Income Taxes, Major Reasons for the Difference in Effective Income Tax Rate from the Federal Tax Rate (Details)	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Effective Income Tax Rate Reconciliation
Federal income tax rate at statutory rate	21.00%	21.00%	35.00%
Increases (decreases) resulting from:
Tax-exempt interest and dividends	(3.30%)	(2.60%)	(4.10%)
State income taxes (net of federal benefit)	3.50%	3.70%	2.90%
Bank owned life insurance	(0.80%)	(0.70%)	(1.70%)
Tax effect of tax credits and benefits, net of related expenses	(0.90%)	(0.70%)	(0.70%)
Tax reserve adjustments / settlements	0.20%	1.50%	(1.20%)
Net tax benefit from stock-based compensation	(0.20%)	(0.50%)	(1.30%)
Tax Act impact on deferred remeasurement	0.00%	0.00%	3.50%
Tax planning in response to the Tax Act	0.00%	(3.60%)	0.00%
FDIC premium	0.50%	0.90%	0.00%
Other	(0.40%)	0.30%	(0.10%)
Effective income tax rate	19.60%	19.30%	32.30%