Income Taxes (Schedule of Components of the Provision for Income Taxes) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Income Tax Expense (Benefit), Continuing Operations, by Jurisdiction [Abstract]
Federal income tax expense, current	$ 467,577	$ 540,090	$ 504,558
Federal income tax expense (benefit), deferred	(13,053)	7,558	(21,973)
Federal income tax expense, total	454,524	547,648	482,585
State income tax expense, current	41,183	49,016	47,242
State income tax expense (benefit), deferred	8,293	2,836	(677)
State income tax expense, total	49,476	51,852	46,565
Current income tax expense	508,760	589,106	551,800
Deferred income tax expense (benefit)	(4,760)	10,394	(22,650)
Provision for income taxes	$ 504,000	$ 599,500	$ 529,150