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Commitments and Contingencies - Additional Information (Detail) (USD $)	12 Months Ended		0 Months Ended								3 Months Ended
	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	$ 4,000,000us-gaap_RemainingAmountsOfRegulatoryAssetsForWhichNoReturnOnInvestmentDuringRecoveryPeriodIsProvided
Accrued Revenue	48,500,000us-gaap_UnbilledContractsReceivable	56,600,000us-gaap_UnbilledContractsReceivable
Regulatory Assets	107,600,000us-gaap_RegulatoryAssetsNoncurrent	100,100,000us-gaap_RegulatoryAssetsNoncurrent
Environmental obligations, spent during the year	10,600,000us-gaap_AccrualForEnvironmentalLossContingenciesPayments	600,000us-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	2,100,000us-gaap_UnbilledContractsReceivable / us-gaap_RestructuringCostAndReserveAxis = us-gaap_OtherRestructuringMember
Regulatory Assets	1,900,000us-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	0utl_ChangeInNetIncomeLossDueToChangeInEstimates / us-gaap_LossContingenciesByNatureOfContingencyAxis = utl_EnvironmentalMattersMember
Maximum
Regulatory Assets [Line Items]
Senior unsecured notes approved amount							50,000,000us-gaap_UnsecuredDebt / us-gaap_RangeAxis = us-gaap_MaximumMember
Northern Utilities Inc
Regulatory Assets [Line Items]
Increase in annual revenue				1,400,000us-gaap_IncreaseDecreaseInRegulatoryClauseRevenue / dei_LegalEntityAxis = utl_NorthernUtilitiesIncMember	1,000,000us-gaap_IncreaseDecreaseInRegulatoryClauseRevenue / dei_LegalEntityAxis = utl_NorthernUtilitiesIncMember
Amendment effective date	May 1, 2014
TIRA initial term	Four years
First annual TIRA Adjustment			1,300,000utl_InfrastructureReplacementAdjustment / dei_LegalEntityAxis = utl_NorthernUtilitiesIncMember
First annual TIRA Adjustment, effective date					May 1, 2014
Environmental obligations, spent during the year	500,000us-gaap_AccrualForEnvironmentalLossContingenciesPayments / dei_LegalEntityAxis = utl_NorthernUtilitiesIncMember	400,000us-gaap_AccrualForEnvironmentalLossContingenciesPayments / dei_LegalEntityAxis = utl_NorthernUtilitiesIncMember
Northern Utilities Inc | Maine
Regulatory Assets [Line Items]
Increase in annual revenue					3,800,000us-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				1,400,000us-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				4,600,000us-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							1,500,000utl_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						100,000us-gaap_ConstructionInProgressExpendituresIncurredButNotYetPaid / dei_LegalEntityAxis = utl_UnitilEnergySystemsIncMember
Recovery of damage of electrical system						2,000,000us-gaap_StormDamageProvision / dei_LegalEntityAxis = utl_UnitilEnergySystemsIncMember
Unitil Energy Systems Inc | Storm Costs
Regulatory Assets [Line Items]
Cost recovery period, years								5 years
Recovery amount								2,300,000us-gaap_AssetRecoveryDamagedPropertyCostsNoncurrent / dei_LegalEntityAxis = utl_UnitilEnergySystemsIncMember / us-gaap_RegulatoryAssetAxis = us-gaap_StormCostsMember
Storm expenditures						2,100,000utl_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									600,000utl_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										5,600,000us-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										500,000us-gaap_PublicUtilitiesAllowanceForFundsUsedDuringConstructionAdditions / dei_LegalEntityAxis = utl_FitchburgGasAndElectricLightCompanyMember
Recovery amount	300,000us-gaap_AssetRecoveryDamagedPropertyCostsNoncurrent / dei_LegalEntityAxis = utl_FitchburgGasAndElectricLightCompanyMember									900,000us-gaap_AssetRecoveryDamagedPropertyCostsNoncurrent / dei_LegalEntityAxis = utl_FitchburgGasAndElectricLightCompanyMember
Storm expenditures						200,000utl_StormExpenditures / dei_LegalEntityAxis = utl_FitchburgGasAndElectricLightCompanyMember
Construction expenditures						100,000us-gaap_ConstructionInProgressExpendituresIncurredButNotYetPaid / dei_LegalEntityAxis = utl_FitchburgGasAndElectricLightCompanyMember
Environmental obligations, spent during the year	10,100,000us-gaap_AccrualForEnvironmentalLossContingenciesPayments / dei_LegalEntityAxis = utl_FitchburgGasAndElectricLightCompanyMember	200,000us-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										5,000,000us-gaap_DeferredStormAndPropertyReserveDeficiencyNoncurrent / dei_LegalEntityAxis = utl_FitchburgGasAndElectricLightCompanyMember / us-gaap_RegulatoryAssetAxis = us-gaap_StormCostsMember
Storm expenditures						300,000utl_StormExpenditures / dei_LegalEntityAxis = utl_FitchburgGasAndElectricLightCompanyMember / us-gaap_RegulatoryAssetAxis = us-gaap_StormCostsMember				5,000,000utl_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											12,000,000us-gaap_AccrualForEnvironmentalLossContingencies / dei_LegalEntityAxis = utl_FitchburgGasAndElectricLightCompanyMember / us-gaap_RegulatoryAssetAxis = utl_EnvironmentalCleanUpCostsMember
Environmental obligations, spent during the year											3,600,000us-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											5,500,000us-gaap_AccrualForEnvironmentalLossContingencies / dei_LegalEntityAxis = utl_FitchburgGasAndElectricLightCompanyMember / us-gaap_LossContingenciesByNatureOfContingencyAxis = utl_RevisedMember / us-gaap_RegulatoryAssetAxis = utl_EnvironmentalCleanUpCostsMember

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