Federal Income Taxes (Effective Tax Rate Reconciliation) (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2012	Sep. 30, 2012	Jun. 30, 2012	Mar. 31, 2012	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
Effective Income Tax Rate, Tax Rate Reconciliation [Abstract]
Tax at statutory rate									$ 28,388	$ 25,133	$ 21,088
Tax-exempt interest income									(2,746)	(2,524)	(2,569)
Income tax credits									(1,615)	(1,561)	(1,374)
Other, net									273	(248)	55
Provision for federal income taxes	$ 6,100	$ 6,400	$ 6,100	$ 5,700	$ 4,000	$ 5,300	$ 6,325	$ 5,175	$ 24,300	$ 20,800	$ 17,200
Effective federal income tax rate									30.00%	29.00%	28.50%