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Debt and Financing Arrangements - Details on Long Term Debt (Detail) - USD ($) $ in Millions | Dec. 31, 2024 | Dec. 31, 2023
--- | --- | ---
Debt Instrument [Line Items] |  |  |
Total Long-Term Debt |  | $ 647.3 | $ 517.2
Less: Unamortized Debt Issuance Costs |  | 4.0 | 3.2
Long-Term Debt |  | 643.3 | 514.0
Less: Current Portion | [1] | 4.9 | 4.9
Total Long-Term Debt, Less Current Portion |  | 638.4 | 509.1
3.70% Senior Notes, Due August 1, 2026 |  |  |
Debt Instrument [Line Items] |  |  |
Total Long-Term Debt |  | 30.0 | 30.0
3.43% Senior Notes, Due December 18, 2029 |  |  |
Debt Instrument [Line Items] |  |  |
Total Long-Term Debt |  | 30.0 | 30.0
5.99% Senior Notes, Due August 21, 2034 |  |  |
Debt Instrument [Line Items] |  |  |
Total Long-Term Debt |  | 20.0 |
Unitil Energy Systems Inc | First Mortgage Bonds 6.96% Senior Secured Notes, Due September 1, 2028 |  |  |
Debt Instrument [Line Items] |  |  |
Total Long-Term Debt |  | 8.0 | 10.0
Unitil Energy Systems Inc | First Mortgage Bonds 8.00% Senior Secured Notes, Due May 1, 2031 |  |  |
Debt Instrument [Line Items] |  |  |
Total Long-Term Debt |  | 10.5 | 12.0
Unitil Energy Systems Inc | First Mortgage Bonds 6.32% Senior Secured Notes, Due September 15, 2036 |  |  |
Debt Instrument [Line Items] |  |  |
Total Long-Term Debt |  | 15.0 | 15.0
Unitil Energy Systems Inc | First Mortgage Bonds 3.58% Senior Secured Notes Due September 15, 2040 |  |  |
Debt Instrument [Line Items] |  |  |
Total Long-Term Debt |  | 27.5 | 27.5
Unitil Energy Systems Inc | First Mortgage Bonds 4.18% Senior Secured Notes Due November 30, 2048 |  |  |
Debt Instrument [Line Items] |  |  |
Total Long-Term Debt |  | 30.0 | 30.0
Unitil Energy Systems Inc | First Mortgage Bonds 5.69% Senior Secured Notes Due August 21, 2054 |  |  |
Debt Instrument [Line Items] |  |  |
Total Long-Term Debt |  | 40.0 |
Fitchburg Gas and Electric Light Company | 3.52% Senior Notes, Due November 1, 2027 |  |  |
Debt Instrument [Line Items] |  |  |
Total Long-Term Debt |  | 10.0 | 10.0
Fitchburg Gas and Electric Light Company | 7.37% Senior Notes, Due January 15, 2029 |  |  |
Debt Instrument [Line Items] |  |  |
Total Long-Term Debt |  | 6.0 | 7.2
Fitchburg Gas and Electric Light Company | 5.90% Notes, Due December 15, 2030 |  |  |
Debt Instrument [Line Items] |  |  |
Total Long-Term Debt |  | 15.0 | 15.0
Fitchburg Gas and Electric Light Company | 7.98% Notes, Due June 1, 2031 |  |  |
Debt Instrument [Line Items] |  |  |
Total Long-Term Debt |  | 14.0 | 14.0
Fitchburg Gas and Electric Light Company | 5.70 % Senior Note Due to July 2, 2033 |  |  |
Debt Instrument [Line Items] |  |  |
Total Long-Term Debt |  | 12.0 | 12.0
Fitchburg Gas and Electric Light Company | 5.54% Senior Notes Due to August 21, 2034 |  |  |
Debt Instrument [Line Items] |  |  |
Total Long-Term Debt |  | 12.5 |
Fitchburg Gas and Electric Light Company | 3.78% Senior Notes, Due September 15, 2040 |  |  |
Debt Instrument [Line Items] |  |  |
Total Long-Term Debt |  | 27.5 | 27.5
Fitchburg Gas and Electric Light Company | 5.99% Senior Notes, Due August 21, 2044 |  |  |
Debt Instrument [Line Items] |  |  |
Total Long-Term Debt |  | 12.5 |
Fitchburg Gas and Electric Light Company | 4.32% Senior Notes, Due November 1, 2047 |  |  |
Debt Instrument [Line Items] |  |  |
Total Long-Term Debt |  | 15.0 | 15.0
Fitchburg Gas and Electric Light Company | 5.96% Senior Note Due to July 2, 2053 |  |  |
Debt Instrument [Line Items] |  |  |
Total Long-Term Debt |  | 13.0 | 13.0
Northern Utilities Inc | 3.52% Senior Notes, Due November 1, 2027 |  |  |
Debt Instrument [Line Items] |  |  |
Total Long-Term Debt |  | 20.0 | 20.0
Northern Utilities Inc | 5.54% Senior Notes Due to August 21, 2034 |  |  |
Debt Instrument [Line Items] |  |  |
Total Long-Term Debt |  | 25.0 |
Northern Utilities Inc | 3.78% Senior Notes, Due September 15, 2040 |  |  |
Debt Instrument [Line Items] |  |  |
Total Long-Term Debt |  | 40.0 | 40.0
Northern Utilities Inc | 4.32% Senior Notes, Due November 1, 2047 |  |  |
Debt Instrument [Line Items] |  |  |
Total Long-Term Debt |  | 30.0 | 30.0
Northern Utilities Inc | 7.72% Senior Notes, Due December 3, 2038 |  |  |
Debt Instrument [Line Items] |  |  |
Total Long-Term Debt |  | 50.0 | 50.0
Northern Utilities Inc | 5.74% Senior Notes, Due August 21, 2039 |  |  |
Debt Instrument [Line Items] |  |  |
Total Long-Term Debt |  | 15.0 |
Northern Utilities Inc | 4.42% Senior Notes, Due October 15, 2044 |  |  |
Debt Instrument [Line Items] |  |  |
Total Long-Term Debt |  | 50.0 | 50.0
Northern Utilities Inc | 4.04% Senior Notes, Due September 12, 2049 |  |  |
Debt Instrument [Line Items] |  |  |
Total Long-Term Debt |  | 40.0 | 40.0
Granite State Gas Transmission Inc | 3.72% Senior Notes, Due November 1, 2027 |  |  |
Debt Instrument [Line Items] |  |  |
Total Long-Term Debt |  | 15.0 | 15.0
Granite State Gas Transmission Inc | 5.74% Senior Notes, Due August 21, 2034 |  |  |
Debt Instrument [Line Items] |  |  |
Total Long-Term Debt |  | 10.0 |
Unitil Realty Corp Member | 2.64% Senior Notes, Due December 18, 2030 |  |  |
Debt Instrument [Line Items] |  |  |
Total Long-Term Debt |  | $ 3.8 | $ 4.0
[1] | The Current Portion of Long-Term Debt includes sinking fund payments.

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