0.07)	$ (0.08)