Summary of Significant Accounting Policies - Additional Information (Detail)			12 Months Ended
	Jan. 01, 2018 USD ($)	Sep. 20, 2017	Dec. 31, 2020 USD ($)	Sep. 30, 2020 USD ($)	Dec. 31, 2019 USD ($)	Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)
Summary Of Significant Accounting Policies [Line Items]
Period for federal funds sold outstanding			1 day
Loan non-accrual classification period			90 days
Days to charged consumer loans to the value of the collateral			90 days
Maximum time for re-evaluation (in months)			12 months
Re-evaluation period for real estate			2 years
Amount of fair value of capital over carrying value				$ 57,800,000
Percentage of fair value of capital over carrying value				26.80%
Number of times fair value of capital in excess of carrying value of goodwill description			over 1.2 times
Other real estate owned			$ 71,000		$ 214,000
Percentage of likelihood of realization of recognized tax benefit, minimum			50.00%
Share stock split ratio		2
Net effect on total stockholder's equity due to reclassification of tax effects			$ 249,160,000		$ 230,258,000	$ 143,287,000	$ 134,107,000
Amount of reclassification effect			$ 0
ASU 2018-02 [Member]
Summary Of Significant Accounting Policies [Line Items]
Reclassification of tax effects from AOCI due to adoption of new Tax Cuts and Job Acts	$ 359,000
ASU 2018-02 [Member] | Reclassification Adjustment [Member]
Summary Of Significant Accounting Policies [Line Items]
Net effect on total stockholder's equity due to reclassification of tax effects	$ 0
ASU 2017-04 [Member]
Summary Of Significant Accounting Policies [Line Items]
Change in Accounting Principle, Accounting Standards Update, Adopted [true false]			true
Change in Accounting Principle, Accounting Standards Update, Adoption Date			Jan. 01, 2020
Change in Accounting Principle, Accounting Standards Update, Immaterial Effect [true false]			true
ASU 2018-13 [Member]
Summary Of Significant Accounting Policies [Line Items]
Change in Accounting Principle, Accounting Standards Update, Adopted [true false]			true
Change in Accounting Principle, Accounting Standards Update, Adoption Date			Jan. 01, 2020
Change in Accounting Principle, Accounting Standards Update, Immaterial Effect [true false]			true
Maximum [Member]
Summary Of Significant Accounting Policies [Line Items]
Maturity period of cash equivalents			3 months