Income Taxes (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Current taxes:
Federal	$ 142,576	$ 161,863	$ 131,000
State	12,800	14,206	12,197
Foreign	38	35	37
Total currently payable	155,414	176,104	143,234
Deferred taxes	(1,410)	728	(3,001)
Provision for income taxes	$ 154,004	$ 176,832	$ 140,233
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)	2.00%	2.10%	2.20%
State tax incentives (as a percent)	(0.10%)	(0.20%)	(0.10%)
Valuation allowance on losses capital in nature (as a percent)	0.90%	(1.00%)	(1.80%)
Other items (as a percent)	0.70%	0.20%	0.40%
Effective income tax rate (as a percent)	38.50%	36.10%	35.70%