Allowance for Credit Losses on Loans and Off-Balance Sheet Credit Exposures - Narrative (Details)	12 Months Ended
Dec. 31, 2024 USD ($) loan	Dec. 31, 2023 USD ($) loan
Financing Receivable, Allowance for Credit Loss [Line Items]
Charge-offs	$ 3,000	$ 8,000
Aggregate interest on nonaccrual loans not recognized	235,000	143,000
Loans modified in prior 12 months that subsequently defaulted	0	0
Consumer
Financing Receivable, Allowance for Credit Loss [Line Items]
Charge-offs	$ 3,000	$ 8,000
Consumer \| Originated 2021
Financing Receivable, Allowance for Credit Loss [Line Items]
Number of loans \| loan	2
Charge-offs	$ 1,000
Consumer \| Originated 2022
Financing Receivable, Allowance for Credit Loss [Line Items]
Number of loans \| loan		2
Charge-offs		$ 8,000
Consumer \| Originated 2024
Financing Receivable, Allowance for Credit Loss [Line Items]
Number of loans \| loan	2
Charge-offs	$ 2,000
Residential real estate
Financing Receivable, Allowance for Credit Loss [Line Items]
Charge-offs	0	0
Residential real estate \| Non-revolving residential real estate
Financing Receivable, Allowance for Credit Loss [Line Items]
Charge-offs	$ 0	0
Number of residential real estate loans in process of foreclosure \| loan	1
Recorded investment in residential real estate loans in process of foreclosure	$ 8,000
Residential real estate \| Revolving residential real estate
Financing Receivable, Allowance for Credit Loss [Line Items]
Charge-offs	$ 0	$ 0
Number of residential real estate loans in process of foreclosure \| loan		1
Recorded investment in residential real estate loans in process of foreclosure		$ 17,000

Financing Receivable, Excluding Accrued Interest, Allowance For Credit Loss, Write Off, Number Of Loans

No definition available.

Name:	unb_FinancingReceivableExcludingAccruedInterestAllowanceForCreditLossWriteOffNumberOfLoans
Namespace Prefix:	unb_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	duration

- Definition

Financing Receivable Impaired Interest Lost On Nonaccrual Loans

+ References

No definition available.

+ Details

Name:	unb_FinancingReceivableImpairedInterestLostOnNonaccrualLoans
Namespace Prefix:	unb_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

Number of consumer mortgage loans secured by residential real estate properties for which formal foreclosure proceedings are in process.

+ References

No definition available.

+ Details

Name:	unb_MortgageLoansinProcessofForeclosureNumber
Namespace Prefix:	unb_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	instant

- 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
-Name Regulation S-K (SK)
-Number 229
-Section 1405
-Paragraph c
-Publisher SEC

Reference 2: http://www.xbrl.org/2009/role/commonPracticeRef
-Topic 326
-SubTopic 20
-Name Accounting Standards Codification
-Section 50
-Paragraph 13
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147479319/326-20-50-13

+ Details

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

- Definition

Amount, excluding accrued interest, of decrease in allowance for credit loss on financing receivable from writeoff. Excludes net investment in lease.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Topic 326
-SubTopic 20
-Name Accounting Standards Codification
-Section 50
-Paragraph 6
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147479319/326-20-50-6

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Topic 326
-SubTopic 20
-Name Accounting Standards Codification
-Section 50
-Paragraph 13
-Subparagraph (d)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147479319/326-20-50-13

+ Details

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

- Definition

Amortized cost excluding accrued interest, of financing receivable with payment default in current period and modified within previous 12 months preceding payment default for debtor experiencing financial difficulty.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Topic 310
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 44
-Subparagraph (a)(2)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147481962/310-10-50-44

+ Details

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

- Definition

Recorded investment of consumer mortgage loan receivables secured by residential real estate properties for which formal foreclosure proceedings are in process.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Topic 310
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 35
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147481962/310-10-50-35

+ Details

Name:	us-gaap_MortgageLoansInProcessOfForeclosureAmount
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Details

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

- Details

Name:	unb_FinancingReceivableOriginationYearAxis=unb_Originated2021Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	unb_FinancingReceivableOriginationYearAxis=unb_Originated2022Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	unb_FinancingReceivableOriginationYearAxis=unb_Originated2024Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

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

- Details

Name:	us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis=unb_NonRevolvingResidentialRealEstateMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis=unb_RevolvingResidentialRealEstateMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: