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Financial Instruments with Off-Balance Sheet Credit Exposures - Schedule of Notional Amounts of Financial Instruments with Off-Balance Sheet Credit Risk (Detail) - USD ($) $ in Thousands	Sep. 30, 2025	Dec. 31, 2024
Unadvanced portions of loans [Member]
Fair Value, off-Balance-Sheet Risks, Disclosure Information [Line Items]
Unadvanced portions of loans	$ 45,544	$ 42,883
Commitments to originate loans [Member]
Fair Value, off-Balance-Sheet Risks, Disclosure Information [Line Items]
Unadvanced portions of loans	11,545	9,333
Standby Letters of Credit [Member]
Fair Value, off-Balance-Sheet Risks, Disclosure Information [Line Items]
Unadvanced portions of loans	$ 513	$ 125

X

- Definition

The fair value of financial liabilities, which are not recognized in the financial statements (off-balance sheet) because they fail to meet some other criterion for recognition.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Topic 825
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 11
-Subparagraph (a)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147482907/825-10-50-11

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Topic 825
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 10
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147482907/825-10-50-10

+ Details

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Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

X

- 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

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Topic 825
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 11
-Subparagraph (a)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147482907/825-10-50-11

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Topic 942
-SubTopic 825
-Name Accounting Standards Codification
-Section 50
-Paragraph 1
-Subparagraph (a)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147478898/942-825-50-1

+ Details

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Period Type:	duration

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- Details

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Namespace Prefix:
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Balance Type:
Period Type:

X

- Details

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Namespace Prefix:
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Balance Type:
Period Type:

X

- Details

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