Earnings per Share (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended									12 Months Ended
	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Numerator:
Net income	$ 81,214	$ 83,464	$ 114,501	$ 56,076	$ 91,937	$ 97,480	$ 11,254	$ (44,922)		$ 335,255	$ 155,749	$ 415,522
Denominator:
Weighted-average shares outstanding, basic (in shares)										68,429	68,249	69,221
Effect of dilutive stock awards (in shares)										130	182	156
Adjusted weighted-average shares outstanding, diluted (in shares)										68,559	68,431	69,377
Earnings per Share:
Basic earnings per common share (in dollars per share)	$ 1.19	$ 1.22	$ 1.67	$ 0.82	$ 1.35	$ 1.43	$ 0.16	$ (0.66)		$ 4.90	$ 2.28	$ 6.00
Diluted earnings per common share (in dollars per share)	$ 1.18	$ 1.22	$ 1.67	$ 0.82	$ 1.34	$ 1.42	$ 0.16	$ (0.66)		$ 4.89	$ 2.28	$ 5.99
Antidilutive awards excluded from computation of earnings per common share (in shares)										143	145	85
Non-recurring income tax benefit due to the Tax Cuts and Jobs Act of 2017									$ 196,700
Non-recurring income tax benefit due to the Tax Cuts and Jobs Act of 2017 per basic share (in dollars per share)												$ 2.84
Non-recurring income tax benefit due to the Tax Cuts and Jobs Act of 2017 per diluted share (in dollars per share)												$ 2.84