Loans Narrative Data (Details) $ in Thousands	12 Months Ended
Dec. 31, 2020 USD ($) loan	Dec. 31, 2019 USD ($) loan
Receivables [Abstract]
Number of PPP loans \| loan	679
PPP loans, outstanding amount	$ 66,200
PPP loans, origination fee	2,400
PPP loans, origination fee recognized	953
Loans pledged as collateral	$ 210,000	$ 207,700
Number of residential real estate loans in process of foreclosure \| loan	0	2
Recorded investment in residential real estate loans in process of foreclosure		$ 64
Aggregate interest on nonaccrual loans not recognized	$ 420	$ 271

Financing Receivable Impaired Interest Lost On Nonaccrual Loans

No definition available.

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

Number of loans outstanding under the SBA Paycheck Protection Program as part of the CARES Act.

+ References

No definition available.

+ Details

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

- Definition

Origination fee to be collected related to loans outstanding under the SBA Paycheck Protection Program as part of the CARES Act.

+ References

No definition available.

+ Details

Name:	unb_PaycheckProtectionProgramOriginationFee
Namespace Prefix:	unb_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Origination fee recognized in income on loans advanced under the SBA Paycheck Protection Program as part of the CARES Act.

+ References

No definition available.

+ Details

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

- Definition

Total dollar amount of loans outstanding under the SBA Paycheck Protection Program as part of the CARES Act.

+ References

No definition available.

+ Details

Name:	unb_PaycheckProtectionProgramTotalAmountOfLoansOutstanding
Namespace Prefix:	unb_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Carrying amount as of the balance sheet date of loans that have been pledged as collateral for borrowings. If the secured party has the right by contract or custom to sell or re-pledge the collateral, then the debtor reclassifies that asset and report that asset in its statement of financial position separately from other assets not so encumbered.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 860
-SubTopic 30
-Section 45
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=66007379&loc=d3e113888-111728

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 50
-Paragraph 5
-URI http://asc.fasb.org/extlink&oid=121611835&loc=d3e5086-111524

+ Details

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

- 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://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 50
-Paragraph 35
-URI http://asc.fasb.org/extlink&oid=121611835&loc=SL49126937-111524

+ Details

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

- References

No definition available.

+ Details

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