Net Income per Share (Details) (USD $) In Thousands, except Per Share data, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Numerator:
Net income	$ 55,909	$ 67,000	$ 64,849	$ 37,706	$ 43,193	$ 61,103	$ 42,068	$ 30,554	$ 225,464	$ 176,918	$ 108,460
Denominator:
Weighted-average shares outstanding, basic (in shares)									72,739	72,593	72,386
Effect of dilutive stock awards (in shares)									555	406	205
Adjusted weighted-average shares outstanding, diluted (in shares)									73,294	72,999	72,591
Net Income per Share:
Basic earnings per common share (in dollars per share)	$ 0.77	$ 0.92	$ 0.89	$ 0.52	$ 0.59	$ 0.84	$ 0.58	$ 0.42	$ 3.10	$ 2.44	$ 1.50
Diluted earnings per common share (in dollars per share)	$ 0.76	$ 0.91	$ 0.88	$ 0.51	$ 0.59	$ 0.84	$ 0.58	$ 0.42	$ 3.08	$ 2.42	$ 1.49
Antidilutive awards excluded from computation of earnings per common share (in shares)									29	1	88