Loans and Allowance for Credit Losses - Troubled Debt Restructuring is 90 Days or More Past Due (Detail) $ in Thousands	3 Months Ended	9 Months Ended
Sep. 30, 2020 USD ($) Contract	Sep. 30, 2019 USD ($) Contract	Sep. 30, 2020 USD ($) Contract	Sep. 30, 2019 USD ($) Contract
Financing Receivable, Past Due [Line Items]
Financing Receivable, Troubled Debt Restructuring, Subsequent Default, Number of Contracts \| Contract	3	2	4	3
Financing Receivable, Troubled Debt Restructuring, Subsequent Default \| $	$ 190	$ 49	$ 240	$ 70
Residential real estate [Member]
Financing Receivable, Past Due [Line Items]
Financing Receivable, Troubled Debt Restructuring, Subsequent Default, Number of Contracts \| Contract	0	2	1	3
Financing Receivable, Troubled Debt Restructuring, Subsequent Default \| $	$ 0	$ 49	$ 50	$ 70
Loans to individuals [Member]
Financing Receivable, Past Due [Line Items]
Financing Receivable, Troubled Debt Restructuring, Subsequent Default, Number of Contracts \| Contract	2	0	2	0
Financing Receivable, Troubled Debt Restructuring, Subsequent Default \| $	$ 78	$ 0	$ 78	$ 0
Commercial Real Estate [Member]
Financing Receivable, Past Due [Line Items]
Financing Receivable, Troubled Debt Restructuring, Subsequent Default, Number of Contracts \| Contract	1	0	1	0
Financing Receivable, Troubled Debt Restructuring, Subsequent Default \| $	$ 112	$ 0	$ 112	$ 0

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

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 50
-Paragraph 34
-URI http://asc.fasb.org/extlink&oid=121611835&loc=SL6953682-111524

+ Details

Name:	us-gaap_FinancingReceivableModificationsSubsequentDefaultNumberOfContracts1
Namespace Prefix:	us-gaap_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	duration

- Definition

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

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 50
-Paragraph 34
-Subparagraph (a)(2)
-URI http://asc.fasb.org/extlink&oid=121611835&loc=SL6953682-111524

+ Details

Name:	us-gaap_FinancingReceivableModificationsSubsequentDefaultRecordedInvestment1
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

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.

+ References

No definition available.

+ Details

Name:	us-gaap_FinancingReceivableRecordedInvestmentPastDueLineItems
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Details

Name:	us-gaap_FinancingReceivablePortfolioSegmentAxis=us-gaap_ResidentialRealEstateMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancingReceivablePortfolioSegmentAxis=us-gaap_ConsumerLoanMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancingReceivablePortfolioSegmentAxis=us-gaap_CommercialRealEstateMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: