Earnings Per Share - Basic and Diluted Loss Per Share (Detail) (USD $) In Thousands, except Share data, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2013	Jun. 30, 2012	Jun. 30, 2013	Jun. 30, 2012
Numerator
Net income	$ 224,909	$ 10,638	$ 247,425	$ 12,903
Less: distributed earnings allocated to participating securities		(149)		(308)
Less: undistributed earnings allocated to participating securities	(1,867)		(2,087)
Net income attributable to common stockholders (numerator for basic earnings per share)	223,042	10,489	245,338	12,595
Add back: undistributed earnings allocated to participating securities	1,867		2,087
Less: undistributed earnings reallocated to participating securities	1,849		2,066
Numerator for diluted earnings per share under two class method	$ 223,060	$ 10,489	$ 245,359	$ 12,595
Denominator
Weighted-average common shares outstanding	48,447,005	47,398,088	48,410,486	47,367,051
Add: dilutive effect of stock options	467,979	118,170	506,502	95,493
Denominator for diluted earnings per share under two class method	48,914,984	47,516,258	48,916,988	47,462,544
Basic Earnings Per Common Share	$ 4.60	$ 0.22	$ 5.07	$ 0.27
Diluted Earnings Per Common Share	$ 4.56	$ 0.22	$ 5.02	$ 0.27