Income Taxes (Components of Income Tax Expense) (Details) - USD ($) $ 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
Current income tax (benefit) expense:
Federal									$ 120,968	$ 63,642	$ (62,829)
State									72,837	48,558	(4,750)
Foreign									1,815	1,345	409
Total current income tax expense (benefit)									195,620	113,545	(67,170)
Deferred income tax expense (benefit):
Federal									40,057	25,296	199,858
State									(6,201)	1,883	60,437
Foreign									0	(213)	919
Total deferred income tax expense									33,856	26,966	261,214
Income tax expense	$ 89,229	$ 42,624	$ 39,355	$ 58,268	$ 50,403	$ 13,321	$ 39,632	$ 37,155	$ 229,476	$ 140,511	$ 194,044