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Regulatory Matters - Additional Information (Detail) (USD $) In Millions, unless otherwise specified	0 Months Ended		3 Months Ended	0 Months Ended				12 Months Ended	0 Months Ended
	Nov. 26, 2014	May 30, 2014	Mar. 31, 2015	Apr. 30, 2014	Apr. 25, 2013	Dec. 27, 2013	May 12, 2010	Dec. 31, 2014	Feb. 27, 2015	Apr. 21, 2014	Apr. 15, 2015	Feb. 19, 2015	Jun. 27, 2014	Mar. 31, 2014
Regulatory Matters [Line Items]
Regulatory Liabilities			$ 39.4us-gaap_RegulatoryLiabilityCurrent					$ 8.7us-gaap_RegulatoryLiabilityCurrent						$ 13.1us-gaap_RegulatoryLiabilityCurrent
Fitchburg Gas and Electric Light Company
Regulatory Matters [Line Items]
Increase in annual revenue		5.6us-gaap_IncreaseDecreaseInRegulatoryClauseRevenue / dei_LegalEntityAxis = utl_FitchburgGasAndElectricLightCompanyMember
Amendment effective date			June 1, 2014
Percentage of approved return on equity		9.70%us-gaap_PublicUtilitiesApprovedReturnOnEquityPercentage / dei_LegalEntityAxis = utl_FitchburgGasAndElectricLightCompanyMember
Percentage of approved common equity ratio		48.00%us-gaap_PublicUtilitiesApprovedEquityCapitalStructurePercentage / dei_LegalEntityAxis = utl_FitchburgGasAndElectricLightCompanyMember
Cost recovery period, years		5 years
Annual funding amount approved		0.5us-gaap_PublicUtilitiesAllowanceForFundsUsedDuringConstructionAdditions / dei_LegalEntityAxis = utl_FitchburgGasAndElectricLightCompanyMember
Recovery amount		0.9us-gaap_AssetRecoveryDamagedPropertyCostsNoncurrent / dei_LegalEntityAxis = utl_FitchburgGasAndElectricLightCompanyMember	0.3us-gaap_AssetRecoveryDamagedPropertyCostsNoncurrent / dei_LegalEntityAxis = utl_FitchburgGasAndElectricLightCompanyMember
Storm expenditures	0.2utl_StormExpenditures / dei_LegalEntityAxis = utl_FitchburgGasAndElectricLightCompanyMember
Construction expenditures	0.1us-gaap_ConstructionInProgressExpendituresIncurredButNotYetPaid / dei_LegalEntityAxis = utl_FitchburgGasAndElectricLightCompanyMember
Fitchburg Gas and Electric Light Company | Storm Costs
Regulatory Matters [Line Items]
Cost recovery period, years		3 years
Deferred emergency storm repair costs incurred		5.0us-gaap_DeferredStormAndPropertyReserveDeficiencyNoncurrent / dei_LegalEntityAxis = utl_FitchburgGasAndElectricLightCompanyMember / us-gaap_RegulatoryAssetAxis = us-gaap_StormCostsMember
Storm expenditures	0.3utl_StormExpenditures / dei_LegalEntityAxis = utl_FitchburgGasAndElectricLightCompanyMember / us-gaap_RegulatoryAssetAxis = us-gaap_StormCostsMember	5.0utl_StormExpenditures / dei_LegalEntityAxis = utl_FitchburgGasAndElectricLightCompanyMember / us-gaap_RegulatoryAssetAxis = us-gaap_StormCostsMember
Unitil Energy Systems Inc
Regulatory Matters [Line Items]
Amendment effective date			May 1, 2014
Percent of increase in annual revenue				9.67%utl_PercentageOfRevenueIncrease / dei_LegalEntityAxis = utl_UnitilEnergySystemsIncMember
Adjustment to recover the increased spending for vegetation management program and reliability enhancement program and a proposed storm resiliency program				1.5utl_AdjustmentForRateIncrease / dei_LegalEntityAxis = utl_UnitilEnergySystemsIncMember
Settlement agreement date				May 01, 2016
Interest rate allowed by regulators on the unrecovered amount of storm costs which have been approved for recovery through rate adjustments					4.52%utl_InterestRateAllowedOnUnrecoveredStormCosts / dei_LegalEntityAxis = utl_UnitilEnergySystemsIncMember
Construction expenditures	0.4us-gaap_ConstructionInProgressExpendituresIncurredButNotYetPaid / dei_LegalEntityAxis = utl_UnitilEnergySystemsIncMember
Recovery of damage of electrical system	1.7us-gaap_StormDamageProvision / dei_LegalEntityAxis = utl_UnitilEnergySystemsIncMember
Unitil Energy Systems Inc | Storm Costs
Regulatory Matters [Line Items]
Cost recovery period, years					5 years
Recovery amount					2.3us-gaap_AssetRecoveryDamagedPropertyCostsNoncurrent / dei_LegalEntityAxis = utl_UnitilEnergySystemsIncMember / us-gaap_RegulatoryAssetAxis = us-gaap_StormCostsMember
Storm expenditures	2.1utl_StormExpenditures / dei_LegalEntityAxis = utl_UnitilEnergySystemsIncMember / us-gaap_RegulatoryAssetAxis = us-gaap_StormCostsMember
Northern Utilities Inc
Regulatory Matters [Line Items]
Increase in annual revenue						1.0us-gaap_IncreaseDecreaseInRegulatoryClauseRevenue / dei_LegalEntityAxis = utl_NorthernUtilitiesIncMember
TIRA initial term			Four years
Proposed percentage increase in pipeline rates							55.00%us-gaap_PublicUtilitiesRequestedRateIncreaseDecreasePercentage / dei_LegalEntityAxis = utl_NorthernUtilitiesIncMember
Regulatory Liabilities			22.0us-gaap_RegulatoryLiabilityCurrent / dei_LegalEntityAxis = utl_NorthernUtilitiesIncMember
Northern Utilities Inc | First Annual Adjustment
Regulatory Matters [Line Items]
Annual TIRA Adjustment								1.3utl_InfrastructureReplacementAdjustment / dei_LegalEntityAxis = utl_NorthernUtilitiesIncMember / us-gaap_StatementScenarioAxis = utl_FirstAmendmentMember
Annual TIRA Adjustment, effective date			May 1, 2014
Northern Utilities Inc | Second Annual Adjustment
Regulatory Matters [Line Items]
Annual TIRA Adjustment									1.2utl_InfrastructureReplacementAdjustment / dei_LegalEntityAxis = utl_NorthernUtilitiesIncMember / us-gaap_StatementScenarioAxis = utl_SecondAmendmentMember
Annual TIRA Adjustment, effective date			May 1, 2015
Northern Utilities Inc | 2014 Step Adjustments
Regulatory Matters [Line Items]
Increase in annual revenue										1.4us-gaap_IncreaseDecreaseInRegulatoryClauseRevenue / dei_LegalEntityAxis = utl_NorthernUtilitiesIncMember / us-gaap_StatementScenarioAxis = utl_TwentyFourteenStepAdjustmentsMember
Amendment effective date			May 1, 2014
Northern Utilities Inc | 2015 Step Adjustments
Regulatory Matters [Line Items]
Increase in annual revenue			1.4us-gaap_IncreaseDecreaseInRegulatoryClauseRevenue / dei_LegalEntityAxis = utl_NorthernUtilitiesIncMember / us-gaap_StatementScenarioAxis = utl_TwentyFifteenStepAdjustmentsMember
Amendment effective date			May 1, 2015
Northern Utilities Inc | Subsequent Event
Regulatory Matters [Line Items]
Pipeline refund received											22us-gaap_ProceedsFromLegalSettlements / dei_LegalEntityAxis = utl_NorthernUtilitiesIncMember / us-gaap_SubsequentEventTypeAxis = us-gaap_SubsequentEventMember
Northern Utilities Inc | Maine
Regulatory Matters [Line Items]
Increase in annual revenue						3.8us-gaap_IncreaseDecreaseInRegulatoryClauseRevenue / dei_LegalEntityAxis = utl_NorthernUtilitiesIncMember / us-gaap_StatementGeographicalAxis = stpr_ME
Amendment effective date			January 1, 2014
Northern Utilities Inc | New Hampshire
Regulatory Matters [Line Items]
Increase in annual revenue									1.8us-gaap_IncreaseDecreaseInRegulatoryClauseRevenue / dei_LegalEntityAxis = utl_NorthernUtilitiesIncMember / us-gaap_StatementGeographicalAxis = stpr_NH
Amendment effective date			May 1, 2015
Percent of increase in annual revenue										9.50%utl_PercentageOfRevenueIncrease / dei_LegalEntityAxis = utl_NorthernUtilitiesIncMember / us-gaap_StatementGeographicalAxis = stpr_NH
Northern Utilities Inc | New Hampshire | Natural Gas Distribution
Regulatory Matters [Line Items]
Increase in annual revenue										4.6us-gaap_IncreaseDecreaseInRegulatoryClauseRevenue / dei_LegalEntityAxis = utl_NorthernUtilitiesIncMember / us-gaap_PublicUtilityAxis = us-gaap_GasDistributionMember / us-gaap_StatementGeographicalAxis = stpr_NH
Northern Utilities Inc | Maximum | Portland Natural Gas Transmission System
Regulatory Matters [Line Items]
Grace period to issue refund												60 days
Northern Utilities Inc | Maximum | Northern Utilities' customers
Regulatory Matters [Line Items]
Grace period to issue refund			3 years
Granite State Gas Transmission Inc | Gas Transportation and Storage
Regulatory Matters [Line Items]
Amendment effective date			August 1, 2014
Annual revenue increase in rate adjustments													$ 0.6utl_IncreaseDecreaseInUnearnedRevenue / dei_LegalEntityAxis = utl_GraniteStateGasTransmissionIncMember / us-gaap_PublicUtilityAxis = utl_GasTransportationAndStorageMember

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://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 980

-SubTopic 340

-Section 25

-Paragraph 1

-URI http://asc.fasb.org/extlink&oid=6499739&loc=d3e43603-110378

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- Definition

Future cash outflow to pay for construction in progress expenditures that have occurred.

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-Publisher FASB

-Name Accounting Standards Codification

-Topic 230

-SubTopic 10

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- Definition

Carrying amount as of the balance sheet date of capitalized storm costs of a regulated entity that are expected to be recoverable through rate adjustments after one year or beyond the normal operating cycle, if longer.

+ References

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-Publisher FASB

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-Topic 980

-SubTopic 340

-Section 25

-Paragraph 1

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X

- Definition

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

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-Publisher FASB

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-Paragraph 28

-Subparagraph (a)

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- Definition

Cash received for the settlement of litigation during the current period.

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- Definition

Total increase in earnings in the period representing the cost of equity (based on assumed rate of return) and/or borrowed funds (based on interest rate) used to finance construction of regulated assets, which is expected to be recovered through rate adjustments.

+ References

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- Definition

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

+ References

No definition available.

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- Definition

Percentage of public utility's approved return on equity.

+ References

No definition available.

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X

- Definition

Percentage of public utility's requested rate increase (decrease) with regulatory agency.

+ References

No definition available.

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- Definition

Describes the term of the registration payment arrangement.

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-Publisher FASB

-Name Accounting Standards Codification

-Topic 825

-SubTopic 20

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-Subparagraph (a)(1)

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- Definition

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

+ References

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-Publisher FASB

-Name Accounting Standards Codification

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- Definition

Remaining recovery period of regulatory assets for which no return on investment during recovery period is provided, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

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- Definition

A noncash adjustment to the provision established for storm damage by an oil, gas, or utility company.

+ References

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-Publisher FASB

-Name Accounting Standards Codification

-Topic 230

-SubTopic 10

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-URI http://asc.fasb.org/extlink&oid=31042434&loc=d3e3602-108585

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Balance Type:	debit
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X

- Definition

Adjustment for rate Increase.

+ References

No definition available.

+ Details

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Balance Type:	debit
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X

- Definition

Effective date of rates resulting from rate case.

+ References

No definition available.

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- Definition

Effective Date

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No definition available.

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- Definition

Grace Period for Issuing or Receiving Refunds

+ References

No definition available.

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- Definition

Increase in annual revenue allowed as a result of a rate case filing.

+ References

No definition available.

+ Details

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- Definition

Infrastructure Replacement Adjustment

+ References

No definition available.

+ Details

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- Definition

Represents the interest rate allowed by regulators on the unrecovered amount of storm costs which have been approved for recovery through rate adjustments.

+ References

No definition available.

+ Details

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- Definition

Percentage Of Revenue Increase.

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No definition available.

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- Definition

Settlement Dates.

+ References

No definition available.

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X

- Definition

Expenditures related to specific storms.

+ References

No definition available.

+ Details

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- Details

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- Details

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