Net Income per Share (Details) (USD $) In Thousands, except Share data, unless otherwise specified	3 Months Ended
	Mar. 31, 2013	Mar. 31, 2012
Numerator
Net income	$ 30,554	$ 23,419
Denominator
Weighted-average shares outstanding, basic (in shares)	72,486,000	72,292,000
Effect of dilutive nonvested stock awards (in shares)	318,000	207,000
Adjusted weighted-average shares outstanding, diluted (in shares)	72,804,000	72,499,000
Net Income per Share
Basic earnings per common share (in dollars per share)	$ 0.42	$ 0.32
Diluted earnings per common share (in dollars per share)	$ 0.42	$ 0.32
Antidilutive awards excluded from computation of earnings per common share (in shares)	0	12,000