Allowance for loan losses and credit quality Troubled Debt Restured Loans (Details) $ in Thousands	Sep. 30, 2019 USD ($) loans	Dec. 31, 2018 USD ($) loans
Financing Receivable, Modifications [Line Items]
Troubled Debt Restructured Loans, Number of Loans \| loans	40	42
Troubled Debt Restructured Loans, Principal Balance \| $	$ 2,981	$ 3,309
Residential Real Estate [Member]
Financing Receivable, Modifications [Line Items]
Troubled Debt Restructured Loans, Number of Loans \| loans	26	27
Troubled Debt Restructured Loans, Principal Balance \| $	$ 1,595	$ 1,678
Construction Real Estate [Member]
Financing Receivable, Modifications [Line Items]
Troubled Debt Restructured Loans, Number of Loans \| loans	2	2
Troubled Debt Restructured Loans, Principal Balance \| $	$ 106	$ 119
Commercial Real Estate [Member]
Financing Receivable, Modifications [Line Items]
Troubled Debt Restructured Loans, Number of Loans \| loans	8	9
Troubled Debt Restructured Loans, Principal Balance \| $	$ 992	$ 1,172
Commercial [Member]
Financing Receivable, Modifications [Line Items]
Troubled Debt Restructured Loans, Number of Loans \| loans	4	4
Troubled Debt Restructured Loans, Principal Balance \| $	$ 288	$ 340

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.

No definition available.

+ Details

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

- Definition

Number of financing receivables that have been modified by troubled debt restructurings.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 50
-Paragraph 33
-URI http://asc.fasb.org/extlink&oid=84173941&loc=SL6953676-111524

+ Details

Name:	us-gaap_FinancingReceivableModificationsNumberOfContracts2
Namespace Prefix:	us-gaap_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	duration

- Definition

The amount of the outstanding recorded investment related to financing receivables that have been modified by troubled debt restructurings.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 40
-Section 50
-Paragraph 4
-URI http://asc.fasb.org/extlink&oid=82895541&loc=d3e18845-111554

+ Details

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

- Details

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

- Details

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

- Details

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

- Details

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