Federal and State Income Taxes - Summary of Effective Tax Rate (Details)	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Effective income tax rate reconciliation
Federal statutory rate	21.00%	21.00%	21.00%
State taxes, net of federal income tax benefit	4.60%	4.40%	4.80%
Tax-exempt interest income	(4.00%)	(3.70%)	(5.00%)
Tax credits	(4.20%)	(4.50%)	(4.20%)
Other, net	1.40%	1.60%	1.60%
Effective income tax rate	18.80%	18.80%	18.20%