Net Income per Share (Details) (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	$ 42,068	$ 34,591	$ 72,622	$ 58,010
Denominator
Weighted-average shares outstanding, basic (in shares)	72,593,000	72,379,000	72,540,000	72,336,000
Effect of dilutive nonvested stock awards (in shares)	399,000	205,000	358,000	206,000
Adjusted weighted-average shares outstanding, diluted (in shares)	72,992,000	72,584,000	72,898,000	72,542,000
Net Income per Share
Basic earnings per common share (in dollars per share)	$ 0.58	$ 0.48	$ 1.00	$ 0.80
Diluted earnings per common share (in dollars per share)	$ 0.58	$ 0.48	$ 1.00	$ 0.80
Antidilutive awards excluded from computation of earnings per common share (in shares)			1,545	26,000