Summary of Effective Tax Rate (Details)	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Effective income tax rate reconciliation
Federal statutory rate	35.00%	35.00%	35.00%
State taxes, net of federal income tax benefit	4.00%	4.00%	3.90%
Tax-exempt interest income	(11.50%)	(12.20%)	(14.00%)
Tax credits	(2.80%)	(3.20%)	(4.20%)
Other, net	(0.50%)	0.30%	(0.50%)
Effective tax rate	24.20%	23.90%	20.20%