Net Income per Share (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Numerator:
Net income	$ 74,400	$ 97,114	$ 76,704	$ 69,002	$ 55,909	$ 67,000	$ 64,849	$ 37,706	$ 317,220	$ 225,464	$ 176,918
Denominator:
Weighted-average shares outstanding, basic (in shares)									72,208	72,739	72,593
Effect of dilutive stock awards (in shares)									218	555	406
Adjusted weighted-average shares outstanding, diluted (in shares)									72,426	73,294	72,999
Net Income per Share:
Basic earnings per common share (in dollars per share)	$ 1.04	$ 1.35	$ 1.06	$ 0.94	$ 0.77	$ 0.92	$ 0.89	$ 0.52	$ 4.39	$ 3.10	$ 2.44
Diluted earnings per common share (in dollars per share)	$ 1.04	$ 1.35	$ 1.05	$ 0.94	$ 0.76	$ 0.91	$ 0.88	$ 0.51	$ 4.38	$ 3.08	$ 2.42
Antidilutive awards excluded from computation of earnings per common share (in shares)									52	29	1