Income Taxes - Effective Income Tax Rate Reconciliation (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Reconciliation of effective income tax rate with federal statutory tax rate
Expected tax expense, Amount									$ 13,336	$ 12,895	$ 13,081
Expected tax expense, Percent Of Pretax Income									35.00%	35.00%	35.00%
Tax-exempt interest, Amount									(1,788)	(1,690)	(1,622)
Tax-exempt interest, Percent Of Pretax Income									(4.70%)	(4.60%)	(4.30%)
Interest expense limitation, Amount									96	99	112
Interest expenses limitation, Percent Of Pretax Income									0.30%	0.30%	0.30%
State income taxes, net of federal income tax benefit, Amount									1,134	1,216	1,456
State income taxes, net of federal income tax benefit, Percent Of Pretax Income									3.00%	3.30%	3.90%
Income tax credits, Amount									(1,546)	(1,992)	(2,046)
Income tax credits, Percent Of Pretax Income									(4.10%)	(5.40%)	(5.50%)
Other, Amount									(103)	384	(439)
Other, Percent Of Pretax Income									(0.30%)	1.00%	(1.20%)
Total	$ 1,927	$ 3,625	$ 3,188	$ 2,389	$ 2,354	$ 2,705	$ 2,863	$ 2,990	$ 11,129	$ 10,912	$ 10,542
Total, Percent Of Pretax Income									29.20%	29.60%	28.20%