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				9 Months Ended
	Sep. 30, 2014		Sep. 30, 2013		Sep. 30, 2014		Sep. 30, 2013
Earnings Per Share Reconciliation [Abstract]
Net income, as reported	$ 146,418		$ 19,178		$ 350,328		$ 87,551
Participating basic earnings	(2,479)	[1]	(341)	[1]	(5,940)	[1]	(1,479)	[1]
Basic net income attributed to common shareholders	143,939		18,837		344,388		86,072
Reallocation of participating earnings	9	[1]	1	[1]	28	[1]	6	[1]
Diluted net income attributed to common shareholders	$ 143,948		$ 18,838		$ 344,416		$ 86,078
Net income per common share:
Basic	$ 0.87		$ 0.12		$ 2.11		$ 0.54
Diluted	$ 0.86		$ 0.12		$ 2.10		$ 0.53

[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.