Net Income per Share (Details) (USD $) In Thousands, except Share data, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2012	Sep. 30, 2011	Sep. 30, 2012	Sep. 30, 2011
Numerator
Net income	$ 30,884	$ 27,657	$ 88,894	$ 52,457
Denominator
Weighted-average shares outstanding, basic (in shares)	72,427,000	72,175,000	72,367,000	46,840,000
Effect of dilutive nonvested stock awards (in shares)	231,000	252,000	214,000	289,000
Adjusted weighted-average shares outstanding, diluted (in shares)	72,658,000	72,427,000	72,581,000	47,129,000
Net Income per Share
Basic earnings per common share (in dollars per share)	$ 0.43	$ 0.38	$ 1.23	$ 1.12
Diluted earnings per common share (in dollars per share)	$ 0.43	$ 0.38	$ 1.22	$ 1.11
Antidilutive awards excluded from computation of earnings per common share (in shares)			0	121,000