Income Taxes, Major Reasons for the Difference in Effective Income Tax Rate from the Federal Tax Rate (Details)	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Effective Income Tax Rate Reconciliation
Federal income tax rate at statutory rate	35.00%	35.00%	35.00%
Increases (decreases) resulting from:
Tax-exempt interest and dividends	(4.10%)	(4.80%)	(5.00%)
State income taxes (net of federal benefit)	2.70%	2.30%	2.30%
Bank owned life insurance	(1.70%)	(1.70%)	(1.20%)
Tax effect of tax credits and benefits, net of related expenses	(0.50%)	(0.80%)	(0.50%)
Tax reserve adjustments	(1.20%)	0.30%	(0.60%)
Net tax benefit from stock-based compensation	(1.30%)	(0.40%)	0.00%
Tax Cuts and Jobs Act impact on deferred remeasurement	3.50%	0.00%	0.00%
Other	(0.10%)	0.50%	0.20%
Effective income tax rate	32.30%	30.40%	30.20%