XML 50 R41.htm IDEA: XBRL DOCUMENT
v3.25.2

DEBT AND FINANCING ARRANGEMENTS - Details On Long Term Debt (Parenthetical) (Detail) | 6 Months Ended
--- | ---
Jun. 30, 2025
3.70% Senior Notes, Due August 1, 2026 |
Debt Instrument [Line Items] |
Stated interest rate | 3.70%
Debt instrument due date | Aug. 01, 2026
3.43% Senior Notes, Due December 18, 2029 |
Debt Instrument [Line Items] |
Stated interest rate | 3.43%
Debt instrument due date | Dec. 18, 2029
5.99% Senior Notes, Due August 21, 2034 |
Debt Instrument [Line Items] |
Stated interest rate | 5.99%
Debt instrument due date | Aug. 21, 2034
First Mortgage Bonds 6.96% Senior Secured Notes, Due September 1, 2028 | Unitil Energy Systems Inc [Member] |
Debt Instrument [Line Items] |
Stated interest rate | 6.96%
Debt instrument due date | Sep. 01, 2028
First Mortgage Bonds 8.00% Senior Secured Notes, Due May 1, 2031 | Unitil Energy Systems Inc [Member] |
Debt Instrument [Line Items] |
Stated interest rate | 8.00%
Debt instrument due date | May 01, 2031
First Mortgage Bonds 6.32% Senior Secured Notes, Due September 15, 2036 | Unitil Energy Systems Inc [Member] |
Debt Instrument [Line Items] |
Stated interest rate | 6.32%
Debt instrument due date | Sep. 15, 2036
3.58% Senior Secured Notes, Due September 15, 2040 | Unitil Energy Systems Inc [Member] |
Debt Instrument [Line Items] |
Stated interest rate | 3.58%
Debt instrument due date | Sep. 15, 2040
First Mortgage Bonds 4.18% Senior Secured Notes Due November 30, 2048 | Unitil Energy Systems Inc [Member] |
Debt Instrument [Line Items] |
Stated interest rate | 4.18%
Debt instrument due date | Nov. 30, 2048
5.69% Senior Secured Notes, Due August 21, 2054 | Unitil Energy Systems Inc [Member] |
Debt Instrument [Line Items] |
Stated interest rate | 5.69%
Debt instrument due date | Aug. 21, 2054
3.52% Senior Notes, Due November 1, 2027 | Fitchburg Gas And Electric Light Company [Member] |
Debt Instrument [Line Items] |
Stated interest rate | 3.52%
Debt instrument due date | Nov. 01, 2027
3.52% Senior Notes, Due November 1, 2027 | Northern Utilities Inc [Member] |
Debt Instrument [Line Items] |
Stated interest rate | 3.52%
Debt instrument due date | Nov. 01, 2027
7.37% Notes, Due January 15, 2029 | Fitchburg Gas And Electric Light Company [Member] |
Debt Instrument [Line Items] |
Stated interest rate | 7.37%
Debt instrument due date | Jan. 15, 2029
5.90% Notes, Due December 15, 2030 | Fitchburg Gas And Electric Light Company [Member] |
Debt Instrument [Line Items] |
Stated interest rate | 5.90%
Debt instrument due date | Dec. 15, 2030
7.98% Notes, Due June 1, 2031 | Fitchburg Gas And Electric Light Company [Member] |
Debt Instrument [Line Items] |
Stated interest rate | 7.98%
Debt instrument due date | Jun. 01, 2031
5.70% Notes, Due July 2, 2033 | Fitchburg Gas And Electric Light Company [Member] |
Debt Instrument [Line Items] |
Stated interest rate | 5.70%
Debt instrument due date | Jul. 02, 2033
5.54% Senior Notes, Due August 21, 2034 | Fitchburg Gas And Electric Light Company [Member] |
Debt Instrument [Line Items] |
Stated interest rate | 5.54%
Debt instrument due date | Aug. 21, 2034
5.54% Senior Notes, Due August 21, 2034 | Northern Utilities Inc [Member] |
Debt Instrument [Line Items] |
Stated interest rate | 5.54%
Debt instrument due date | Aug. 21, 2034
3.78% Senior Notes, Due September 15, 2040 | Fitchburg Gas And Electric Light Company [Member] |
Debt Instrument [Line Items] |
Stated interest rate | 3.78%
Debt instrument due date | Sep. 15, 2040
3.78% Senior Notes, Due September 15, 2040 | Northern Utilities Inc [Member] |
Debt Instrument [Line Items] |
Stated interest rate | 3.78%
Debt instrument due date | Sep. 15, 2040
5.99% Senior Notes, Due August 21, 2044 | Fitchburg Gas And Electric Light Company [Member] |
Debt Instrument [Line Items] |
Stated interest rate | 5.99%
Debt instrument due date | Aug. 21, 2044
4.32% Senior Notes, Due November 1, 2047 | Fitchburg Gas And Electric Light Company [Member] |
Debt Instrument [Line Items] |
Stated interest rate | 4.32%
Debt instrument due date | Nov. 01, 2047
4.32% Senior Notes, Due November 1, 2047 | Northern Utilities Inc [Member] |
Debt Instrument [Line Items] |
Stated interest rate | 4.32%
Debt instrument due date | Nov. 01, 2047
5.96% Senior Notes, Due July 2, 2053 | Fitchburg Gas And Electric Light Company [Member] |
Debt Instrument [Line Items] |
Stated interest rate | 5.96%
Debt instrument due date | Jul. 02, 2053
7.72% Senior Notes, Due December 3, 2038 | Northern Utilities Inc [Member] |
Debt Instrument [Line Items] |
Stated interest rate | 7.72%
Debt instrument due date | Dec. 03, 2038
5.74% Senior Notes, Due August 21, 2039 | Northern Utilities Inc [Member] |
Debt Instrument [Line Items] |
Stated interest rate | 5.74%
Debt instrument due date | Aug. 21, 2039
5.74% Senior Notes, Due August 21, 2034 | Granite State Gas Transmission Inc [Member] |
Debt Instrument [Line Items] |
Stated interest rate | 5.74%
Debt instrument due date | Aug. 21, 2034
4.42% Senior Notes, Due October 15, 2044 | Northern Utilities Inc [Member] |
Debt Instrument [Line Items] |
Stated interest rate | 4.42%
Debt instrument due date | Oct. 15, 2044
4.04% Senior Notes, Due September 12, 2049 | Northern Utilities Inc [Member] |
Debt Instrument [Line Items] |
Stated interest rate | 4.04%
Debt instrument due date | Sep. 12, 2049
3.72% Senior Notes, Due November 1, 2027 | Granite State Gas Transmission Inc [Member] |
Debt Instrument [Line Items] |
Stated interest rate | 3.72%
Debt instrument due date | Nov. 01, 2027
2.64% Senior Secured Notes, Due December 18, 2030 | Unitil Realty Corp [Member] |
Debt Instrument [Line Items] |
Stated interest rate | 2.64%
Debt instrument due date | Dec. 18, 2030

X
- Definition Contractual interest rate for funds borrowed, under the debt agreement. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Topic 210 -SubTopic 10 -Name Accounting Standards Codification -Section S99 -Paragraph 1 -Subparagraph (SX 210.5-02(22)(a)(1)) -Publisher FASB -URI https://asc.fasb.org/1943274/2147480566/210-10-S99-1 Reference 2: http://www.xbrl.org/2003/role/disclosureRef -Topic 470 -SubTopic 20 -Name Accounting Standards Codification -Section 50 -Paragraph 1B -Subparagraph (b) -Publisher FASB -URI https://asc.fasb.org/1943274/2147481139/470-20-50-1B + Details
Name: | us-gaap_DebtInstrumentInterestRateStatedPercentage
Namespace Prefix: | us-gaap_
Data Type: | dtr-types:percentItemType
Balance Type: | na
Period Type: | instant

X
- Definition Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table. + References Reference 1: http://www.xbrl.org/2003/role/exampleRef -Topic 835 -SubTopic 30 -Name Accounting Standards Codification -Section 55 -Paragraph 8 -Publisher FASB -URI https://asc.fasb.org/1943274/2147482949/835-30-55-8 Reference 2: http://www.xbrl.org/2009/role/commonPracticeRef -Topic 235 -SubTopic 10 -Name Accounting Standards Codification -Section S99 -Paragraph 1 -Subparagraph (SX 210.4-08(f)) -Publisher FASB -URI https://asc.fasb.org/1943274/2147480678/235-10-S99-1 Reference 3: http://www.xbrl.org/2003/role/disclosureRef -Topic 235 -SubTopic 10 -Name Accounting Standards Codification -Section S99 -Paragraph 3 -Subparagraph (SX 210.12-04(a)) -Publisher FASB -URI https://asc.fasb.org/1943274/2147480678/235-10-S99-3 Reference 4: http://www.xbrl.org/2003/role/disclosureRef -Topic 470 -SubTopic 20 -Name Accounting Standards Codification -Section 50 -Paragraph 1B -Subparagraph (a) -Publisher FASB -URI https://asc.fasb.org/1943274/2147481139/470-20-50-1B Reference 5: http://www.xbrl.org/2003/role/exampleRef -Topic 470 -SubTopic 20 -Name Accounting Standards Codification -Section 55 -Paragraph 69B -Publisher FASB -URI https://asc.fasb.org/1943274/2147481568/470-20-55-69B Reference 6: http://www.xbrl.org/2003/role/exampleRef -Topic 470 -SubTopic 20 -Name Accounting Standards Codification -Section 55 -Paragraph 69C -Publisher FASB -URI https://asc.fasb.org/1943274/2147481568/470-20-55-69C Reference 7: http://www.xbrl.org/2003/role/exampleRef -Topic 470 -SubTopic 20 -Name Accounting Standards Codification -Section 55 -Paragraph 69E -Publisher FASB -URI https://asc.fasb.org/1943274/2147481568/470-20-55-69E Reference 8: http://www.xbrl.org/2003/role/exampleRef -Topic 470 -SubTopic 20 -Name Accounting Standards Codification -Section 55 -Paragraph 69F -Publisher FASB -URI https://asc.fasb.org/1943274/2147481568/470-20-55-69F Reference 9: http://www.xbrl.org/2003/role/disclosureRef -Topic 470 -SubTopic 20 -Name Accounting Standards Codification -Section 50 -Paragraph 1B -Subparagraph (b) -Publisher FASB -URI https://asc.fasb.org/1943274/2147481139/470-20-50-1B Reference 10: http://www.xbrl.org/2003/role/disclosureRef -Topic 470 -SubTopic 20 -Name Accounting Standards Codification -Section 50 -Paragraph 1B -Subparagraph (c) -Publisher FASB -URI https://asc.fasb.org/1943274/2147481139/470-20-50-1B Reference 11: http://www.xbrl.org/2003/role/exampleRef -Topic 470 -SubTopic 20 -Name Accounting Standards Codification -Section 50 -Paragraph 1B -Subparagraph (d) -Publisher FASB -URI https://asc.fasb.org/1943274/2147481139/470-20-50-1B Reference 12: http://www.xbrl.org/2003/role/disclosureRef -Topic 470 -SubTopic 20 -Name Accounting Standards Codification -Section 50 -Paragraph 1B -Subparagraph (e) -Publisher FASB -URI https://asc.fasb.org/1943274/2147481139/470-20-50-1B Reference 13: http://www.xbrl.org/2003/role/disclosureRef -Topic 470 -SubTopic 20 -Name Accounting Standards Codification -Section 50 -Paragraph 1B -Subparagraph (f) -Publisher FASB -URI https://asc.fasb.org/1943274/2147481139/470-20-50-1B Reference 14: http://www.xbrl.org/2003/role/disclosureRef -Topic 470 -SubTopic 20 -Name Accounting Standards Codification -Section 50 -Paragraph 1B -Subparagraph (h) -Publisher FASB -URI https://asc.fasb.org/1943274/2147481139/470-20-50-1B Reference 15: http://www.xbrl.org/2003/role/disclosureRef -Topic 470 -SubTopic 20 -Name Accounting Standards Codification -Section 50 -Paragraph 1D -Subparagraph (a) -Publisher FASB -URI https://asc.fasb.org/1943274/2147481139/470-20-50-1D Reference 16: http://www.xbrl.org/2003/role/disclosureRef -Topic 470 -SubTopic 20 -Name Accounting Standards Codification -Section 50 -Paragraph 1D -Subparagraph (b) -Publisher FASB -URI https://asc.fasb.org/1943274/2147481139/470-20-50-1D Reference 17: http://www.xbrl.org/2003/role/disclosureRef -Topic 470 -SubTopic 20 -Name Accounting Standards Codification -Section 50 -Paragraph 1D -Subparagraph (c) -Publisher FASB -URI https://asc.fasb.org/1943274/2147481139/470-20-50-1D Reference 18: http://www.xbrl.org/2003/role/disclosureRef -Topic 470 -SubTopic 20 -Name Accounting Standards Codification -Section 50 -Paragraph 1E -Subparagraph (a) -Publisher FASB -URI https://asc.fasb.org/1943274/2147481139/470-20-50-1E Reference 19: http://www.xbrl.org/2003/role/disclosureRef -Topic 470 -SubTopic 20 -Name Accounting Standards Codification -Section 50 -Paragraph 1E -Subparagraph (c) -Publisher FASB -URI https://asc.fasb.org/1943274/2147481139/470-20-50-1E Reference 20: http://www.xbrl.org/2003/role/disclosureRef -Topic 470 -SubTopic 20 -Name Accounting Standards Codification -Section 50 -Paragraph 1E -Subparagraph (d) -Publisher FASB -URI https://asc.fasb.org/1943274/2147481139/470-20-50-1E Reference 21: http://www.xbrl.org/2003/role/disclosureRef -Topic 470 -SubTopic 20 -Name Accounting Standards Codification -Section 50 -Paragraph 1F -Subparagraph (a) -Publisher FASB -URI https://asc.fasb.org/1943274/2147481139/470-20-50-1F Reference 22: http://www.xbrl.org/2003/role/disclosureRef -Topic 470 -SubTopic 20 -Name Accounting Standards Codification -Section 50 -Paragraph 1F -Subparagraph (b) -Publisher FASB -URI https://asc.fasb.org/1943274/2147481139/470-20-50-1F Reference 23: http://www.xbrl.org/2003/role/disclosureRef -Topic 470 -SubTopic 20 -Name Accounting Standards Codification -Section 50 -Paragraph 1F -Subparagraph (b)(1) -Publisher FASB -URI https://asc.fasb.org/1943274/2147481139/470-20-50-1F Reference 24: http://www.xbrl.org/2003/role/disclosureRef -Topic 470 -SubTopic 20 -Name Accounting Standards Codification -Section 50 -Paragraph 1F -Subparagraph (b)(2) -Publisher FASB -URI https://asc.fasb.org/1943274/2147481139/470-20-50-1F Reference 25: http://www.xbrl.org/2003/role/disclosureRef -Topic 470 -SubTopic 20 -Name Accounting Standards Codification -Section 50 -Paragraph 1I -Subparagraph (c) -Publisher FASB -URI https://asc.fasb.org/1943274/2147481139/470-20-50-1I Reference 26: http://www.xbrl.org/2009/role/commonPracticeRef -Topic 835 -SubTopic 30 -Name Accounting Standards Codification -Section 45 -Paragraph 2 -Publisher FASB -URI https://asc.fasb.org/1943274/2147482925/835-30-45-2 Reference 27: http://www.xbrl.org/2009/role/commonPracticeRef -Topic 835 -SubTopic 30 -Name Accounting Standards Codification -Section 50 -Paragraph 1 -Publisher FASB -URI https://asc.fasb.org/1943274/2147482900/835-30-50-1 + Details
Name: | us-gaap_DebtInstrumentLineItems
Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

X
- Definition Date when the debt instrument is scheduled to be fully repaid, in YYYY-MM-DD format. + References Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef -Topic 820 -SubTopic 10 -Name Accounting Standards Codification -Section 50 -Paragraph 2 -Subparagraph (bbb)(2)(i) -Publisher FASB -URI https://asc.fasb.org/1943274/2147482106/820-10-50-2 Reference 2: http://www.xbrl.org/2009/role/commonPracticeRef -Topic 820 -SubTopic 10 -Name Accounting Standards Codification -Section 50 -Paragraph 2 -Subparagraph (bbb)(2) -Publisher FASB -URI https://asc.fasb.org/1943274/2147482106/820-10-50-2 Reference 3: http://www.xbrl.org/2003/role/disclosureRef -Topic 210 -SubTopic 10 -Name Accounting Standards Codification -Section S99 -Paragraph 1 -Subparagraph (SX 210.5-02(22)(a)(2)) -Publisher FASB -URI https://asc.fasb.org/1943274/2147480566/210-10-S99-1 Reference 4: http://www.xbrl.org/2003/role/exampleRef -Topic 470 -SubTopic 20 -Name Accounting Standards Codification -Section 50 -Paragraph 1B -Subparagraph (d) -Publisher FASB -URI https://asc.fasb.org/1943274/2147481139/470-20-50-1B + Details
Name: | us-gaap_DebtInstrumentMaturityDate
Namespace Prefix: | us-gaap_
Data Type: | xbrli:dateItemType
Balance Type: | na
Period Type: | duration

X
- Details
Name: | us-gaap_DebtInstrumentAxis=utl_SeniorUnsecuredNotesThreePointSevenZeroPercentDueAugustOneTwentyTwentySixMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_DebtInstrumentAxis=utl_SeniorUnsecuredNotesThreePointFourThreePercentDueDecemberEighteenTwentyTwentyNineMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_DebtInstrumentAxis=utl_SeniorUnsecuredNotesFivePointNineNinePercentDueAugustTwentyOneTwentyThirtyFourMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_DebtInstrumentAxis=utl_FirstMortgageBondsSixPointNineSixPercentSeniorSecuredNotesDueSeptemberOneTwentyTwentyEightMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | dei_LegalEntityAxis=utl_UnitilEnergySystemsIncMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_DebtInstrumentAxis=utl_FirstMortgageBondsEightPointZeroZeroPercentSeniorSecuredNotesDueMayOneTwentyThirtyOneMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_DebtInstrumentAxis=utl_FirstMortgageBondsSixPointThreeTwoPercentSeniorSecuredNotesDueSeptemberFifteenTwentyThirtySixMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_DebtInstrumentAxis=utl_FirstMortgageBondsThreePointFiftyEightPercentSeniorSecuredNotesDueSeptemberFifteenTwentyFortyMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_DebtInstrumentAxis=utl_FirstMortgageBondsFourPointEighteenPercentSeniorSecuredNotesDueNovemberThirtyTwentyFortyEightMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_DebtInstrumentAxis=utl_FirstMortgageBondsFivePointSixNinePercentSeniorSecuredNotesDueAugustTwentyOneTwentyFiftyFourMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_DebtInstrumentAxis=utl_SeniorNotesThreePointFiveTwoPercentDueNovemberOneTwentyTwentySevenMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | dei_LegalEntityAxis=utl_FitchburgGasAndElectricLightCompanyMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | dei_LegalEntityAxis=utl_NorthernUtilitiesIncMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_DebtInstrumentAxis=utl_SeniorNotesSevenPointThreeSevenPercentDueJanuaryFifteenTwentyTwentyNineMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_DebtInstrumentAxis=utl_SeniorNotesFivePointNineZeroPercentDueDecemberFifteenTwentyThirtyMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_DebtInstrumentAxis=utl_SeniorNotesSevenPointNineEightPercentDueJuneOneTwentyThirtyOneMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_DebtInstrumentAxis=utl_SeniorNotesFivePointSevenZeroPercentDueJulyTwoTwentyThirtyThreeMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_DebtInstrumentAxis=utl_SeniorNotesFivePointFiveFourPercentDueAugustTwentyOneTwentyThirtyFourMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_DebtInstrumentAxis=utl_SeniorNotesThreePointSeventyEightPercentDueSeptemberFifteenTwentyFortyMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_DebtInstrumentAxis=utl_SeniorNotesFivePointNineNinePercentDueAugustTwentyOneTwentyFortyFourMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_DebtInstrumentAxis=utl_SeniorNotesFourPointThreeTwoPercentDueNovemberOneTwentyFortySevenMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_DebtInstrumentAxis=utl_SeniorNotesFivePointNineSixPercentDueJulyTwoTwentyFiftThreeMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_DebtInstrumentAxis=utl_SeniorNotesSevenPointSevenTwoPercentDueDecemberThreeTwentyThirtyEightMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_DebtInstrumentAxis=utl_SeniorNotesFivePointSevenFourPercentDueAugustTwentyOneTwentyThirtyNineMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_DebtInstrumentAxis=utl_SeniorNotesFivePointSevenFourPercentDueAugustTwentyOneTwentyThirtyFourMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | dei_LegalEntityAxis=utl_GraniteStateGasTransmissionIncMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_DebtInstrumentAxis=utl_SeniorNotesFourPointFourTwoPercentDueOctoberFifteenTwentyFortyFourMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_DebtInstrumentAxis=utl_SeniorNotesFourPointZeroFourPercentDueSeptemberTwelveTwentyFortyNineMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_DebtInstrumentAxis=utl_SeniorNotesThreePointSevenTwoPercentDueNovemberOneTwentyTwentySevenMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_DebtInstrumentAxis=utl_SeniorNotesTwoPointSixFourPercentDueDecemberEighteenTwentyThirtyMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | dei_LegalEntityAxis=utl_UnitilRealtyCorpMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |