Income Taxes (Details)	9 Months Ended
	Sep. 30, 2013	Sep. 30, 2012
Analysis of the difference between the statutory and effective tax rates
U.S. federal statutory tax rate (as a percent)	35.00%	35.00%
Tax exempt interest income (as a percent)	(2.00%)	(3.20%)
Tax credits (as a percent)	(2.20%)	(2.90%)
Cash surrender value of life insurance (as a percent)	(1.80%)	(1.80%)
State income taxes (as a percent)	0.70%	1.00%
Non-taxable gain on acquisition	(0.50%)
Non-deductible acquisition costs	0.30%
Other, net (as a percent)	0.30%	(0.50%)
Effective tax rate (as a percent)	29.80%	27.60%