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Details on Long Term Debt (Detail) (USD $) In Millions, unless otherwise specified | Dec. 31, 2014 | Dec. 31, 2013
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Debt Instrument [Line Items] |  |
Long-Term Debt | $ 332.9 us-gaap_LongTermDebt | $ 287.3 us-gaap_LongTermDebt
Less: Current Portion | 4.0 us-gaap_LongTermDebtCurrent | 2.5 us-gaap_LongTermDebtCurrent
Total Long-Term Debt, Less Current Portion | 328.9 us-gaap_LongTermDebtNoncurrent | 284.8 us-gaap_LongTermDebtNoncurrent
6.33% Notes, Due May 1, 2022 |  |
Debt Instrument [Line Items] |  |
Senior Notes | 20.0 us-gaap_SeniorNotes / us-gaap_DebtInstrumentAxis = utl_SeniorNotesSixPointThreeThreePercentDueMayOneTwentyTwentyTwoMember | 20.0 us-gaap_SeniorNotes / us-gaap_DebtInstrumentAxis = utl_SeniorNotesSixPointThreeThreePercentDueMayOneTwentyTwentyTwoMember
Unitil Energy Systems Inc | First Mortgage Bonds 5.24% Series, Due March 2, 2020 |  |
Debt Instrument [Line Items] |  |
Long-Term Debt | 15.0 us-gaap_LongTermDebt / us-gaap_DebtInstrumentAxis = utl_FirstMortgageBondsFivePointTwoFourPercentDueMarchTwoTwentyTwentyMember / dei_LegalEntityAxis = utl_UnitilEnergySystemsIncMember | 15.0 us-gaap_LongTermDebt / us-gaap_DebtInstrumentAxis = utl_FirstMortgageBondsFivePointTwoFourPercentDueMarchTwoTwentyTwentyMember / dei_LegalEntityAxis = utl_UnitilEnergySystemsIncMember
Unitil Energy Systems Inc | First Mortgage Bonds 8.49% Series, Due October 14, 2024 |  |
Debt Instrument [Line Items] |  |
Long-Term Debt | 15.0 us-gaap_LongTermDebt / us-gaap_DebtInstrumentAxis = utl_FirstMortgageBondsEightPointFourNinePercentDueOctoberFourteenTwentyTwentyFourMember / dei_LegalEntityAxis = utl_UnitilEnergySystemsIncMember | 15.0 us-gaap_LongTermDebt / us-gaap_DebtInstrumentAxis = utl_FirstMortgageBondsEightPointFourNinePercentDueOctoberFourteenTwentyTwentyFourMember / dei_LegalEntityAxis = utl_UnitilEnergySystemsIncMember
Unitil Energy Systems Inc | First Mortgage Bonds 6.96% Series, Due September 1, 2028 |  |
Debt Instrument [Line Items] |  |
Long-Term Debt | 20.0 us-gaap_LongTermDebt / us-gaap_DebtInstrumentAxis = utl_FirstMortgageBondsSixPointNineSixPercentDueSeptemberOneTwentyTwentyEightMember / dei_LegalEntityAxis = utl_UnitilEnergySystemsIncMember | 20.0 us-gaap_LongTermDebt / us-gaap_DebtInstrumentAxis = utl_FirstMortgageBondsSixPointNineSixPercentDueSeptemberOneTwentyTwentyEightMember / dei_LegalEntityAxis = utl_UnitilEnergySystemsIncMember
Unitil Energy Systems Inc | First Mortgage Bonds 8.00% Series, Due May 1, 2031 |  |
Debt Instrument [Line Items] |  |
Long-Term Debt | 15.0 us-gaap_LongTermDebt / us-gaap_DebtInstrumentAxis = utl_FirstMortgageBondsEightPointZeroZeroPercentDueMayOneTwentyThirtyOneMember / dei_LegalEntityAxis = utl_UnitilEnergySystemsIncMember | 15.0 us-gaap_LongTermDebt / us-gaap_DebtInstrumentAxis = utl_FirstMortgageBondsEightPointZeroZeroPercentDueMayOneTwentyThirtyOneMember / dei_LegalEntityAxis = utl_UnitilEnergySystemsIncMember
Unitil Energy Systems Inc | First Mortgage Bonds 6.32% Series, Due September 15, 2036 |  |
Debt Instrument [Line Items] |  |
Long-Term Debt | 15.0 us-gaap_LongTermDebt / us-gaap_DebtInstrumentAxis = utl_FirstMortgageBondsSixPointThreeTwoPercentDueSeptemberFifteenTwentyThirtySixMember / dei_LegalEntityAxis = utl_UnitilEnergySystemsIncMember | 15.0 us-gaap_LongTermDebt / us-gaap_DebtInstrumentAxis = utl_FirstMortgageBondsSixPointThreeTwoPercentDueSeptemberFifteenTwentyThirtySixMember / dei_LegalEntityAxis = utl_UnitilEnergySystemsIncMember
Fitchburg Gas and Electric Light Company | 6.75% Notes, Due November 30, 2023 |  |
Debt Instrument [Line Items] |  |
Long-Term Debt | 15.2 us-gaap_LongTermDebt / us-gaap_DebtInstrumentAxis = utl_NotesSixPointSevenFivePercentDueNovemberThirtyTwentyTwentyThreeMember / dei_LegalEntityAxis = utl_FitchburgGasAndElectricLightCompanyMember | 19.0 us-gaap_LongTermDebt / us-gaap_DebtInstrumentAxis = utl_NotesSixPointSevenFivePercentDueNovemberThirtyTwentyTwentyThreeMember / dei_LegalEntityAxis = utl_FitchburgGasAndElectricLightCompanyMember
Fitchburg Gas and Electric Light Company | 7.37% Notes, Due January 15, 2029 |  |
Debt Instrument [Line Items] |  |
Long-Term Debt | 12.0 us-gaap_LongTermDebt / us-gaap_DebtInstrumentAxis = utl_NotesSevenPointThreeSevenPercentDueJanuaryFifteenTwentyTwentyNineMember / dei_LegalEntityAxis = utl_FitchburgGasAndElectricLightCompanyMember | 12.0 us-gaap_LongTermDebt / us-gaap_DebtInstrumentAxis = utl_NotesSevenPointThreeSevenPercentDueJanuaryFifteenTwentyTwentyNineMember / dei_LegalEntityAxis = utl_FitchburgGasAndElectricLightCompanyMember
Fitchburg Gas and Electric Light Company | 7.98% Notes, Due June 1, 2031 |  |
Debt Instrument [Line Items] |  |
Long-Term Debt | 14.0 us-gaap_LongTermDebt / us-gaap_DebtInstrumentAxis = utl_NotesSevenPointNineEightPercentDueJuneOneTwentyThirtyOneMember / dei_LegalEntityAxis = utl_FitchburgGasAndElectricLightCompanyMember | 14.0 us-gaap_LongTermDebt / us-gaap_DebtInstrumentAxis = utl_NotesSevenPointNineEightPercentDueJuneOneTwentyThirtyOneMember / dei_LegalEntityAxis = utl_FitchburgGasAndElectricLightCompanyMember
Fitchburg Gas and Electric Light Company | 6.79% Notes, Due October 15, 2025 |  |
Debt Instrument [Line Items] |  |
Long-Term Debt | 10.0 us-gaap_LongTermDebt / us-gaap_DebtInstrumentAxis = utl_NotesSixPointSevenNinePercentDueOctoberFifteenTwentyTwentyFiveMember / dei_LegalEntityAxis = utl_FitchburgGasAndElectricLightCompanyMember | 10.0 us-gaap_LongTermDebt / us-gaap_DebtInstrumentAxis = utl_NotesSixPointSevenNinePercentDueOctoberFifteenTwentyTwentyFiveMember / dei_LegalEntityAxis = utl_FitchburgGasAndElectricLightCompanyMember
Fitchburg Gas and Electric Light Company | 5.90% Notes, Due December 15, 2030 |  |
Debt Instrument [Line Items] |  |
Long-Term Debt | 15.0 us-gaap_LongTermDebt / us-gaap_DebtInstrumentAxis = utl_NotesFivePointNineZeroPercentDueDecemberFifteenTwentyThirtyMember / dei_LegalEntityAxis = utl_FitchburgGasAndElectricLightCompanyMember | 15.0 us-gaap_LongTermDebt / us-gaap_DebtInstrumentAxis = utl_NotesFivePointNineZeroPercentDueDecemberFifteenTwentyThirtyMember / dei_LegalEntityAxis = utl_FitchburgGasAndElectricLightCompanyMember
Northern Utilities Inc | 6.95% Senior Notes, Series A, Due December 3, 2018 |  |
Debt Instrument [Line Items] |  |
Senior Notes | 30.0 us-gaap_SeniorNotes / us-gaap_DebtInstrumentAxis = utl_SeniorNotesSixPointNineFivePercentDueDecemberThreeTwentyEighteenMember / dei_LegalEntityAxis = utl_NorthernUtilitiesIncMember | 30.0 us-gaap_SeniorNotes / us-gaap_DebtInstrumentAxis = utl_SeniorNotesSixPointNineFivePercentDueDecemberThreeTwentyEighteenMember / dei_LegalEntityAxis = utl_NorthernUtilitiesIncMember
Northern Utilities Inc | 5.29% Senior Notes, Due March 2, 2020 |  |
Debt Instrument [Line Items] |  |
Senior Notes | 25.0 us-gaap_SeniorNotes / us-gaap_DebtInstrumentAxis = utl_SeniorNotesFivePointTwoNinePercentDueMarchTwoTwentyTwentyMember / dei_LegalEntityAxis = utl_NorthernUtilitiesIncMember | 25.0 us-gaap_SeniorNotes / us-gaap_DebtInstrumentAxis = utl_SeniorNotesFivePointTwoNinePercentDueMarchTwoTwentyTwentyMember / dei_LegalEntityAxis = utl_NorthernUtilitiesIncMember
Northern Utilities Inc | 7.72% Senior Notes, Series B, Due December 3, 2038 |  |
Debt Instrument [Line Items] |  |
Senior Notes | 50.0 us-gaap_SeniorNotes / us-gaap_DebtInstrumentAxis = utl_SeniorNotesSevenPointSevenTwoPercentDueDecemberThreeTwentyThirtyEightMember / dei_LegalEntityAxis = utl_NorthernUtilitiesIncMember | 50.0 us-gaap_SeniorNotes / us-gaap_DebtInstrumentAxis = utl_SeniorNotesSevenPointSevenTwoPercentDueDecemberThreeTwentyThirtyEightMember / dei_LegalEntityAxis = utl_NorthernUtilitiesIncMember
Northern Utilities Inc | 4.42% Senior Notes, Due October 15, 2044 |  |
Debt Instrument [Line Items] |  |
Senior Notes | 50.0 us-gaap_SeniorNotes / us-gaap_DebtInstrumentAxis = utl_SeniorNotesFourPointFourTwoPercentDueOctoberFifteenTwentyFortyFourMember / dei_LegalEntityAxis = utl_NorthernUtilitiesIncMember |
Granite State Gas Transmission Inc | 7.15% Senior Notes, Due December 15, 2018 |  |
Debt Instrument [Line Items] |  |
Senior Notes | 10.0 us-gaap_SeniorNotes / us-gaap_DebtInstrumentAxis = utl_SeniorNotesSevenPointOneFivePercentDueDecemberFifteenTwentyEighteenMember / dei_LegalEntityAxis = utl_GraniteStateGasTransmissionIncMember | 10.0 us-gaap_SeniorNotes / us-gaap_DebtInstrumentAxis = utl_SeniorNotesSevenPointOneFivePercentDueDecemberFifteenTwentyEighteenMember / dei_LegalEntityAxis = utl_GraniteStateGasTransmissionIncMember
Unitil Realty Corp | 8.00% Senior Secured Notes, Due August 1, 2017 |  |
Debt Instrument [Line Items] |  |
Senior Secured Notes | $ 1.7 us-gaap_SecuredDebt / us-gaap_DebtInstrumentAxis = utl_SeniorSecuredNotesEightPointZeroZeroPercentDueThroughAugustOneTwentySeventeenMember / dei_LegalEntityAxis = utl_UnitilRealtyCorpMember | $ 2.3 us-gaap_SecuredDebt / us-gaap_DebtInstrumentAxis = utl_SeniorSecuredNotesEightPointZeroZeroPercentDueThroughAugustOneTwentySeventeenMember / dei_LegalEntityAxis = utl_UnitilRealtyCorpMember

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- Definition Carrying amount of long-term debt, net of unamortized discount or premium, including current and noncurrent amounts. Includes, but not limited to, notes payable, bonds payable, debentures, mortgage loans and commercial paper. Excludes capital lease obligations. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 03 -Paragraph 16 -Article 7 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.5-02.22) -URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 942 -SubTopic 210 -Section S99 -Paragraph 1 -Subparagraph (SX 210.9-03.16) -URI http://asc.fasb.org/extlink&oid=6876686&loc=d3e534808-122878 Reference 4: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 03 -Paragraph 16 -Article 9 Reference 5: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 02 -Paragraph 20, 22 -Article 5 Reference 6: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 210 -Section S99 -Paragraph 1 -Subparagraph (SX 210.7-03.16) -URI http://asc.fasb.org/extlink&oid=6879938&loc=d3e572229-122910 + Details
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- Definition Amount of long-term debt, after unamortized discount or premium, scheduled to be repaid within one year or the normal operating cycle, if longer. Includes, but not limited to, notes payable, bonds payable, debentures, mortgage loans and commercial paper. Excludes capital lease obligations. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.5-02.19,20) -URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 02 -Paragraph 20 -Article 5 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 02 -Paragraph 19 -Article 5 + Details
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- Definition Carrying value as of the balance sheet date, including the current and noncurrent portions, of collateralized debt obligations (with maturities initially due after one year or beyond the operating cycle, if longer). Such obligations include mortgage loans, chattel loans, and any other borrowings secured by assets of the borrower. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 942 -SubTopic 210 -Section S99 -Paragraph 1 -Subparagraph (SX 210.9-03.16) -URI http://asc.fasb.org/extlink&oid=6876686&loc=d3e534808-122878 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 03 -Paragraph 16 -Article 9 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 02 -Paragraph 19, 20, 22 -Article 5 Reference 4: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 210 -Section S99 -Paragraph 1 -Subparagraph (SX 210.7-03.16) -URI http://asc.fasb.org/extlink&oid=6879938&loc=d3e572229-122910 + Details
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- Definition Including the current and noncurrent portions, carrying value as of the balance sheet date of Notes with the highest claim on the assets of the issuer in case of bankruptcy or liquidation (with maturities initially due after one year or beyond the operating cycle if longer). Senior note holders are paid off in full before any payments are made to junior note holders. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 942 -SubTopic 210 -Section S99 -Paragraph 1 -Subparagraph (SX 210.9-03.16) -URI http://asc.fasb.org/extlink&oid=6876686&loc=d3e534808-122878 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 210 -Section S99 -Paragraph 1 -Subparagraph (SX 210.7-03.16(a)) -URI http://asc.fasb.org/extlink&oid=6879938&loc=d3e572229-122910 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 03 -Paragraph 16 -Article 9 + Details
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