Selected Quarterly Financial Data (Detail) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Quarterly Financial Information Disclosure [Abstract]
Net revenues	$ 4,530	$ 2,043	$ 2,163	$ 3,009	$ 12,404	$ 162,354			$ 11,745	$ 174,758
Net income (loss)	$ (32,784)	$ (32,886)	$ (35,339)	$ (16,324)	$ 53,976	$ 126,520	$ (29,959)	$ (24,873)	$ (117,333)	$ 125,664	$ (368,445)
Net income (loss) per share — basic	$ (0.28)	$ (0.31)	$ (0.35)	$ (0.17)	$ 0.56	$ 1.32	$ (0.33)	$ (0.29)	$ (1.13)	$ 1.37	$ (4.54)
Net income (loss) per share — diluted	$ (0.28)	$ (0.31)	$ (0.35)	$ (0.17)	$ 0.56	$ 1.31	$ (0.33)	$ (0.29)	$ (1.13)	$ 1.36	$ (4.54)
Weighted average common shares used to compute basic net income (loss) per share	116,451	104,703	99,864	95,744	95,676	95,627	91,061	85,771	104,245	92,053	81,233
Weighted average common shares used to compute diluted net income (loss) per share	116,451	104,703	99,864	95,744	96,510	96,548	91,061	85,771	104,245	92,085	81,233