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
Federal									$ (32,341)	$ 7,329	$ 6,163
State									5,949	5,501	4,424
Total current income tax expense (benefit)									(26,392)	12,830	10,587
Deferred
Federal									229,917	117,463	108,877
State									1,139	11,374	13,027
Total deferred income tax expense									231,056	128,837	121,904
Total income tax expense	$ 74,361	$ 54,668	$ 41,788	$ 33,847	$ 39,519	$ 37,375	$ 33,574	$ 31,199	204,664	141,667	132,491
Share-based compensation tax benefit (deficiency)									$ 5,000	$ (790)	$ 1,656