XML 60 R46.htm IDEA: XBRL DOCUMENT v3.19.1

Regulatory Matters - Additional Information (Detail)													1 Months Ended		3 Months Ended
	May 01, 2019 USD ($)	Feb. 28, 2019 USD ($)	Oct. 31, 2018 USD ($)	May 07, 2018 USD ($)	May 02, 2018 USD ($)	May 01, 2018 USD ($)	Apr. 30, 2018 USD ($)	Feb. 28, 2018 USD ($)	Jan. 01, 2018 USD ($)	Aug. 01, 2017 USD ($)	Jun. 30, 2017 MW	Apr. 20, 2017 USD ($)	Oct. 31, 2018 USD ($)	Dec. 31, 2017	Mar. 31, 2019	Jun. 30, 2027 MW	Dec. 31, 2022 MWh	Apr. 03, 2019 USD ($)	Jul. 31, 2018 MWh MW	Jul. 23, 2018 MW
Regulatory Assets [Line Items]
Annual revenue adjustment						$ 2,300,000
Tax Year 2018
Regulatory Assets [Line Items]
Corporate income tax rate														21.00%	21.00%
Scenario Forecast
Regulatory Assets [Line Items]
Annual revenue adjustment	$ 1,400,000
Unitil Energy Systems Inc | New Hampshire
Regulatory Assets [Line Items]
Permanent rate increase												$ 4,100,000
Increase (decrease) in annual revenue		$ 340,000					$ 300,000
Reduction of federal tax decrease							2,200,000
Reconciliation adjustment of recouped difference between temporary rates and final rates							1,400,000
Unitil Energy Systems Inc | New Hampshire | Second rate Step Adjustments [Member]
Regulatory Assets [Line Items]
Increase (decrease) in annual revenue							$ 3,300,000
Fitchburg Gas and Electric Light Company
Regulatory Assets [Line Items]
Increase (decrease) in annual revenue			$ 800,000			$ 900,000
Annual revenue adjustment													$ 400,000
Amendment effective date													Nov. 01, 2018
Fitchburg Gas and Electric Light Company | Subsequent Event
Regulatory Assets [Line Items]
Recovery amount																		$ 400,000
Fitchburg Gas and Electric Light Company | Offshore Wind Energy
Regulatory Assets [Line Items]
Power generation facility | MW																			400	400
Remuneration percentage																			2.75%
Fitchburg Gas and Electric Light Company | Qualified Clean Energy
Regulatory Assets [Line Items]
Power generation capacity | MWh																			9,554,940
Fitchburg Gas and Electric Light Company | Minimum | Offshore Wind Energy
Regulatory Assets [Line Items]
Power generation facility | MW											400
Pro rata percentage share of contracts											1.00%
Fitchburg Gas and Electric Light Company | Scenario Forecast
Regulatory Assets [Line Items]
Power generation capacity | MWh																	9,450,000
Fitchburg Gas and Electric Light Company | Scenario Forecast | Offshore Wind Energy
Regulatory Assets [Line Items]
Power generation facility | MW																1,600
Northern Utilities Inc
Regulatory Assets [Line Items]
Increase (decrease) in annual revenue					$ 2,600,000				$ 3,200,000	$ 2,600,000
Increase in annual base rate		2.10%		2.40%
Annual TIRA Adjustment		$ 1,000,000		$ 1,100,000
TIRA initial term															Four years
Northern Utilities Inc | Revenue Offset [Member]
Regulatory Assets [Line Items]
Increase (decrease) in annual revenue					$ (1,700,000)
Northern Utilities Inc | Maine
Regulatory Assets [Line Items]
Increase (decrease) in annual revenue								$ 2,100,000
Percentage of approved return on equity								9.50%
Percentage of approved return on equity, reflecting on equity								50.00%
Percentage of approved return on equity, reflecting on debt								50.00%
TIRA extended term															8 years
Northern Utilities Inc | Maine | Revenue Offset [Member]
Regulatory Assets [Line Items]
Increase (decrease) in annual revenue								$ 2,200,000

X

- Definition

Carrying amount as of the balance sheet date of the portion of the reserved funds established by a utility from customer payments to cover the repair costs of damages from major storms, which repairs are expected to be incurred beyond one year or the normal operating cycle, if longer.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 980
-SubTopic 340
-Section 25
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=77907324&loc=d3e43603-110378

+ Details

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

X

- Definition

Amount of current federal tax expense (benefit) pertaining to income (loss) from continuing operations.

+ 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.7)
-URI http://asc.fasb.org/extlink&oid=34349781&loc=d3e330036-122817

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 235
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.4-08(h))
-URI http://asc.fasb.org/extlink&oid=26873400&loc=d3e23780-122690

Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 740
-SubTopic 10
-Section 50
-Paragraph 9
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=84230637&loc=d3e32639-109319

+ Details

Name:	us-gaap_CurrentFederalTaxExpenseBenefit
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

X

- 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=34349781&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

X

- Definition

The increase (decrease) during the reporting period in unrecovered fuel costs for a regulated utility company.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 230
-SubTopic 10
-Section 45
-Paragraph 28
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=109223946&loc=d3e3602-108585

+ Details

Name:	us-gaap_IncreaseDecreaseInRegulatoryClauseRevenue
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

X

- Definition

Percentage of public utility's approved debt to capital, including debt and equity.

+ References

No definition available.

+ Details

Name:	us-gaap_PublicUtilitiesApprovedDebtCapitalStructurePercentage
Namespace Prefix:	us-gaap_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

X

- Definition

Percentage of public utility's approved equity to capital, including debt and equity.

+ References

No definition available.

+ Details

Name:	us-gaap_PublicUtilitiesApprovedEquityCapitalStructurePercentage
Namespace Prefix:	us-gaap_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

X

- Definition

Percentage of public utility's approved return on equity.

+ References

No definition available.

+ Details

Name:	us-gaap_PublicUtilitiesApprovedReturnOnEquityPercentage
Namespace Prefix:	us-gaap_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

X

- Definition

Describes the term of the registration payment arrangement.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 825
-SubTopic 20
-Section 50
-Paragraph 1
-Subparagraph (a)(1)
-URI http://asc.fasb.org/extlink&oid=99383193&loc=d3e16207-108621

+ Details

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

X

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

X

- Definition

Contracts pro rata share percentage.

+ References

No definition available.

+ Details

Name:	utl_ContractsProRataSharePercentage
Namespace Prefix:	utl_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

X

- Definition

Facility Power Capacity

+ References

No definition available.

+ Details

Name:	utl_FacilityPowerCapacity
Namespace Prefix:	utl_
Data Type:	num:powerItemType
Balance Type:	na
Period Type:	instant

X

- Definition

Increase in annual base rate.

+ References

No definition available.

+ Details

Name:	utl_IncreaseInAnnualBaseRate
Namespace Prefix:	utl_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

X

- Definition

Infrastructure replacement adjustment.

+ References

No definition available.

+ Details

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

X

- Definition

Permanent adjustment for rate increase.

+ References

No definition available.

+ Details

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

X

- Definition

Power Generation Capacity

+ References

No definition available.

+ Details

Name:	utl_PowerGenerationCapacity
Namespace Prefix:	utl_
Data Type:	num:energyItemType
Balance Type:	na
Period Type:	instant

X

- Definition

Public utilities approved rate increase decrease effective date.

+ References

No definition available.

+ Details

Name:	utl_PublicUtilitiesApprovedRateIncreaseDecreaseEffectiveDate
Namespace Prefix:	utl_
Data Type:	xbrli:dateItemType
Balance Type:	na
Period Type:	duration

X

- Definition

Reconciliation adjustment of recouped the difference between temporary rates and final rates.

+ References

No definition available.

+ Details

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

X

- Definition

Describes the extended term of the registration payment arrangement.

+ References

No definition available.

+ Details

Name:	utl_RegistrationPaymentArrangementExtensionTerm
Namespace Prefix:	utl_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration

X

- Definition

Remuneration percentage.

+ References

No definition available.

+ Details

Name:	utl_RemunerationPercentage
Namespace Prefix:	utl_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

X

- Definition

Revenue adjustment.

+ References

No definition available.

+ Details

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

X

- Details

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

X

- Details

Name:	us-gaap_StatementScenarioAxis=us-gaap_ScenarioForecastMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

X

- Details

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

X

- Details

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

X

- Details

Name:	us-gaap_VariableRateAxis=utl_SecondRateStepAdjustmentsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

X

- Details

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

X

- Details

Name:	us-gaap_SubsequentEventTypeAxis=us-gaap_SubsequentEventMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

X

- Details

Name:	us-gaap_EnergyAxis=utl_OffshoreEnergyMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

X

- Details

Name:	us-gaap_EnergyAxis=utl_QualifiedCleanEnergyMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

X

- Details

Name:	srt_RangeAxis=srt_MinimumMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

X

- Details

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

X

- Details

Name:	us-gaap_RegulatoryAssetAxis=utl_RevenueOffsetMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

X

- Details

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