Income Taxes (Details) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Currently payable:
Federal	$ 104,922	$ 93,677	$ 85,394
State	9,335	9,033	6,730
Total currently payable	114,257	102,710	92,124
Deferred taxes	(5,782)	2,004	(5,191)
Provision for income taxes	$ 108,475	$ 104,714	$ 86,933
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)	2.20%	2.40%	1.90%
State tax incentives (as a percent)	(0.20%)	(0.20%)	(0.20%)
Valuation allowance on losses capital in nature (as a percent)	(0.80%)	(0.20%)	(1.10%)
Other items (as a percent)	(0.20%)	0.80%	0.60%
Effective income tax rate (as a percent)	36.00%	37.80%	36.20%