Loans and Allowance for Credit Losses - Additional Information (Detail) - USD ($)	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018	Dec. 31, 2018
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	$ 300,000		$ 300,000		$ 1,600,000
Off balance sheet reserve to nonperforming loans	12,000		12,000		0
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Sales	0	$ 12,289,000	0	$ 12,289,000
Total gains or losses included in earnings	0	5,262,000	$ (47,000)	8,102,000
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	68,000	$ 24,000	$ 100,000	100,000
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			$ 400,000	$ 1,200,000