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

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 740
-SubTopic 10
-Section 50
-Paragraph 6
-URI http://asc.fasb.org/extlink&oid=84230637&loc=d3e32621-109319

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 740
-SubTopic 10
-Section 25
-Paragraph 20
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=117330534&loc=d3e28680-109314

Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 740
-SubTopic 10
-Section 50
-Paragraph 8
-URI http://asc.fasb.org/extlink&oid=84230637&loc=d3e32632-109319

+ Details

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

- Definition

Amount before allocation of valuation allowances of deferred tax asset attributable to deductible alternative minimum tax credit carryforwards.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 740
-SubTopic 10
-Section 50
-Paragraph 6
-URI http://asc.fasb.org/extlink&oid=84230637&loc=d3e32621-109319

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 740
-SubTopic 10
-Section 50
-Paragraph 3
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=84230637&loc=d3e32559-109319

Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 740
-SubTopic 10
-Section 50
-Paragraph 8
-URI http://asc.fasb.org/extlink&oid=84230637&loc=d3e32632-109319

+ Details

Name:	us-gaap_DeferredTaxAssetsTaxCreditCarryforwardsAlternativeMinimumTax
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://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 740
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SAB TOPIC 6.I)
-URI http://asc.fasb.org/extlink&oid=116825942&loc=d3e330036-122817

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 740
-SubTopic 10
-Section 50
-Paragraph 12
-URI http://asc.fasb.org/extlink&oid=84230637&loc=d3e32687-109319

Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 740
-SubTopic 10
-Section 50
-Paragraph 13
-URI http://asc.fasb.org/extlink&oid=84230637&loc=d3e32698-109319

+ Details

Name:	us-gaap_EffectiveIncomeTaxRateReconciliationAtFederalStatutoryIncomeTaxRate
Namespace Prefix:	us-gaap_
Data Type:	num: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
-Publisher FASB
-Name Accounting Standards Codification
-Topic 980
-SubTopic 405
-Section 25
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=6500807&loc=d3e48068-110394

+ Details

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

- Definition

Deferred income tax provision.

+ References

No definition available.

+ Details

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

- Definition

Deferred tax assets, tax credit carryforwards, additional alternative minimum tax.

+ References

No definition available.

+ Details

Name:	utl_DeferredTaxAssetsTaxCreditCarryforwardsAdditionalAlternativeMinimumTax
Namespace Prefix:	utl_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Estimated flow back period.

+ References

No definition available.

+ Details

Name:	utl_EstimatedFlowBackPeriod
Namespace Prefix:	utl_
Data Type:	xbrli:durationItemType
Balance Type:	na
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 in federal tax provisions.

+ References

No definition available.

+ Details

Name:	utl_NetOperatingLossCarryforwardsInFederalTaxProvisions
Namespace Prefix:	utl_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
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

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

Tax Cuts and Jobs Act of 2017 change in tax rate regulatory liability recovery refund.

+ References

No definition available.

+ Details

Name:	utl_TaxCutsAndJobsActOf2017ChangeInTaxRateRegulatoryLiabilityRecoveryRefund
Namespace Prefix:	utl_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Details

Name:	srt_StatementScenarioAxis=srt_ScenarioForecastMember
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_TaxCreditCarryforwardAxis=utl_ReconcilingItemsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_TaxCreditCarryforwardAxis=utl_NetOperatingLossCarryforwardAssetsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	dei_LegalEntityAxis=utl_FitchburgGasAndElectricLightCompanyMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

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

- Details

Name:	us-gaap_RegulatoryLiabilityAxis=utl_GasRatePayersMember
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:

- Details

Name:	us-gaap_IncomeTaxAuthorityNameAxis=us-gaap_InternalRevenueServiceIRSMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_TaxPeriodAxis=us-gaap_TaxYear2019Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

Income Taxes - Additional Information (Detail) - USD ($) $ in Millions	1 Months Ended	9 Months Ended	12 Months Ended
	Dec. 31, 2017	Sep. 30, 2019	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017	Jan. 01, 2020
Income Taxes [Line Items]
Corporate federal income tax			21.00%	21.00%	34.00%
Regulatory Liability			$ 54.0	$ 58.5
Regulatory liability, expected flow back to customers			$ 47.1	48.9
Average Rate Assumption Method estimated flow back period		15 years	15 years
Tax provision			$ 1.7
Deferred tax assets, operating loss carryforwards, federal			1.6
Alternative minimum tax credit carryforwards			0.9
Additional Alternative minimum tax credit carryforwards			2.3
Scenario Forecast
Income Taxes [Line Items]
Alternative minimum tax credit carryforwards						$ 0.3
Income Tax Related Liabilities
Income Taxes [Line Items]
Regulatory Liability	$ 48.9		47.6	47.0	$ 48.9
Reconciling Mechanisms
Income Taxes [Line Items]
Regulatory liability, expected pass back to ratepayers			1.1		$ 1.8
Tax provision			0.0	0.7
Net Operating Loss Carryforward Assets
Income Taxes [Line Items]
Regulatory liability, expected pass back to ratepayers			0.3	$ 2.0
Fitchburg Gas and Electric Light Company \| Electric Ratepayers
Income Taxes [Line Items]
Regulatory liability, expected flow back to customers			10.1
Fitchburg Gas and Electric Light Company \| Gas Ratepayers
Income Taxes [Line Items]
Regulatory liability, expected flow back to customers			$ 10.4
Tax Year 2018
Income Taxes [Line Items]
Corporate federal income tax	21.00%		21.00%	21.00%	21.00%
Tax Year 2018 \| Internal Revenue Service [Member]
Income Taxes [Line Items]
Net Operating Loss Carryforwards Utilized For Income Taxes			$ 5.7
Tax Year 2019
Income Taxes [Line Items]
Net operating loss carryforwards in federal tax provisions			$ 3.5