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Debt and Financing Arrangements - Additional Information (Detail) (USD $) | 12 Months Ended | 0 Months Ended |  |
--- | --- | --- | --- | ---
Dec. 31, 2014 | Dec. 31, 2013 | Dec. 31, 2012 | Oct. 04, 2013 | Apr. 30, 2014 | Oct. 15, 2014
Line of Credit Facility [Line Items] |  |  |  |  |  |
Long term debt repayments | $ 4,400,000 us-gaap_RepaymentsOfLongTermDebt | $ 500,000 us-gaap_RepaymentsOfLongTermDebt | $ 500,000 us-gaap_RepaymentsOfLongTermDebt |  |  |
Weighted average interest rate on short term borrowings | 1.60%us-gaap_ShortTermDebtWeightedAverageInterestRate | 1.80%us-gaap_ShortTermDebtWeightedAverageInterestRate | 2.00%us-gaap_ShortTermDebtWeightedAverageInterestRate |  |  |
Standard & Poor's Ratings Services, Credit rating | BBB+ |  |  |  |  |
Proceeds from capital lease obligation | 6,900,000 us-gaap_ProceedsFromLongTermCapitalLeaseObligations |  |  |  |  |
Accounts Payable | 44,200,000 us-gaap_AccountsPayableCurrent | 38,100,000 us-gaap_AccountsPayableCurrent |  |  |  |
Total rental expense under operating leases | 1,300,000 us-gaap_LeaseAndRentalExpense | 1,200,000 us-gaap_LeaseAndRentalExpense | 1,300,000 us-gaap_LeaseAndRentalExpense |  |  |
Net Utility Plant | 733,700,000 us-gaap_PublicUtilitiesPropertyPlantAndEquipmentNet | 665,600,000 us-gaap_PublicUtilitiesPropertyPlantAndEquipmentNet |  |  |  |
Future minimum capital lease payments due current | 471,000 us-gaap_CapitalLeasesFutureMinimumPaymentsDueCurrent |  |  |  |  |
Future minimum capital lease payments due noncurrent | 7,500,000 utl_OperatingAndCapitalLeasesFutureMinimumPaymentsDue |  |  |  |  |
Guarantee outstanding | 39,100,000 utl_LetterOfCreditFacilityAndBankGuaranteesOutstanding |  |  |  |  |
Capital Lease Obligations |  |  |  |  |  |
Line of Credit Facility [Line Items] |  |  |  |  |  |
Revolving credit facility |  |  |  |  | 15,000,000 us-gaap_LineOfCreditFacilityMaximumBorrowingCapacity / us-gaap_LongtermDebtTypeAxis = us-gaap_CapitalLeaseObligationsMember |
Assets under Capital Leases |  |  |  |  |  |
Line of Credit Facility [Line Items] |  |  |  |  |  |
Net Utility Plant | 9,700,000 us-gaap_PublicUtilitiesPropertyPlantAndEquipmentNet / us-gaap_PropertyPlantAndEquipmentByTypeAxis = us-gaap_AssetsHeldUnderCapitalLeasesMember | 1,400,000 us-gaap_PublicUtilitiesPropertyPlantAndEquipmentNet / us-gaap_PropertyPlantAndEquipmentByTypeAxis = us-gaap_AssetsHeldUnderCapitalLeasesMember |  |  |  |
Net Utility Plant, accumulated amortization | 800,000 us-gaap_AccumulatedDepreciationDepletionAndAmortizationPropertyPlantAndEquipment / us-gaap_PropertyPlantAndEquipmentByTypeAxis = us-gaap_AssetsHeldUnderCapitalLeasesMember | 800,000 us-gaap_AccumulatedDepreciationDepletionAndAmortizationPropertyPlantAndEquipment / us-gaap_PropertyPlantAndEquipmentByTypeAxis = us-gaap_AssetsHeldUnderCapitalLeasesMember |  |  |  |
Unitil Corporation | Maximum |  |  |  |  |  |
Line of Credit Facility [Line Items] |  |  |  |  |  |
Total funded indebtedness as percentage of capitalization | 70.00%utl_PercentageOfIndebtednessToNetCapital / dei_LegalEntityAxis = utl_UnitilCorporationMember / us-gaap_RangeAxis = us-gaap_MaximumMember |  |  |  |  |
Fitchburg Gas and Electric Light Company |  |  |  |  |  |
Line of Credit Facility [Line Items] |  |  |  |  |  |
Total funded indebtedness as percentage of capitalization | 65.00%utl_PercentageOfIndebtednessToNetCapital / dei_LegalEntityAxis = utl_FitchburgGasAndElectricLightCompanyMember |  |  |  |  |
Northern Utilities Inc |  |  |  |  |  |
Line of Credit Facility [Line Items] |  |  |  |  |  |
Total funded indebtedness as percentage of capitalization | 65.00%utl_PercentageOfIndebtednessToNetCapital / dei_LegalEntityAxis = utl_NorthernUtilitiesIncMember |  |  |  |  |
Natural gas storage inventory | 15,100,000 us-gaap_EnergyRelatedInventoryNaturalGasInStorage / dei_LegalEntityAxis = utl_NorthernUtilitiesIncMember | 12,500,000 us-gaap_EnergyRelatedInventoryNaturalGasInStorage / dei_LegalEntityAxis = utl_NorthernUtilitiesIncMember |  |  |  |
Accounts Payable | 1,000,000 us-gaap_AccountsPayableCurrent / dei_LegalEntityAxis = utl_NorthernUtilitiesIncMember | 2,700,000 us-gaap_AccountsPayableCurrent / dei_LegalEntityAxis = utl_NorthernUtilitiesIncMember |  |  |  |
Granite State Gas Transmission Inc |  |  |  |  |  |
Line of Credit Facility [Line Items] |  |  |  |  |  |
Total funded indebtedness as percentage of capitalization | 65.00%utl_PercentageOfIndebtednessToNetCapital / dei_LegalEntityAxis = utl_GraniteStateGasTransmissionIncMember |  |  |  |  |
Master Lease Agreement |  |  |  |  |  |
Line of Credit Facility [Line Items] |  |  |  |  |  |
Future minimum capital lease payments due noncurrent | 6,900,000 utl_OperatingAndCapitalLeasesFutureMinimumPaymentsDue / us-gaap_LeaseArrangementTypeAxis = utl_MasterLeaseAgreementsMember |  |  |  |  |
Revolving Credit Facility |  |  |  |  |  |
Line of Credit Facility [Line Items] |  |  |  |  |  |
Revolving credit facility expiration date |  |  |  | Oct. 04, 2018 |  |
Revolving credit facility | 120,000,000 us-gaap_LineOfCreditFacilityMaximumBorrowingCapacity / us-gaap_CreditFacilityAxis = us-gaap_RevolvingCreditFacilityMember | 120,000,000 us-gaap_LineOfCreditFacilityMaximumBorrowingCapacity / us-gaap_CreditFacilityAxis = us-gaap_RevolvingCreditFacilityMember |  | 120,000,000 us-gaap_LineOfCreditFacilityMaximumBorrowingCapacity / us-gaap_CreditFacilityAxis = us-gaap_RevolvingCreditFacilityMember |  |
Sublimit for the issuance of standby letters of credit |  |  |  | 25,000,000 utl_SublimitForIssuanceOfStandbyLettersOfCredit / us-gaap_CreditFacilityAxis = us-gaap_RevolvingCreditFacilityMember |  |
Credit Facility by an aggregate additional amount |  |  |  | 30,000,000 utl_LineOfCreditFacilityMaximumIncreaseToAggregateCommitments / us-gaap_CreditFacilityAxis = us-gaap_RevolvingCreditFacilityMember |  |
Percentage of capitalization | The affirmative and negative covenants under the Credit Facility shall apply to Unitil until the Credit Facility terminates and all amounts borrowed under the Credit Facility are paid in full (or with respect to letters of credit, they are cash collateralized). The only financial covenant in the Credit Facility provides that Unitil's Funded Debt to Capitalization (as each term is defined in the Credit Facility) cannot exceed 65% tested on a quarterly basis. At September 30, 2014, the Company was in compliance with the covenants contained in the Credit Facility in effect on that date. |  |  |  |  |
Revolving Credit Facility | London Interbank Offered Rate |  |  |  |  |  |
Line of Credit Facility [Line Items] |  |  |  |  |  |
Credit facility, daily fluctuating rate of interest |  |  |  | 1.375%us-gaap_DebtInstrumentBasisSpreadOnVariableRate1 / us-gaap_CreditFacilityAxis = us-gaap_RevolvingCreditFacilityMember / us-gaap_VariableRateAxis = us-gaap_LondonInterbankOfferedRateLIBORMember |  |
Bonds |  |  |  |  |  |
Line of Credit Facility [Line Items] |  |  |  |  |  |
Long term debt repayments | 4,386,919 us-gaap_RepaymentsOfLongTermDebt / us-gaap_DebtInstrumentAxis = us-gaap_BondsMember | 541,938 us-gaap_RepaymentsOfLongTermDebt / us-gaap_DebtInstrumentAxis = us-gaap_BondsMember | 500,405 us-gaap_RepaymentsOfLongTermDebt / us-gaap_DebtInstrumentAxis = us-gaap_BondsMember |  |  |
Debt repayment, 2015 | 4,035,633 us-gaap_LongTermDebtMaturitiesRepaymentsOfPrincipalInNextTwelveMonths / us-gaap_DebtInstrumentAxis = us-gaap_BondsMember |  |  |  |  |
Debt repayment, 2016 | 17,421,724 us-gaap_LongTermDebtMaturitiesRepaymentsOfPrincipalInYearTwo / us-gaap_DebtInstrumentAxis = us-gaap_BondsMember |  |  |  |  |
Debt repayment, 2017 | 17,160,985 us-gaap_LongTermDebtMaturitiesRepaymentsOfPrincipalInYearThree / us-gaap_DebtInstrumentAxis = us-gaap_BondsMember |  |  |  |  |
Debt repayment, 2018 | 30,133,332 us-gaap_LongTermDebtMaturitiesRepaymentsOfPrincipalInYearFour / us-gaap_DebtInstrumentAxis = us-gaap_BondsMember |  |  |  |  |
Debt repayment, 2019 | 18,800,000 us-gaap_LongTermDebtMaturitiesRepaymentsOfPrincipalInYearFive / us-gaap_DebtInstrumentAxis = us-gaap_BondsMember |  |  |  |  |
Debt repayment, Thereafter | 245,400,000 us-gaap_LongTermDebtMaturitiesRepaymentsOfPrincipalAfterYearFive / us-gaap_DebtInstrumentAxis = us-gaap_BondsMember |  |  |  |  |
4.42% Senior Unsecured Notes due October 15, 2044 | Northern Utilities Inc |  |  |  |  |  |
Line of Credit Facility [Line Items] |  |  |  |  |  |
Long-term debt, aggregate principal amount |  |  |  |  |  | 50,000,000 us-gaap_DebtInstrumentFaceAmount / us-gaap_DebtInstrumentAxis = utl_SeniorUnsecuredNotesFourPointFourTwoPercentDueOctoberFifteenTwentyFortyFourMember / dei_LegalEntityAxis = utl_NorthernUtilitiesIncMember
Long-term debt, stated interest rate |  |  |  |  |  | 4.42%us-gaap_DebtInstrumentInterestRateStatedPercentage / us-gaap_DebtInstrumentAxis = utl_SeniorUnsecuredNotesFourPointFourTwoPercentDueOctoberFifteenTwentyFortyFourMember / dei_LegalEntityAxis = utl_NorthernUtilitiesIncMember
Long-term debt, maturity date | Oct. 15, 2044 |  |  |  |  |
8.00% Senior Secured Notes, Due August 1, 2017 | Unitil Realty Corp |  |  |  |  |  |
Line of Credit Facility [Line Items] |  |  |  |  |  |
Long-term debt, stated interest rate | 8.00%us-gaap_DebtInstrumentInterestRateStatedPercentage / us-gaap_DebtInstrumentAxis = utl_SeniorSecuredNotesEightPointZeroZeroPercentDueThroughAugustOneTwentySeventeenMember / dei_LegalEntityAxis = utl_UnitilRealtyCorpMember | 8.00%us-gaap_DebtInstrumentInterestRateStatedPercentage / us-gaap_DebtInstrumentAxis = utl_SeniorSecuredNotesEightPointZeroZeroPercentDueThroughAugustOneTwentySeventeenMember / dei_LegalEntityAxis = utl_UnitilRealtyCorpMember |  |  |  |
Senior Secured Notes | 1,700,000 us-gaap_SecuredDebt / us-gaap_DebtInstrumentAxis = utl_SeniorSecuredNotesEightPointZeroZeroPercentDueThroughAugustOneTwentySeventeenMember / dei_LegalEntityAxis = utl_UnitilRealtyCorpMember | 2,300,000 us-gaap_SecuredDebt / us-gaap_DebtInstrumentAxis = utl_SeniorSecuredNotesEightPointZeroZeroPercentDueThroughAugustOneTwentySeventeenMember / dei_LegalEntityAxis = utl_UnitilRealtyCorpMember |  |  |  |
8.00% Senior Secured Notes, Due August 1, 2017 | Unitil Realty Corp | Debt Outstanding, Principal Amount |  |  |  |  |  |
Line of Credit Facility [Line Items] |  |  |  |  |  |
Senior Secured Notes | 1,700,000 us-gaap_SecuredDebt / us-gaap_DebtInstrumentAxis = utl_SeniorSecuredNotesEightPointZeroZeroPercentDueThroughAugustOneTwentySeventeenMember / utl_DebtTypeAxis = utl_DebtOutstandingPrincipalAmountMember / dei_LegalEntityAxis = utl_UnitilRealtyCorpMember |  |  |  |  |
7.15% Senior Notes, Due December 15, 2018 | Granite State Gas Transmission Inc |  |  |  |  |  |
Line of Credit Facility [Line Items] |  |  |  |  |  |
Long-term debt, stated interest rate | 7.15%us-gaap_DebtInstrumentInterestRateStatedPercentage / us-gaap_DebtInstrumentAxis = utl_SeniorNotesSevenPointOneFivePercentDueDecemberFifteenTwentyEighteenMember / dei_LegalEntityAxis = utl_GraniteStateGasTransmissionIncMember | 7.15%us-gaap_DebtInstrumentInterestRateStatedPercentage / us-gaap_DebtInstrumentAxis = utl_SeniorNotesSevenPointOneFivePercentDueDecemberFifteenTwentyEighteenMember / dei_LegalEntityAxis = utl_GraniteStateGasTransmissionIncMember |  |  |  |
Senior Notes | 10,000,000 us-gaap_SeniorNotes / us-gaap_DebtInstrumentAxis = utl_SeniorNotesSevenPointOneFivePercentDueDecemberFifteenTwentyEighteenMember / dei_LegalEntityAxis = utl_GraniteStateGasTransmissionIncMember | 10,000,000 us-gaap_SeniorNotes / us-gaap_DebtInstrumentAxis = utl_SeniorNotesSevenPointOneFivePercentDueDecemberFifteenTwentyEighteenMember / dei_LegalEntityAxis = utl_GraniteStateGasTransmissionIncMember |  |  |  |
7.15% Senior Notes, Due December 15, 2018 | Granite State Gas Transmission Inc | Debt Outstanding, Principal Amount |  |  |  |  |  |
Line of Credit Facility [Line Items] |  |  |  |  |  |
Senior Notes | $ 10,000,000 us-gaap_SeniorNotes / us-gaap_DebtInstrumentAxis = utl_SeniorNotesSevenPointOneFivePercentDueDecemberFifteenTwentyEighteenMember / utl_DebtTypeAxis = utl_DebtOutstandingPrincipalAmountMember / dei_LegalEntityAxis = utl_GraniteStateGasTransmissionIncMember |  |  |  |  |

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