Federal Income Taxes (Effective Tax Rate Reconciliation) (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Effective Income Tax Rate, Tax Rate Reconciliation [Abstract]
Tax at statutory rate									$ 43,341	$ 31,367	$ 28,388
Tax-exempt interest income									(3,918)	(3,118)	(2,746)
Income tax credits, net									(2,557)	(1,624)	(1,615)
Other, net									134	875	273
Provision for federal income taxes	$ 10,200	$ 9,600	$ 9,100	$ 8,100	$ 7,050	$ 7,450	$ 7,100	$ 5,900	$ 37,000	$ 27,500	$ 24,300
Effective federal income tax rate									29.90%	30.70%	30.00%