Net Income per Share (Details) (USD $) In Thousands, except Per Share data, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012
Numerator
Net income	$ 61,103	$ 30,884	$ 133,725	$ 88,894
Denominator
Weighted-average shares outstanding, basic (in shares)	72,632	72,427	72,571	72,367
Effect of dilutive nonvested stock awards (in shares)	371	231	363	214
Adjusted weighted-average shares outstanding, diluted (in shares)	73,003	72,658	72,934	72,581
Net Income per Share
Basic earnings per common share (in dollars per share)	$ 0.84	$ 0.43	$ 1.84	$ 1.23
Diluted earnings per common share (in dollars per share)	$ 0.84	$ 0.43	$ 1.83	$ 1.22
Antidilutive awards excluded from computation of earnings per common share (in shares)	1	0	1	0