(Loss) Income Per Common Share - Computations of Basic and Diluted (Loss) Income Per Common Share (Detail) - USD ($) $ / shares in Units, $ in Thousands		3 Months Ended		6 Months Ended
		Jun. 30, 2015	Jun. 30, 2014	Jun. 30, 2015	Jun. 30, 2014
Earnings Per Share Reconciliation [Abstract]
Net (loss) income, as reported		$ (118,594)	$ 171,389	$ (90,910)	$ 203,910
Participating earnings	[1]	(111)	(2,868)	(224)	(3,460)
Basic net (loss) income attributed to common shareholders		(118,705)	168,521	(91,134)	200,450
Reallocation of participating earnings	[1]	0	15	0	19
Diluted net (loss) income attributed to common shareholders		$ (118,705)	$ 168,536	$ (91,134)	$ 200,469
Net (loss) income per common share:
Basic		$ (0.71)	$ 1.04	$ (0.55)	$ 1.24
Diluted		$ (0.71)	$ 1.04	$ (0.55)	$ 1.24

[1] Restricted Stock Awards represent participating securities because they participate in nonforfeitable dividends or distributions with common equity owners. Income allocable to participating securities represents the distributed and undistributed earnings attributable to the participating securities. Participating securities, however, do not participate in undistributed net losses.