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	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Earnings Per Share [Abstract]
Net loss—basic and diluted	$ (18,000)	$ (22,645)	$ (116,445)
Weighted average shares outstanding—basic and diluted	381,759	342,538	297,237
Net loss per share—basic and diluted	$ (0.05)	$ (0.07)	$ (0.39)