Summary of Effective Tax Rate (Details)	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Effective income tax rate reconciliation
Federal statutory rate	35.00%	35.00%	35.00%
State taxes, net of federal income tax benefit	3.70%	4.00%	4.00%
Tax-exempt interest income	(12.60%)	(11.50%)	(12.20%)
Tax credits	(3.00%)	(2.80%)	(3.20%)
Other, net	(0.50%)	(0.50%)	0.30%
Effective tax rate	22.60%	24.20%	23.90%