Components of Basic and Diluted Net Income (Loss) Per Common Share Computations (Detail) - USD ($) $ / shares in Units, $ 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
Basic EPS:
Net (loss) income (numerator)	$ (32,784)	$ (32,886)	$ (35,339)	$ (16,324)	$ 53,976	$ 126,520	$ (29,959)	$ (24,873)	$ (117,333)	$ 125,664	$ (368,445)
Weighted average common shares (denominator)	116,451,000	104,703,000	99,864,000	95,744,000	95,676,000	95,627,000	91,061,000	85,771,000	104,245,000	92,053,000	81,233,000
Net (loss) income 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)
Diluted EPS:
Net (loss) income (numerator)	$ (32,784)	$ (32,886)	$ (35,339)	$ (16,324)	$ 53,976	$ 126,520	$ (29,959)	$ (24,873)	$ (117,333)	$ 125,664	$ (368,445)
Weighted average common shares (denominator)	116,451,000	104,703,000	99,864,000	95,744,000	95,676,000	95,627,000	91,061,000	85,771,000	104,245,000	92,053,000	81,233,000
Effect of dilutive securities - common shares issuable										32,000
Adjusted weighted average common shares (denominator)	116,451,000	104,703,000	99,864,000	95,744,000	96,510,000	96,548,000	91,061,000	85,771,000	104,245,000	92,085,000	81,233,000
Net (loss) income 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)