Net Income Per Common Share - Computations of Basic and Diluted (Loss) Income Per Common Share (Detail) (USD $) In Thousands, except Per Share data, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2014		Dec. 31, 2013		Dec. 31, 2012
Earnings Per Share Reconciliation [Abstract]
Net income, as reported	$ 284,054	$ 146,418	$ 171,389	$ 32,521	$ 28,171	$ 19,178	$ 143,983	$ (75,610)	$ 634,382		$ 115,722		$ 13,002
Participating basic earnings									(10,725)	[1]	(1,975)	[1]	(460)	[1]
Basic net income attributed to common shareholders									623,657		113,747		12,542
Reallocation of participating earnings									48	[1]	9	[1]
Diluted net income attributed to common shareholders									$ 623,705		$ 113,756		$ 12,542
Net income per common share:
Basic	$ 1.68	$ 0.87	$ 1.04	$ 0.20	$ 0.17	$ 0.12	$ 0.88	$ (0.47)	$ 3.81		$ 0.71		$ 0.08
Diluted	$ 1.68	$ 0.86	$ 1.04	$ 0.20	$ 0.17	$ 0.12	$ 0.88	$ (0.47)	$ 3.79		$ 0.70		$ 0.08

[1] Restricted stock Liability 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.