Selected Quarterly Financial Data (Detail) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Quarterly Financial Information Disclosure [Abstract]
Total revenues	$ 16,031	$ 4,469	$ 3,893	$ 3,465	$ 4,530	$ 2,043	$ 2,163	$ 3,009	$ 27,859	$ 11,745	$ 174,758
Net income (loss)	$ (9,747)	$ (24,168)	$ (22,675)	$ (30,385)	$ (32,784)	$ (32,886)	$ (35,339)	$ (16,324)	$ (86,975)	$ (117,333)	$ 125,664
Net income (loss) per share — basic	$ (0.06)	$ (0.16)	$ (0.16)	$ (0.25)	$ (0.28)	$ (0.31)	$ (0.35)	$ (0.17)	$ (0.60)	$ (1.13)	$ 1.37
Net income (loss) per share — diluted	$ (0.06)	$ (0.16)	$ (0.16)	$ (0.25)	$ (0.28)	$ (0.31)	$ (0.35)	$ (0.17)	$ (0.60)	$ (1.13)	$ 1.36
Weighted average common shares used to compute basic net income (loss) per share	161,397	153,597	140,054	120,911	116,451	104,703	99,864	95,744	144,136	104,245	92,053
Weighted average common shares used to compute diluted net income (loss) per share	161,397	153,597	140,054	120,911	116,451	104,703	99,864	95,744	144,136	104,245	92,085
Net (loss) income (numerator)	$ (9,747)	$ (24,168)	$ (22,675)	$ (30,385)	$ (32,784)	$ (32,886)	$ (35,339)	$ (16,324)	$ (86,975)	$ (117,333)	$ 125,664