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DEBT AND FINANCING ARRANGEMENTS - Additional Information (Detail) - USD ($) $ in Millions |  | 3 Months Ended | 6 Months Ended | 12 Months Ended |  |  |
--- | --- | --- | --- | --- | --- | --- | ---
Sep. 29, 2022 | Jun. 30, 2025 | Jun. 30, 2024 | Jun. 30, 2025 | Jun. 30, 2024 | Dec. 31, 2024 | Jul. 08, 2025 | Jan. 29, 2025 | Aug. 21, 2024
Line of Credit Facility [Line Items] |  |  |  |  |  |  |  |  |
Weighted average interest rate on short term borrowings |  | 5.60% | 6.70% | 5.60% | 6.70% | 6.50% |  |  |
Repayments of debt |  |  |  |  |  | $ 1.0 |  |  |
Capital lease obligation, current |  | $ 0.1 | $ 0.1 | $ 0.1 | $ 0.1 | 0.1 |  |  |
Capital lease obligation, noncurrent |  | 0.3 | 0.3 | 0.3 | 0.3 | 0.4 |  |  |
Total rental expense under operating leases |  | 0.6 | 0.5 | 1.1 | 1.1 |  |  |  |
Net Utility Plant |  | 1,644.4 | 1,454.8 | 1,644.4 | 1,454.8 | 1,539.6 |  |  |
Operating lease obligations |  |  |  | 1.1 | 1.1 |  |  |  |
Operating Lease Obligations |  | 1.7 | 1.7 | 1.7 | 1.7 | 1.8 |  |  |
Operating Lease Obligations |  | $ 4.0 | $ 3.4 | $ 4.0 | $ 3.4 | 4.9 |  |  |
Operating lease, weighted average remaining lease term |  | 3 years 9 months 18 days | 3 years 7 months 6 days | 3 years 9 months 18 days | 3 years 7 months 6 days |  |  |  |
Operating lease, weighted average discount rate percentage |  | 5.00% | 4.60% | 5.00% | 4.60% |  |  |  |
Assets under Capital Leases [Member] |  |  |  |  |  |  |  |  |
Line of Credit Facility [Line Items] |  |  |  |  |  |  |  |  |
Net Utility Plant |  | $ 0.6 | $ 0.7 | $ 0.6 | $ 0.7 | 0.6 |  |  |
Net Utility Plant, accumulated amortization |  | 0.2 | 0.2 | 0.2 | 0.2 | 0.1 |  |  |
Bangor Natural Gas [Member] |  |  |  |  |  |  |  |  |
Line of Credit Facility [Line Items] |  |  |  |  |  |  |  |  |
Net Utility Plant |  | 66.7 |  | $ 66.7 |  |  |  |  |
Northern Utilities Inc [Member] |  |  |  |  |  |  |  |  |
Line of Credit Facility [Line Items] |  |  |  |  |  |  |  |  |
Total funded indebtedness as percentage of capitalization |  |  |  | 65.00% |  |  |  |  |
Revolving Credit Facility |  |  |  |  |  |  |  |  |
Line of Credit Facility [Line Items] |  |  |  |  |  |  |  |  |
Revolving credit facility |  | 275.0 | $ 200.0 | $ 275.0 | $ 200.0 | 200.0 |  |  |
Proceeds from lines of credit |  |  |  | 215.5 |  |  |  |  |
Repayments of lines of credit |  |  |  | 149.9 |  |  |  |  |
Credit Facility |  |  |  |  |  |  |  |  |
Line of Credit Facility [Line Items] |  |  |  |  |  |  |  |  |
Revolving credit facility | $ 75.0 |  |  |  |  |  |  |  |
Credit Facility | Third Amendment Credit Facility |  |  |  |  |  |  |  |  |
Line of Credit Facility [Line Items] |  |  |  |  |  |  |  |  |
Sublimit for the issuance of standby letters of credit | $ 25.0 |  |  |  |  |  |  |  |
Revolving credit facility |  |  |  |  |  | $ 200.0 |  | $ 275.0 |
Credit Facility | Revolving Credit Facility |  |  |  |  |  |  |  |  |
Line of Credit Facility [Line Items] |  |  |  |  |  |  |  |  |
Natural gas storage inventory |  | $ 8.7 |  | $ 8.7 |  |  |  |  |
Debt Instrument, Covenant Description |  |  |  | The affirmative and negative covenants under the Credit Facility shall apply to Unitil until the Credit Facility terminates and all amounts borrowed under 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. |  |  |  |  |
Credit Facility | Revolving Credit Facility | Third Amendment Credit Facility |  |  |  |  |  |  |  |  |
Line of Credit Facility [Line Items] |  |  |  |  |  |  |  |  |
Credit facility, daily fluctuating rate of interest | 0.10% |  |  |  |  |  |  |  |
Credit Facility | Revolving Credit Facility | Third Amendment Credit Facility | Maximum [Member] |  |  |  |  |  |  |  |  |
Line of Credit Facility [Line Items] |  |  |  |  |  |  |  |  |
Debt instrument variable interest rate additional spread | 1.375% |  |  |  |  |  |  |  |
Credit Facility | Revolving Credit Facility | Third Amendment Credit Facility | Minimum [Member] |  |  |  |  |  |  |  |  |
Line of Credit Facility [Line Items] |  |  |  |  |  |  |  |  |
Debt instrument variable interest rate additional spread | 1.125% |  |  |  |  |  |  |  |
Senior Notes Five Point Nine Nine Percent Due Twenty Thirty Four [Member] | Fitchburg |  |  |  |  |  |  |  |  |
Line of Credit Facility [Line Items] |  |  |  |  |  |  |  |  |
Long-term debt, aggregate principal amount |  |  |  |  |  |  |  |  | $ 12.5
Long-term debt, stated interest rate |  |  |  |  |  |  |  |  | 5.54%
Senior Notes Five Point Nine Nine Percent Due Twenty Thirty Four [Member] | Unitil Corporation [Member] |  |  |  |  |  |  |  |  |
Line of Credit Facility [Line Items] |  |  |  |  |  |  |  |  |
Long-term debt, aggregate principal amount |  |  |  |  |  |  |  |  | $ 20.0
Long-term debt, stated interest rate |  |  |  |  |  |  |  |  | 5.99%
Senior Notes Five Point Nine Nine Percent Due Twenty Thirty Four [Member] | Northern Utilities Inc [Member] |  |  |  |  |  |  |  |  |
Line of Credit Facility [Line Items] |  |  |  |  |  |  |  |  |
Long-term debt, aggregate principal amount |  |  |  |  |  |  |  |  | $ 25.0
Long-term debt, stated interest rate |  |  |  |  |  |  |  |  | 5.54%
Senior Notes Five Point Nine Nine Percent Due Twenty Thirty Four [Member] | Granite State Gas Transmission Inc [Member] |  |  |  |  |  |  |  |  |
Line of Credit Facility [Line Items] |  |  |  |  |  |  |  |  |
Long-term debt, aggregate principal amount |  |  |  |  |  |  |  |  | $ 10.0
Debt Instrument, Interest Rate, Effective Percentage |  |  |  |  |  |  |  |  | 5.74%
Senior Notes Five Point Nine Nine Percent Due Twenty Forty Four [Member] | Fitchburg |  |  |  |  |  |  |  |  |
Line of Credit Facility [Line Items] |  |  |  |  |  |  |  |  |
Long-term debt, aggregate principal amount |  |  |  |  |  |  |  |  | $ 12.5
Long-term debt, stated interest rate |  |  |  |  |  |  |  |  | 5.99%
Senior Notes Five Point Six Nine Percent Due Twenty Fifty Four [Member] | Unitil Energy Systems Inc |  |  |  |  |  |  |  |  |
Line of Credit Facility [Line Items] |  |  |  |  |  |  |  |  |
Long-term debt, aggregate principal amount |  |  |  |  |  |  |  |  | $ 40.0
Long-term debt, stated interest rate |  |  |  |  |  |  |  |  | 5.69%
Senior Notes Five Point Seven Four Percent Due Twenty Thirty Nine [Member] | Northern Utilities Inc [Member] |  |  |  |  |  |  |  |  |
Line of Credit Facility [Line Items] |  |  |  |  |  |  |  |  |
Long-term debt, aggregate principal amount |  |  |  |  |  |  |  |  | $ 15.0
Long-term debt, stated interest rate |  |  |  |  |  |  |  |  | 5.74%
Subsequent Event [Member] | Bangor Natural Gas [Member] |  |  |  |  |  |  |  |  |
Line of Credit Facility [Line Items] |  |  |  |  |  |  |  |  |
Long-term debt, aggregate principal amount |  |  |  |  |  |  | $ 32.0 |  |
Subsequent Event [Member] | Senior Notes Five Point Seven Zero Percent Due Twenty Thirty [Member] | Bangor Natural Gas [Member] |  |  |  |  |  |  |  |  |
Line of Credit Facility [Line Items] |  |  |  |  |  |  |  |  |
Long-term debt, aggregate principal amount |  |  |  |  |  |  | $ 14.0 |  |
Long-term debt, stated interest rate |  |  |  |  |  |  | 5.70% |  |
Subsequent Event [Member] | Senior Notes Six Point Three One Percent Due Twenty Thirty Five [Member] | Bangor Natural Gas [Member] |  |  |  |  |  |  |  |  |
Line of Credit Facility [Line Items] |  |  |  |  |  |  |  |  |
Long-term debt, aggregate principal amount |  |  |  |  |  |  | $ 18.0 |  |
Long-term debt, stated interest rate |  |  |  |  |  |  | 6.31% |  |

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