Stock Based Compensation RSUs Granted and Unvested (Details) - 2014 Equity Plan [Member] - Restricted Stock Units (RSUs) [Member] - $ / shares	9 Months Ended
Sep. 30, 2019	Feb. 06, 2019
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of RSUs Granted	10,528
Weighted Average Grant Date Fair Value		$ 47.60
Number of Unvested RSUs	6,043
2016 Award [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of RSUs Granted	3,569
Weighted Average Grant Date Fair Value	$ 45.45
Number of Unvested RSUs	478
2017 Award [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of RSUs Granted	3,225
Weighted Average Grant Date Fair Value	$ 52.95
Number of Unvested RSUs	1,831
2018 Award [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of RSUs Granted	3,734
Weighted Average Grant Date Fair Value	$ 47.75
Number of Unvested RSUs	3,734

The number of non-vested equity-based payment instruments, excluding stock (or unit) options, that validly exist and are outstanding as of the balance sheet date.

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 718
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (c)(2)(i)-(ii)
-URI http://asc.fasb.org/extlink&oid=109197908&loc=d3e5070-113901

+ Details

Name:	us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedNumber
Namespace Prefix:	us-gaap_
Data Type:	xbrli:sharesItemType
Balance Type:	na
Period Type:	instant

- Definition

The weighted average fair value of nonvested awards on equity-based plans excluding option plans (for example, phantom stock or unit plan, stock or unit appreciation rights plan, revenue or profit achievement stock award plan) for which the employer is contingently obligated to issue equity instruments or transfer assets to an employee who has not yet satisfied service or performance criteria necessary to gain title to proceeds from the sale of the award or underlying shares or units.

+ References

+ Details

Name:	us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValue
Namespace Prefix:	us-gaap_
Data Type:	num:perShareItemType
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

No definition available.

+ Details

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

- Definition

Net number of non-option equity instruments granted to participants.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 718
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (c)(1)(iv)(1)
-URI http://asc.fasb.org/extlink&oid=109197908&loc=d3e5070-113901

+ Details

Name:	us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNonOptionEquityInstrumentsGranted
Namespace Prefix:	us-gaap_
Data Type:	xbrli:sharesItemType
Balance Type:	na
Period Type:	duration

- Details

Name:	us-gaap_PlanNameAxis=unb_A2014EquityPlanMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_AwardTypeAxis=us-gaap_RestrictedStockUnitsRSUMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_AwardDateAxis=unb_A2016AwardMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_AwardDateAxis=unb_A2017AwardMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_AwardDateAxis=unb_A2018AwardMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: