v3.25.2

INCOME TAXES - Additional Information (Detail) - USD ($) $ in Millions	6 Months Ended		12 Months Ended
	Jun. 30, 2025	Jun. 30, 2024	Dec. 31, 2024	Dec. 31, 2023	Dec. 31, 2017
Income Taxes [Line Items]
Corporate federal income tax	21.00%	21.00%
Regulatory Liability	$ 73.5	$ 57.8	$ 64.0
Regulatory liability, expected flow back to customers					$ 47.1
State Taxes Credit to Offset Future Taxes			1.6
Net Operating Loss Carryforwards Utilized For Income Taxes				$ 2.9
Offset future years income				$ 7.3
Deferred tax assets, operating loss carryforwards, federal			6.6
Federal Tax Credits to Offset Current Taxes			1.3
Gas Ratepayers \| Massachusetts And Maine [Member]
Income Taxes [Line Items]
Regulatory liability, expected flow back to customers	12.7
Income Tax Related Liabilities
Income Taxes [Line Items]
Regulatory Liability	$ 49.7	$ 37.2	$ 50.4		$ 48.9
Tax Year 2018
Income Taxes [Line Items]
Corporate federal income tax					21.00%

Amount before allocation of valuation allowances of deferred tax asset attributable to deductible domestic operating loss carryforwards. Excludes state and local operating loss carryforwards.

+ References

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Topic 740
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 6
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147482685/740-10-50-6

+ Details

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

- Definition

Percentage of domestic federal statutory tax rate applicable to pretax income (loss).

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Topic 740
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 12
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147482685/740-10-50-12

Reference 2: http://www.xbrl.org/2003/role/exampleRef
-Topic 740
-SubTopic 10
-Name Accounting Standards Codification
-Section 55
-Paragraph 231
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147482663/740-10-55-231

Reference 3: http://www.xbrl.org/2003/role/disclosureRef
-Topic 740
-SubTopic 10
-Name Accounting Standards Codification
-Section S99
-Paragraph 1
-Subparagraph (SAB Topic 6.I.1.Q1)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147479360/740-10-S99-1

Reference 4: http://www.xbrl.org/2003/role/disclosureRef
-Topic 235
-SubTopic 10
-Name Accounting Standards Codification
-Section S99
-Paragraph 1
-Subparagraph (SX 210.4-08(h)(2))
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147480678/235-10-S99-1

+ Details

Name:	us-gaap_EffectiveIncomeTaxRateReconciliationAtFederalStatutoryIncomeTaxRate
Namespace Prefix:	us-gaap_
Data Type:	dtr-types:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

The amount for the individual regulatory liability as itemized in a table of regulatory liabilities as of the end of the period.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Topic 980
-SubTopic 405
-Name Accounting Standards Codification
-Section 25
-Paragraph 1
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147477571/980-405-25-1

+ Details

Name:	us-gaap_RegulatoryLiabilities
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Federal Tax Credits to Offset Current Taxes

+ References

No definition available.

+ Details

Name:	utl_FederalTaxCreditsToOffsetCurrentTaxes
Namespace Prefix:	utl_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

Income Taxes [Line Items]

+ References

No definition available.

+ Details

Name:	utl_IncomeTaxesLineItems
Namespace Prefix:	utl_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

Net operating loss carryforwards utilized for income taxes.

+ References

No definition available.

+ Details

Name:	utl_NetOperatingLossCarryforwardsUtilizedForIncomeTaxes
Namespace Prefix:	utl_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

Offset Future Payments

+ References

No definition available.

+ Details

Name:	utl_OffsetFuturePayments
Namespace Prefix:	utl_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

Regulatory liability recovery refund.

+ References

No definition available.

+ Details

Name:	utl_RegulatoryLiabilityRecoveryRefund
Namespace Prefix:	utl_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

State Taxes Credit to Offset Future Taxes

+ References

No definition available.

+ Details

Name:	utl_StateTaxesCreditToOffsetFutureTaxes
Namespace Prefix:	utl_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Details

Name:	us-gaap_RegulatoryLiabilityAxis=utl_GasRatePayersMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	srt_StatementGeographicalAxis=utl_MassachusettsAndMaineMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_RegulatoryLiabilityAxis=utl_IncomeTaxRelatedLiabilitiesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_TaxPeriodAxis=us-gaap_TaxYear2018Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

UNITIL CORP 10-Q