Summary of Effective Tax Rate (Details)	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Effective income tax rate reconciliation
Federal statutory rate	35.00%	35.00%	35.00%
State taxes, net of federal income tax benefit	4.60%	3.80%	3.70%
Tax rate change	10.90%	0.00%	0.00%
Tax-exempt interest income	(10.50%)	(12.20%)	(12.60%)
Tax credits	(3.20%)	(2.10%)	(3.00%)
Other, net	(1.10%)	0.20%	(0.50%)
Effective tax rate	35.70%	24.70%	22.60%