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		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Earnings Per Share [Abstract]
Net loss—basic and diluted	$ (3,462)	$ (24,471)	$ (29,713)	$ (82,776)
Weighted average shares outstanding—basic and diluted	350,994	295,595	336,938	291,526
Net loss per share—basic and diluted	$ (0.01)	$ (0.08)	$ (0.09)	$ (0.28)