Net Income per Share (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Numerator:
Net income	$ 48,493	$ 81,382	$ 73,084	$ 61,920	$ 74,400	$ 97,114	$ 76,704	$ 69,002	$ 264,879	$ 317,220	$ 225,464
Denominator:
Weighted-average shares outstanding, basic (in shares)									70,344	72,208	72,739
Effect of dilutive stock awards (in shares)									164	218	555
Adjusted weighted-average shares outstanding, diluted (in shares)									70,508	72,426	73,294
Net Income per Share:
Basic earnings per common share (in dollars per share)	$ 0.70	$ 1.17	$ 1.03	$ 0.87	$ 1.04	$ 1.35	$ 1.06	$ 0.94	$ 3.77	$ 4.39	$ 3.10
Diluted earnings per common share (in dollars per share)	$ 0.70	$ 1.17	$ 1.03	$ 0.86	$ 1.04	$ 1.35	$ 1.05	$ 0.94	$ 3.76	$ 4.38	$ 3.08
Antidilutive awards excluded from computation of earnings per common share (in shares)									66	52	29