Income Taxes (Reconciliation of the Provision for Income Taxes to the Amounts Computed at the Federal Statutory Rate) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Effective Income Tax Rate Reconciliation, Amount [Abstract]
Federal income taxes at statutory rate	$ 573,231	$ 573,020	$ 511,128
State income taxes, net of federal tax benefit	39,062	35,285	32,137
Excess tax benefit from share-based compensation	(48,688)	0	0
Revaluation of deferred tax liability	(53,240)	0	0
Other items, net	(6,365)	(8,805)	(14,115)
Provision for income taxes	$ 504,000	$ 599,500	$ 529,150