Income Taxes (Details) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009
Currently payable:
Federal	$ 95,224	$ 87,350	$ 48,249
State	9,651	7,381	4,312
Total currently payable	104,875	94,731	52,561
Deferred taxes	2,394	(5,206)	4,090
Provision for income taxes	$ 107,269	$ 89,525	$ 56,651
Reconciliation of statutory federal income tax rate with the entity's effective income tax rate
Statutory federal income tax rate (as a percent)	35.00%	35.00%	35.00%
State income taxes, net of federal tax benefits (as a percent)	2.50%	2.10%	1.90%
State tax incentives (as a percent)	(0.20%)	(0.20%)	(0.70%)
Sale of ACF (as a percent)			(6.00%)
Valuation allowance on losses capital in nature (as a percent)	(0.10%)	(1.10%)	4.10%
Other items (as a percent)	0.70%	0.50%	0.60%
Effective income tax rate (as a percent)	37.90%	36.30%	34.90%