Net Income per Share (Details) (USD $) In Thousands, except Per Share data, unless otherwise specified	3 Months Ended
	Mar. 31, 2014	Mar. 31, 2013
Numerator
Net income	$ 37,706	$ 30,554
Denominator
Weighted-average shares outstanding, basic (in shares)	72,684	72,486
Effect of dilutive stock awards (in shares)	570	318
Adjusted weighted-average shares outstanding, diluted (in shares)	73,254	72,804
Net Income per Share
Basic earnings per common share (in dollars per share)	$ 0.52	$ 0.42
Diluted earnings per common share (in dollars per share)	$ 0.51	$ 0.42
Antidilutive awards excluded from computation of earnings per common share (in shares)	56	0