Income Taxes - Summary of Reconciliation Between the Statutory Tax Rate and Actual Effective Tax Rate (Detail)	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Income Tax Disclosure [Abstract]
Statutory tax rate	21.00%	21.00%	21.00%
State taxes, net of federal benefit	1.60%	1.10%	1.10%
Tax-exempt interest	(2.80%)	(2.20%)	(2.10%)
Cash surrender value of life insurance	(0.80%)	(0.50%)	(0.50%)
Tax credits	(6.30%)	(4.20%)	(2.80%)
TCJA deferred tax corporate rate change	2.50%	1.40%	0.70%
Tax Cuts and Jobs Act revaluation of net deferred tax assets	0	0	(0.004)
Other items	1.50%	1.10%	0.60%
Actual effective tax rate	16.70%	17.70%	17.60%