Loans and Allowance for Credit Losses - Additional Information (Detail)	3 Months Ended		6 Months Ended
	Jun. 30, 2020 USD ($)	Jun. 30, 2019 USD ($)	Jun. 30, 2020 USD ($)	Jun. 30, 2019 USD ($)	Dec. 31, 2019 USD ($)
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	$ 1,100,000		$ 1,100,000		$ 1,700,000
Off balance sheet reserve to nonperforming loans	23,000		23,000		12,000
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Sales	0	$ 0	0	$ 0
Total gains or losses included in earnings	0	$ 0	0	0
Total Deferrals Granted [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans and Leases Receivable, Gross	$ 1,400,000,000		$ 1,400,000,000
Loans and Leases Receivable, Number of Contracts	6,500		6,500
Loans Still in Deferral Period [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans and Leases Receivable, Gross	$ 611,100,000		$ 611,100,000
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			$ 71,000	117,000
Nonaccrual Commercial Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans Held for sale, Amount	$ 0		0		$ 0
Loans and Leases Receivable, Gain (Loss) on Sales, Net			$ 0	$ 400,000