Earnings per Share (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Numerator:
Net income	$ 91,937	$ 97,480	$ 11,254	$ (44,922)	$ 246,975	$ 60,044	$ 77,241	$ 31,261	$ 155,749	$ 415,522	$ 263,483
Denominator:
Weighted-average shares outstanding, basic (in shares)									68,249	69,221	70,344
Effect of dilutive stock awards (in shares)									182	156	164
Adjusted weighted-average shares outstanding, diluted (in shares)									68,431	69,377	70,508
Earnings per Share:
Basic earnings per common share (in dollars per share)	$ 1.35	$ 1.43	$ 0.16	$ (0.66)	$ 3.59	$ 0.87	$ 1.11	$ 0.45	$ 2.28	$ 6.00	$ 3.75
Diluted earnings per common share (in dollars per share)	$ 1.34	$ 1.42	$ 0.16	$ (0.66)	$ 3.58	$ 0.86	$ 1.11	$ 0.45	$ 2.28	$ 5.99	$ 3.74
Antidilutive awards excluded from computation of earnings per common share (in shares)									145	85	66
Non-recurring income tax benefit due to the Tax Cuts and Jobs Act of 2017					$ 196,700
Tax Cuts And Jobs Act Of 2017, Income Tax Expense (Benefit), Per Basic Share					$ (2.86)					$ (2.84)
Non-recurring income tax benefit due to the Tax Cuts and Jobs Act of 2017 per diluted share (in dollars per share)					$ 2.85					$ 2.84