Net Income per Share (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Numerator:
Net income	$ 250,338	$ 60,190	$ 78,143	$ 31,935	$ 48,493	$ 81,382	$ 73,084	$ 61,920	$ 420,606	$ 264,879	$ 317,220
Denominator:
Weighted-average shares outstanding, basic (in shares)									69,221	70,344	72,208
Effect of dilutive stock awards (in shares)									156	164	218
Adjusted weighted-average shares outstanding, diluted (in shares)									69,377	70,508	72,426
Net Income per Share:
Basic earnings per common share (in dollars per share)	$ 3.64	$ 0.87	$ 1.13	$ 0.46	$ 0.70	$ 1.17	$ 1.03	$ 0.87	$ 6.08	$ 3.77	$ 4.39
Diluted earnings per common share (in dollars per share)	$ 3.63	$ 0.87	$ 1.12	$ 0.46	$ 0.70	$ 1.17	$ 1.03	$ 0.86	$ 6.06	$ 3.76	$ 4.38
Antidilutive awards excluded from computation of earnings per common share (in shares)									85	66	52
Non-recurring income tax benefit due to the Tax Cuts and Jobs Act of 2017	$ 199,300
Non-recurring income tax benefit due to the Tax Cuts and Jobs Act of 2017 per basic share (in dollars per share)	$ 2.90								$ 2.88
Non-recurring income tax benefit due to the Tax Cuts and Jobs Act of 2017 per diluted share (in dollars per share)	$ 2.89								$ 2.87