Loans - Additional Information (Detail) - USD ($)	9 Months Ended	12 Months Ended
Sep. 30, 2020	Dec. 31, 2019
Loans and Leases Receivable Disclosure [Line Items]
Loans 90 days or more past due and accruing interest	$ 0	$ 0
Internal risk ratings of loans	331,915,000	312,370,000
Additional committed impared loans	0	0
Loans modified as troubled debt restructurings	0	0
Troubled debt restructurings within past twelve months for which there was a default	0	0
Loans and leases receivable, net amount	328,820,000	$ 310,674,000
Covid Nineteen [Member] \| Payment Deferral [Member]
Loans and Leases Receivable Disclosure [Line Items]
Financing recievable threshold past due		30 days
Loans and leases receivable, net amount	16,400,000
Financing receivable deferral amount	$ 155,000
Covid Nineteen [Member] \| Maximum [Member]
Loans and Leases Receivable Disclosure [Line Items]
Financing receivable moratorium period	3 months
Covid Nineteen [Member] \| Minimum [Member]
Loans and Leases Receivable Disclosure [Line Items]
Financing receivable moratorium period	1 month
Doubtful [Member]
Loans and Leases Receivable Disclosure [Line Items]
Internal risk ratings of loans	$ 0	$ 0
Other Financial Institutions [Member]
Loans and Leases Receivable Disclosure [Line Items]
Participation loans transferred	$ 28,600,000	$ 26,200,000

Additional committed impaired loans.

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Financing receivable, moratorium period.

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Participation loans transferred.

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Amount, after troubled debt restructuring, of modified financing receivable.

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Amount of financing receivable modified as troubled debt restructuring within previous 12 months, with subsequent payment default.

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Amount of financing receivable 90 days or more past due and still accruing.

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Threshold period for when financing receivable is considered past due, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days. Excludes threshold period past due to write off as uncollectible.

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Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

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Amount before allowance of loans and leases held in portfolio, including but not limited to, commercial and consumer loans. Includes deferred interest and fees, undisbursed portion of loan balance, unamortized costs and premiums and discounts from face amounts. Excludes loans and leases covered under loss sharing agreements.

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Amount after allowance and deduction of deferred interest and fees, unamortized costs and premiums and discounts from face amounts, of loans and leases held in portfolio, including but not limited to, commercial and consumer loans. Excludes loans and leases covered under loss sharing agreements and loans held for sale.

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