Loans and Allowance for Credit Losses - Additional Information (Detail) - USD ($)	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019	Dec. 31, 2019
Accounts, Notes, Loans and Financing Receivable [Line Items]
Non Accrual Status Of Loans After Number Of Days Past Due	90 days
Default Status Of TDRs After Number Of Days Past Due	90 days
Unfunded commitments related to nonperforming loans	$ 3,700,000		$ 1,700,000
Off balance sheet reserve to nonperforming loans	23,000		12,000.0
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Sales	0	$ 0
Total gains or losses included in earnings	0	0
Section4013LoanModifications	$ 1,100,000,000
Loans to individuals [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Non Accrual Status Of Loans After Number Of Days Past Due	150 days
Impaired Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans with modifications to rate and payment due to reamortization	$ 18,000	$ 600,000
Nonaccrual Commercial Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans Held for sale, Amount	0		$ 0
Loans and Leases Receivable, Gain (Loss) on Sales, Net	$ 0