Federal Income Taxes (Effective Tax Rate Reconciliation) (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2012	Sep. 30, 2012	Jun. 30, 2012	Mar. 31, 2012	Dec. 31, 2011	Sep. 30, 2011	Jun. 30, 2011	Mar. 31, 2011	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Effective Income Tax Rate, Tax Rate Reconciliation [Abstract]
Tax at statutory rate									$ 25,133	$ 21,088	$ 10,916
Tax-exempt interest income									(2,524)	(2,569)	(2,169)
Income tax credits									(1,561)	(1,374)	(1,232)
Other, net									(248)	55	585
Total provision for federal income taxes	$ 4,000	$ 5,300	$ 6,325	$ 5,175	$ 4,475	$ 4,075	$ 4,750	$ 3,900	$ 20,800	$ 17,200	$ 8,100
Effective Income Tax Rate Reconciliation, at Federal Statutory Income Tax Rate									29.00%	28.50%	26.00%