Income Taxes (Components of Income Tax Expense) (Details) - USD ($) $ 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
Current
Federal									$ 75,582	$ (32,341)	$ 7,329
State									7,081	5,949	5,501
Total current income tax expense (benefit)									82,663	(26,392)	12,830
Deferred
Federal									24,894	229,917	117,463
State									11,321	1,139	11,374
Total deferred income tax expense									36,215	231,056	128,837
Total income tax expense	$ 38,784	$ 18,949	$ 30,936	$ 30,209	$ 74,361	$ 54,668	$ 41,788	$ 33,847	118,878	204,664	141,667
Share-based compensation net tax deficiency									$ 3,300	$ 5,000	$ (790)