Income Taxes - Reconciliation from Federal Statutory Income Tax Rate to Effective Tax Rate (Detail)	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Income Tax Disclosure [Abstract]
Federal statutory tax rate	21.00%	21.00%	21.00%
Net tax-exempt interest income on securities and loans of state and political subdivisions	(4.20%)	(3.30%)	(3.20%)
State income taxes, net of federal tax effect	1.90%	1.70%	1.70%
Bank-owned life insurance	(1.10%)	(0.60%)	(0.80%)
General business credits	(3.70%)	(2.20%)	(1.60%)
All other—net	2.00%	1.20%	0.90%
Effective tax rate	15.90%	17.80%	18.00%