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				6 Months Ended
	Jun. 30, 2013		Jun. 30, 2012		Jun. 30, 2013		Jun. 30, 2012
Net income, as reported	$ 143,983		$ 55,676		$ 68,373		$ 13,876
Participating basic earnings	(2,335)	[1]	(999)	[1]	(1,124)	[1]	(246)	[1]
Basic net income attributed to common shareholders	141,648		54,677		67,249		13,630
Reallocation of participating earnings	12	[1]	3	[1]	5	[1]
Diluted net income attributed to common shareholders	$ 141,660		$ 54,680		$ 67,254		$ 13,630
Net income per common share:
Basic	$ 0.88		$ 0.34		$ 0.42		$ 0.09
Diluted	$ 0.88		$ 0.34		$ 0.42		$ 0.09

[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.