Allowance for Credit Losses on Loans and Off-Balance Sheet Credit Exposures - Schedule Of Financing Receivable Credit Quality Indicators (Details) - USD ($) $ in Thousands	Jun. 30, 2024	Dec. 31, 2023
Financing Receivable, Credit Quality Indicator [Line Items]
2024	$ 82,673	$ 227,925
2023	173,588	246,472
2022	226,097	177,006
2021	168,624	71,494
2020	67,852	61,135
Prior	238,177	199,256
Revolving	48,978	44,982
Total	1,005,989	1,028,270
Residential Real Estate \| Non-revolving residential real estate
Financing Receivable, Credit Quality Indicator [Line Items]
2024	37,535	88,573
2023	83,105	119,253
2022	116,461	88,808
2021	86,106	31,998
2020	30,951	9,377
Prior	63,067	59,400
Revolving	0	0
Total	417,225	397,409
Residential Real Estate \| Revolving residential real estate
Financing Receivable, Credit Quality Indicator [Line Items]
2024	0	0
2023	0	0
2022	0	0
2021	0	0
2020	0	0
Prior	0	0
Revolving	21,486	18,902
Total	21,486	18,902
Residential Real Estate \| Pass \| Non-revolving residential real estate
Financing Receivable, Credit Quality Indicator [Line Items]
2024	34,289	84,211
2023	78,804	112,215
2022	109,246	83,137
2021	81,046	29,704
2020	28,712	8,991
Prior	57,257	53,021
Revolving	0	0
Total	389,354	371,279
Residential Real Estate \| Pass \| Revolving residential real estate
Financing Receivable, Credit Quality Indicator [Line Items]
2024	0	0
2023	0	0
2022	0	0
2021	0	0
2020	0	0
Prior	0	0
Revolving	19,579	17,133
Total	19,579	17,133
Residential Real Estate \| Satisfactory/Monitor \| Non-revolving residential real estate
Financing Receivable, Credit Quality Indicator [Line Items]
2024	3,246	4,362
2023	4,301	7,038
2022	7,215	5,671
2021	5,060	2,280
2020	2,239	386
Prior	5,653	5,873
Revolving	0	0
Total	27,714	25,610
Residential Real Estate \| Satisfactory/Monitor \| Revolving residential real estate
Financing Receivable, Credit Quality Indicator [Line Items]
2024	0	0
2023	0	0
2022	0	0
2021	0	0
2020	0	0
Prior	0	0
Revolving	1,865	1,724
Total	1,865	1,724
Residential Real Estate \| Substandard \| Non-revolving residential real estate
Financing Receivable, Credit Quality Indicator [Line Items]
2024	0	0
2023	0	0
2022	0	0
2021	0	14
2020	0	0
Prior	157	506
Revolving	0	0
Total	157	520
Residential Real Estate \| Substandard \| Revolving residential real estate
Financing Receivable, Credit Quality Indicator [Line Items]
2024	0	0
2023	0	0
2022	0	0
2021	0	0
2020	0	0
Prior	0	0
Revolving	42	45
Total	42	45
Construction Real Estate \| Commercial construction real estate
Financing Receivable, Credit Quality Indicator [Line Items]
2024	3,249	14,048
2023	22,955	9,440
2022	9,498	11,088
2021	9,524	971
2020	935	437
Prior	1,011	989
Revolving	0	0
Total	47,172	36,973
Construction Real Estate \| Residential construction real estate
Financing Receivable, Credit Quality Indicator [Line Items]
2024	12,668	28,474
2023	28,500	20,018
2022	5,166	1,066
2021	742	1,981
2020	929	0
Prior	0	123
Revolving	0	0
Total	48,005	51,662
Construction Real Estate \| Pass \| Commercial construction real estate
Financing Receivable, Credit Quality Indicator [Line Items]
2024	1,846	3,736
2023	3,133	5,767
2022	5,407	2,736
2021	2,301	616
2020	586	437
Prior	924	873
Revolving	0	0
Total	14,197	14,165
Construction Real Estate \| Pass \| Residential construction real estate
Financing Receivable, Credit Quality Indicator [Line Items]
2024	12,041	24,496
2023	26,890	17,904
2022	4,409	498
2021	174	0
2020	0	0
Prior	0	123
Revolving	0	0
Total	43,514	43,021
Construction Real Estate \| Satisfactory/Monitor \| Commercial construction real estate
Financing Receivable, Credit Quality Indicator [Line Items]
2024	1,403	10,312
2023	19,822	3,673
2022	4,091	8,352
2021	7,223	355
2020	349	0
Prior	87	116
Revolving	0	0
Total	32,975	22,808
Construction Real Estate \| Satisfactory/Monitor \| Residential construction real estate
Financing Receivable, Credit Quality Indicator [Line Items]
2024	627	3,978
2023	1,610	2,114
2022	757	568
2021	568	1,981
2020	929	0
Prior	0	0
Revolving	0	0
Total	4,491	8,641
Construction Real Estate \| Substandard \| Commercial construction real estate
Financing Receivable, Credit Quality Indicator [Line Items]
2024	0	0
2023	0	0
2022	0	0
2021	0	0
2020	0	0
Prior	0	0
Revolving	0	0
Total	0	0
Construction Real Estate \| Substandard \| Residential construction real estate
Financing Receivable, Credit Quality Indicator [Line Items]
2024	0	0
2023	0	0
2022	0	0
2021	0	0
2020	0	0
Prior	0	0
Revolving	0	0
Total	0	0
Commercial Real Estate \| Non-residential commercial real estate
Financing Receivable, Credit Quality Indicator [Line Items]
2024	13,475	23,885
2023	24,557	68,991
2022	65,818	48,381
2021	47,214	25,652
2020	25,029	29,822
Prior	100,101	81,234
Revolving	20,611	20,183
Total	296,805	298,148
Commercial Real Estate \| Multi-family residential real estate
Financing Receivable, Credit Quality Indicator [Line Items]
2024	1,644	1,091
2023	1,122	17,849
2022	17,478	22,071
2021	20,338	7,725
2020	7,585	18,159
Prior	54,419	38,449
Revolving	0	0
Total	102,586	105,344
Commercial Real Estate \| Pass \| Non-residential commercial real estate
Financing Receivable, Credit Quality Indicator [Line Items]
2024	3,985	5,977
2023	5,338	44,428
2022	41,803	34,562
2021	33,385	18,327
2020	17,906	23,650
Prior	70,274	57,197
Revolving	15,134	16,629
Total	187,825	200,770
Commercial Real Estate \| Pass \| Multi-family residential real estate
Financing Receivable, Credit Quality Indicator [Line Items]
2024	1,075	250
2023	291	5,364
2022	4,526	10,208
2021	9,892	2,061
2020	1,985	8,226
Prior	42,522	34,993
Revolving	0	0
Total	60,291	61,102
Commercial Real Estate \| Satisfactory/Monitor \| Non-residential commercial real estate
Financing Receivable, Credit Quality Indicator [Line Items]
2024	9,490	17,908
2023	19,219	24,563
2022	24,015	13,819
2021	13,829	5,552
2020	5,428	6,172
Prior	29,439	23,521
Revolving	5,419	3,486
Total	106,839	95,021
Commercial Real Estate \| Satisfactory/Monitor \| Multi-family residential real estate
Financing Receivable, Credit Quality Indicator [Line Items]
2024	569	841
2023	831	12,485
2022	12,952	11,863
2021	10,446	5,664
2020	5,600	9,933
Prior	11,635	2,126
Revolving	0	0
Total	42,033	42,912
Commercial Real Estate \| Substandard \| Non-residential commercial real estate
Financing Receivable, Credit Quality Indicator [Line Items]
2024	0	0
2023	0	0
2022	0	0
2021	0	1,773
2020	1,695	0
Prior	388	516
Revolving	58	68
Total	2,141	2,357
Commercial Real Estate \| Substandard \| Multi-family residential real estate
Financing Receivable, Credit Quality Indicator [Line Items]
2024	0	0
2023	0	0
2022	0	0
2021	0	0
2020	0	0
Prior	262	1,330
Revolving	0	0
Total	262	1,330
Commercial
Financing Receivable, Credit Quality Indicator [Line Items]
2024	2,109	3,872
2023	4,717	7,637
2022	6,659	4,475
2021	3,907	1,138
2020	826	2,981
Prior	15,333	14,476
Revolving	6,851	5,869
Total	40,402	40,448
Commercial \| Pass
Financing Receivable, Credit Quality Indicator [Line Items]
2024	1,338	1,958
2023	2,311	6,394
2022	5,415	2,125
2021	1,821	671
2020	456	2,849
Prior	9,003	7,755
Revolving	5,331	4,992
Total	25,675	26,744
Commercial \| Satisfactory/Monitor
Financing Receivable, Credit Quality Indicator [Line Items]
2024	771	1,914
2023	2,406	1,243
2022	1,244	2,350
2021	2,086	467
2020	370	132
Prior	6,328	6,717
Revolving	1,296	648
Total	14,501	13,471
Commercial \| Substandard
Financing Receivable, Credit Quality Indicator [Line Items]
2024	0	0
2023	0	0
2022	0	0
2021	0	0
2020	0	0
Prior	2	4
Revolving	224	229
Total	226	233
Consumer
Financing Receivable, Credit Quality Indicator [Line Items]
2024	734	1,586
2023	1,128	342
2022	224	131
2021	74	98
2020	80	229
Prior	255	175
Revolving	30	28
Total	2,525	2,589
Consumer \| Pass
Financing Receivable, Credit Quality Indicator [Line Items]
2024	734	1,566
2023	1,128	342
2022	224	131
2021	74	98
2020	80	229
Prior	255	175
Revolving	30	28
Total	2,525	2,569
Consumer \| Satisfactory/Monitor
Financing Receivable, Credit Quality Indicator [Line Items]
2024	0	20
2023	0	0
2022	0	0
2021	0	0
2020	0	0
Prior	0	0
Revolving	0	0
Total	0	20
Consumer \| Substandard
Financing Receivable, Credit Quality Indicator [Line Items]
2024	0	0
2023	0	0
2022	0	0
2021	0	0
2020	0	0
Prior	0	0
Revolving	0	0
Total	0	0
Municipal
Financing Receivable, Credit Quality Indicator [Line Items]
2024	11,259	66,396
2023	7,504	2,942
2022	4,793	986
2021	719	1,931
2020	1,517	130
Prior	3,991	4,410
Revolving	0	0
Total	29,783	76,795
Municipal \| Pass
Financing Receivable, Credit Quality Indicator [Line Items]
2024	11,259	66,396
2023	7,504	2,942
2022	4,793	986
2021	719	1,931
2020	1,517	130
Prior	3,991	4,410
Revolving	0	0
Total	29,783	76,795
Municipal \| Satisfactory/Monitor
Financing Receivable, Credit Quality Indicator [Line Items]
2024	0	0
2023	0	0
2022	0	0
2021	0	0
2020	0	0
Prior	0	0
Revolving	0	0
Total	0	0
Municipal \| Substandard
Financing Receivable, Credit Quality Indicator [Line Items]
2024	0	0
2023	0	0
2022	0	0
2021	0	0
2020	0	0
Prior	0	0
Revolving	0	0
Total	$ 0	$ 0

Amortized cost excluding accrued interest, before allowance for credit loss, of financing receivable. Excludes net investment in lease.

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Topic 326
-SubTopic 20
-Name Accounting Standards Codification
-Section 45
-Paragraph 1
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147479344/326-20-45-1

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Topic 326
-SubTopic 20
-Name Accounting Standards Codification
-Section 50
-Paragraph 3B
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147479319/326-20-50-3B

Reference 3: http://www.xbrl.org/2003/role/disclosureRef
-Topic 326
-SubTopic 20
-Name Accounting Standards Codification
-Section 50
-Paragraph 5
-Subparagraph (b)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147479319/326-20-50-5

+ Details

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

- Definition

Amortized cost excluding accrued interest, of financing receivable originated more than five years prior to current fiscal year. Excludes net investment in lease.

+ References

Reference 1: http://www.xbrl.org/2003/role/exampleRef
-Topic 326
-SubTopic 20
-Name Accounting Standards Codification
-Section 55
-Paragraph 79
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147479294/326-20-55-79

Reference 2: 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 3: http://www.xbrl.org/2003/role/disclosureRef
-Topic 326
-SubTopic 20
-Name Accounting Standards Codification
-Section 50
-Paragraph 3B
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147479319/326-20-50-3B

+ Details

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

- Definition

Amortized cost excluding accrued interest, of financing receivable that can be withdrawn, repaid, and redrawn.

+ References

+ Details

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

- Definition

Amortized cost excluding accrued interest, of financing receivable originated four years prior to current fiscal year. Excludes net investment in lease.

+ References

+ Details

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

- Definition

Amortized cost excluding accrued interest, of financing receivable originated three years prior to current fiscal year. Excludes net investment in lease.

+ References

+ Details

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

- Definition

Amortized cost excluding accrued interest, of financing receivable originated in current fiscal year. Excludes net investment in lease.

+ References

+ Details

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

- Definition

Amortized cost excluding accrued interest, of financing receivable originated two years prior to current fiscal year. Excludes net investment in lease.

+ References

+ Details

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

- Definition

Amortized cost excluding accrued interest of financing receivable originated in fiscal year prior to current fiscal year. Excludes net investment in lease.

+ References

+ Details

Name:	us-gaap_FinancingReceivableExcludingAccruedInterestYearTwoOriginatedFiscalYearBeforeCurrentFiscalYear
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
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://fasb.org/us-gaap/role/ref/otherTransitionRef
-Topic 310
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 29
-Subparagraph (b)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147481962/310-10-50-29

Reference 2: http://www.xbrl.org/2009/role/commonPracticeRef
-Topic 326
-SubTopic 20
-Name Accounting Standards Codification
-Section 50
-Paragraph 5
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147479319/326-20-50-5

Reference 3: http://www.xbrl.org/2003/role/exampleRef
-Topic 326
-SubTopic 20
-Name Accounting Standards Codification
-Section 55
-Paragraph 79
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147479294/326-20-55-79

+ Details

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

- 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:

- Details

Name:	us-gaap_InternalCreditAssessmentAxis=us-gaap_PassMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_InternalCreditAssessmentAxis=unb_SatisfactoryandMonitorMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_InternalCreditAssessmentAxis=us-gaap_SubstandardMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

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

- Details

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

- Details

Name:	us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis=unb_ResidentialConstructionRealEstateMember
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_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis=unb_NonResidentialCommercialRealEstateMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis=unb_MultiFamilyResidentialRealEstateMember
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:

- Details

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

- Details

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