Income Taxes - Income Tax Provision Differences from Expected Statutory Provision (Detail)	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Effective Income Tax Rate Reconciliation, Percent [Abstract]
Expected provision at statutory rate	21.00%	21.00%	21.00%
Difference resulting from:
Tax exempt interest income, net of disallowance	(3.80%)	(3.20%)	(4.00%)
Increase in value of bank owned life insurance assets	(1.10%)	(0.80%)	(1.10%)
Stock-based compensation	0.20%	0.00%	(0.20%)
State income taxes, net of federal benefits	1.10%	0.40%	1.60%
Adjustment to deferred tax assets and liabilities for enacted changes in tax laws and rates	0	0	(0.005)
Changes in valuation allowance	0.20%	0.20%	0.10%
Other	(0.10%)	0.30%	(0.20%)
Provision for income taxes - effective income tax rate	17.50%	17.90%	16.70%