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Commitments and Contingencies - Additional Information (Detail) (USD $) | 12 Months Ended | 0 Months Ended | 3 Months Ended
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Dec. 31, 2014 | Dec. 31, 2013 | Apr. 29, 2014 | Apr. 21, 2014 | Dec. 27, 2013 | Nov. 26, 2014 | Apr. 30, 2014 | Apr. 25, 2013 | Jun. 27, 2014 | May 30, 2014 | Sep. 30, 2014
Regulatory Assets [Line Items] |  |  |  |  |  |  |  |  |  |  |
Regulatory Assets, Remaining Balance | $ 4,000,000 us-gaap_RemainingAmountsOfRegulatoryAssetsForWhichNoReturnOnInvestmentDuringRecoveryPeriodIsProvided |  |  |  |  |  |  |  |  |  |
Accrued Revenue | 48,500,000 us-gaap_UnbilledContractsReceivable | 56,600,000 us-gaap_UnbilledContractsReceivable |  |  |  |  |  |  |  |  |
Regulatory Assets | 107,600,000 us-gaap_RegulatoryAssetsNoncurrent | 100,100,000 us-gaap_RegulatoryAssetsNoncurrent |  |  |  |  |  |  |  |  |
Environmental obligations, spent during the year | 10,600,000 us-gaap_AccrualForEnvironmentalLossContingenciesPayments | 600,000 us-gaap_AccrualForEnvironmentalLossContingenciesPayments |  |  |  |  |  |  |  |  |
Maine | Environmental Restoration Costs |  |  |  |  |  |  |  |  |  |  |
Regulatory Assets [Line Items] |  |  |  |  |  |  |  |  |  |  |
Amortization period for environmental costs | 5 years |  |  |  |  |  |  |  |  |  |
New Hampshire | Environmental Restoration Costs |  |  |  |  |  |  |  |  |  |  |
Regulatory Assets [Line Items] |  |  |  |  |  |  |  |  |  |  |
Amortization period for environmental costs | 7 years |  |  |  |  |  |  |  |  |  |
Other Restructuring |  |  |  |  |  |  |  |  |  |  |
Regulatory Assets [Line Items] |  |  |  |  |  |  |  |  |  |  |
Accrued Revenue | 2,100,000 us-gaap_UnbilledContractsReceivable / us-gaap_RestructuringCostAndReserveAxis = us-gaap_OtherRestructuringMember |  |  |  |  |  |  |  |  |  |
Regulatory Assets | 1,900,000 us-gaap_RegulatoryAssetsNoncurrent / us-gaap_RestructuringCostAndReserveAxis = us-gaap_OtherRestructuringMember |  |  |  |  |  |  |  |  |  |
Cost recovery period, years | 7 years |  |  |  |  |  |  |  |  |  |
Environmental Matters |  |  |  |  |  |  |  |  |  |  |
Regulatory Assets [Line Items] |  |  |  |  |  |  |  |  |  |  |
Income (loss) incurred in excess of recorded amount | 0 utl_ChangeInNetIncomeLossDueToChangeInEstimates / us-gaap_LossContingenciesByNatureOfContingencyAxis = utl_EnvironmentalMattersMember |  |  |  |  |  |  |  |  |  |
Maximum |  |  |  |  |  |  |  |  |  |  |
Regulatory Assets [Line Items] |  |  |  |  |  |  |  |  |  |  |
Senior unsecured notes approved amount |  |  |  |  |  |  | 50,000,000 us-gaap_UnsecuredDebt / us-gaap_RangeAxis = us-gaap_MaximumMember |  |  |  |
Northern Utilities Inc |  |  |  |  |  |  |  |  |  |  |
Regulatory Assets [Line Items] |  |  |  |  |  |  |  |  |  |  |
Increase in annual revenue |  |  |  | 1,400,000 us-gaap_IncreaseDecreaseInRegulatoryClauseRevenue / dei_LegalEntityAxis = utl_NorthernUtilitiesIncMember | 1,000,000 us-gaap_IncreaseDecreaseInRegulatoryClauseRevenue / dei_LegalEntityAxis = utl_NorthernUtilitiesIncMember |  |  |  |  |  |
Amendment effective date | May 1, 2014 |  |  |  |  |  |  |  |  |  |
TIRA initial term | Four years |  |  |  |  |  |  |  |  |  |
First annual TIRA Adjustment |  |  | 1,300,000 utl_InfrastructureReplacementAdjustment / dei_LegalEntityAxis = utl_NorthernUtilitiesIncMember |  |  |  |  |  |  |  |
First annual TIRA Adjustment, effective date |  |  |  |  | May 1, 2014 |  |  |  |  |  |
Environmental obligations, spent during the year | 500,000 us-gaap_AccrualForEnvironmentalLossContingenciesPayments / dei_LegalEntityAxis = utl_NorthernUtilitiesIncMember | 400,000 us-gaap_AccrualForEnvironmentalLossContingenciesPayments / dei_LegalEntityAxis = utl_NorthernUtilitiesIncMember |  |  |  |  |  |  |  |  |
Northern Utilities Inc | Maine |  |  |  |  |  |  |  |  |  |  |
Regulatory Assets [Line Items] |  |  |  |  |  |  |  |  |  |  |
Increase in annual revenue |  |  |  |  | 3,800,000 us-gaap_IncreaseDecreaseInRegulatoryClauseRevenue / dei_LegalEntityAxis = utl_NorthernUtilitiesIncMember / us-gaap_StatementGeographicalAxis = stpr_ME |  |  |  |  |  |
Amendment effective date | January 1, 2014 |  |  |  |  |  |  |  |  |  |
Northern Utilities Inc | New Hampshire |  |  |  |  |  |  |  |  |  |  |
Regulatory Assets [Line Items] |  |  |  |  |  |  |  |  |  |  |
Increase in annual revenue |  |  |  | 1,400,000 us-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 Assets [Line Items] |  |  |  |  |  |  |  |  |  |  |
Increase in annual revenue |  |  |  | 4,600,000 us-gaap_IncreaseDecreaseInRegulatoryClauseRevenue / dei_LegalEntityAxis = utl_NorthernUtilitiesIncMember / us-gaap_PublicUtilityAxis = us-gaap_GasDistributionMember / us-gaap_StatementGeographicalAxis = stpr_NH |  |  |  |  |  |  |
Unitil Energy Systems Inc |  |  |  |  |  |  |  |  |  |  |
Regulatory Assets [Line Items] |  |  |  |  |  |  |  |  |  |  |
Amendment effective date | May 1, 2014 |  |  |  |  |  |  |  |  |  |
Adjustment to recover the increased spending for vegetation management program and reliability enhancement program and a proposed storm resiliency program |  |  |  |  |  |  | 1,500,000 utl_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 |  |  |  |  |  | 100,000 us-gaap_ConstructionInProgressExpendituresIncurredButNotYetPaid / dei_LegalEntityAxis = utl_UnitilEnergySystemsIncMember |  |  |  |  |
Recovery of damage of electrical system |  |  |  |  |  | 2,000,000 us-gaap_StormDamageProvision / dei_LegalEntityAxis = utl_UnitilEnergySystemsIncMember |  |  |  |  |
Unitil Energy Systems Inc | Storm Costs |  |  |  |  |  |  |  |  |  |  |
Regulatory Assets [Line Items] |  |  |  |  |  |  |  |  |  |  |
Cost recovery period, years |  |  |  |  |  |  |  | 5 years |  |  |
Recovery amount |  |  |  |  |  |  |  | 2,300,000 us-gaap_AssetRecoveryDamagedPropertyCostsNoncurrent / dei_LegalEntityAxis = utl_UnitilEnergySystemsIncMember / us-gaap_RegulatoryAssetAxis = us-gaap_StormCostsMember |  |  |
Storm expenditures |  |  |  |  |  | 2,100,000 utl_StormExpenditures / dei_LegalEntityAxis = utl_UnitilEnergySystemsIncMember / us-gaap_RegulatoryAssetAxis = us-gaap_StormCostsMember |  |  |  |  |
Granite State Gas Transmission Inc | Gas Transportation and Storage |  |  |  |  |  |  |  |  |  |  |
Regulatory Assets [Line Items] |  |  |  |  |  |  |  |  |  |  |
Amendment effective date | August 1, 2014 |  |  |  |  |  |  |  |  |  |
Annual revenue increase in rate adjustments |  |  |  |  |  |  |  |  | 600,000 utl_IncreaseDecreaseInUnearnedRevenue / dei_LegalEntityAxis = utl_GraniteStateGasTransmissionIncMember / us-gaap_PublicUtilityAxis = utl_GasTransportationAndStorageMember |  |
Fitchburg Gas and Electric Light Company |  |  |  |  |  |  |  |  |  |  |
Regulatory Assets [Line Items] |  |  |  |  |  |  |  |  |  |  |
Cost recovery period, years |  |  |  |  |  |  |  |  |  | 5 years |
Increase in annual revenue |  |  |  |  |  |  |  |  |  | 5,600,000 us-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 |
Annual funding amount approved |  |  |  |  |  |  |  |  |  | 500,000 us-gaap_PublicUtilitiesAllowanceForFundsUsedDuringConstructionAdditions / dei_LegalEntityAxis = utl_FitchburgGasAndElectricLightCompanyMember |
Recovery amount | 300,000 us-gaap_AssetRecoveryDamagedPropertyCostsNoncurrent / dei_LegalEntityAxis = utl_FitchburgGasAndElectricLightCompanyMember |  |  |  |  |  |  |  |  | 900,000 us-gaap_AssetRecoveryDamagedPropertyCostsNoncurrent / dei_LegalEntityAxis = utl_FitchburgGasAndElectricLightCompanyMember |
Storm expenditures |  |  |  |  |  | 200,000 utl_StormExpenditures / dei_LegalEntityAxis = utl_FitchburgGasAndElectricLightCompanyMember |  |  |  |  |
Construction expenditures |  |  |  |  |  | 100,000 us-gaap_ConstructionInProgressExpendituresIncurredButNotYetPaid / dei_LegalEntityAxis = utl_FitchburgGasAndElectricLightCompanyMember |  |  |  |  |
Environmental obligations, spent during the year | 10,100,000 us-gaap_AccrualForEnvironmentalLossContingenciesPayments / dei_LegalEntityAxis = utl_FitchburgGasAndElectricLightCompanyMember | 200,000 us-gaap_AccrualForEnvironmentalLossContingenciesPayments / dei_LegalEntityAxis = utl_FitchburgGasAndElectricLightCompanyMember |  |  |  |  |  |  |  |  |
Fitchburg Gas and Electric Light Company | Environmental Restoration Costs |  |  |  |  |  |  |  |  |  |  |
Regulatory Assets [Line Items] |  |  |  |  |  |  |  |  |  |  |
Amortization period for environmental costs |  |  |  |  |  |  |  |  |  |  | 7 years
Fitchburg Gas and Electric Light Company | Storm Costs |  |  |  |  |  |  |  |  |  |  |
Regulatory Assets [Line Items] |  |  |  |  |  |  |  |  |  |  |
Cost recovery period, years |  |  |  |  |  |  |  |  |  | 3 years |
Deferred emergency storm repair costs incurred |  |  |  |  |  |  |  |  |  | 5,000,000 us-gaap_DeferredStormAndPropertyReserveDeficiencyNoncurrent / dei_LegalEntityAxis = utl_FitchburgGasAndElectricLightCompanyMember / us-gaap_RegulatoryAssetAxis = us-gaap_StormCostsMember |
Storm expenditures |  |  |  |  |  | 300,000 utl_StormExpenditures / dei_LegalEntityAxis = utl_FitchburgGasAndElectricLightCompanyMember / us-gaap_RegulatoryAssetAxis = us-gaap_StormCostsMember |  |  |  | 5,000,000 utl_StormExpenditures / dei_LegalEntityAxis = utl_FitchburgGasAndElectricLightCompanyMember / us-gaap_RegulatoryAssetAxis = us-gaap_StormCostsMember |
Fitchburg Gas and Electric Light Company | Environmental Clean Up Costs |  |  |  |  |  |  |  |  |  |  |
Regulatory Assets [Line Items] |  |  |  |  |  |  |  |  |  |  |
Environmental obligations |  |  |  |  |  |  |  |  |  |  | 12,000,000 us-gaap_AccrualForEnvironmentalLossContingencies / dei_LegalEntityAxis = utl_FitchburgGasAndElectricLightCompanyMember / us-gaap_RegulatoryAssetAxis = utl_EnvironmentalCleanUpCostsMember
Environmental obligations, spent during the year |  |  |  |  |  |  |  |  |  |  | 3,600,000 us-gaap_AccrualForEnvironmentalLossContingenciesPayments / dei_LegalEntityAxis = utl_FitchburgGasAndElectricLightCompanyMember / us-gaap_RegulatoryAssetAxis = utl_EnvironmentalCleanUpCostsMember
Fitchburg Gas and Electric Light Company | Revised Estimate | Environmental Clean Up Costs |  |  |  |  |  |  |  |  |  |  |
Regulatory Assets [Line Items] |  |  |  |  |  |  |  |  |  |  |
Environmental obligations |  |  |  |  |  |  |  |  |  |  | 5,500,000 us-gaap_AccrualForEnvironmentalLossContingencies / dei_LegalEntityAxis = utl_FitchburgGasAndElectricLightCompanyMember / us-gaap_LossContingenciesByNatureOfContingencyAxis = utl_RevisedMember / us-gaap_RegulatoryAssetAxis = utl_EnvironmentalCleanUpCostsMember

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- 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 + Details
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- Definition Future cash outflow to pay for construction in progress expenditures that have occurred. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 230 -SubTopic 10 -Section 50 -Paragraph 5 -URI http://asc.fasb.org/extlink&oid=6367179&loc=d3e4332-108586 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 230 -SubTopic 10 -Section 50 -Paragraph 4 -URI http://asc.fasb.org/extlink&oid=6367179&loc=d3e4313-108586 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 230 -SubTopic 10 -Section 50 -Paragraph 3 -URI http://asc.fasb.org/extlink&oid=6367179&loc=d3e4304-108586 + Details
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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 Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 230 -SubTopic 10 -Section 45 -Paragraph 28 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=31042434&loc=d3e3602-108585 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 980 -SubTopic 360 -Section 25 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=6500269&loc=d3e45485-110386 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 980 -SubTopic 340 -Section 50 -Paragraph 3 -URI http://asc.fasb.org/extlink&oid=6499975&loc=d3e44278-110382 + Details
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