Income Taxes (Reconciliation of the Provision for Income Taxes to the Amounts Computed at the Federal Statutory Rate) (Details) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Income Taxes
Federal, current	$ 311,631	$ 228,443	$ 146,259
Federal, deferred	10,030	55,175	88,395
Federal, total	321,661	283,618	234,654
State, current	35,982	25,537	24,484
State, deferred	(1,868)	(1,055)	10,862
State, total	34,114	24,482	35,346
Federal and state tax expense, current	347,613	253,980	170,743
Federal and state tax expense, deferred	8,162	54,120	99,257
Provision for income taxes	$ 355,775	$ 308,100	$ 270,000