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Debt and Financing Arrangements - Additional Information (Detail) - USD ($) shares in Millions, $ in Millions |  |  |  | 3 Months Ended | 9 Months Ended | 12 Months Ended
--- | --- | --- | --- | --- | --- | ---
Aug. 21, 2024 | Jul. 06, 2023 | Sep. 29, 2022 | Sep. 30, 2024 | Sep. 30, 2023 | Sep. 30, 2024 | Sep. 30, 2023 | Dec. 31, 2023
Line of Credit Facility [Line Items] |  |  |  |  |  |  |  |
Weighted average interest rate on short term borrowings |  |  |  | 6.60% | 6.40% | 6.60% | 6.20% | 6.40%
Capital lease obligation, current |  |  |  | $ 0.1 | $ 0.1 | $ 0.1 | $ 0.1 | $ 0.1
Capital lease obligation, noncurrent |  |  |  | 0.3 | 0.4 | 0.3 | 0.4 | 0.4
Accounts Payable |  |  |  | 36.2 | 35.6 | 36.2 | 35.6 | 47.7
Total rental expense under operating leases |  |  |  | 0.5 | 0.6 | 1.6 | 1.6 |
Net Utility Plant |  |  |  | 1,502.4 | 1,390.1 | 1,502.4 | 1,390.1 | 1,420.9
Operating lease obligations |  |  |  |  |  | 1.6 | 1.6 |
Other current operating lease obligation |  |  |  | 1.6 | 1.8 | 1.6 | 1.8 | 1.9
Other noncurrent operating lease obligation |  |  |  | $ 3.0 | $ 3.7 | $ 3.0 | $ 3.7 | 3.7
Operating lease, weighted average remaining lease term |  |  |  | 3 years 6 months | 3 years 8 months 12 days | 3 years 6 months | 3 years 8 months 12 days |
Operating lease, weighted average discount rate percentage |  |  |  | 4.60% | 4.40% | 4.60% | 4.40% |
Repayments of debt |  |  |  |  |  | $ 1.0 |  |
Assets under Capital Leases [Member] |  |  |  |  |  |  |  |
Line of Credit Facility [Line Items] |  |  |  |  |  |  |  |
Net Utility Plant |  |  |  | $ 0.7 | $ 0.7 | 0.7 | $ 0.7 | 0.7
Net Utility Plant, accumulated amortization |  |  |  | 0.3 | 0.2 | 0.3 | 0.2 | 0.2
Revolving Credit Facility |  |  |  |  |  |  |  |
Line of Credit Facility [Line Items] |  |  |  |  |  |  |  |
Revolving credit facility |  |  |  | 200.0 | $ 200.0 | 200.0 | $ 200.0 | $ 200.0
Proceeds from lines of credit |  |  |  |  |  | 229.9 |  |
Repayments of lines of credit |  |  |  |  |  | 327.6 |  |
Accounts Payable |  |  |  | 0.1 |  | $ 0.1 |  |
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 |  |  |  |  |  |  |  |
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 |  |  |  |  |
Credit Facility | Revolving Credit Facility |  |  |  |  |  |  |  |
Line of Credit Facility [Line Items] |  |  |  |  |  |  |  |
Natural gas storage inventory |  |  |  | $ 8.8 |  | $ 8.8 |  |
Notes due date |  | Jul. 02, 2053 |  |  |  |  |  |
Cost of issue |  |  |  |  |  | $ 0.2 |  |
Issued note |  | 13.0 |  |  |  |  |  |
Note issue percentage | 5.99% | 5.96% |  |  |  |  |  |
Note issued | 12.5 |  |  |  |  |  |  |
Credit Facility | Revolving Credit Facility | Fitchburg |  |  |  |  |  |  |  |
Line of Credit Facility [Line Items] |  |  |  |  |  |  |  |
Notes due date |  | Jul. 02, 2033 |  |  |  |  |  |
Note issue percentage | 5.54% | 5.70% |  |  |  |  |  |
Note issued | 12.5 | 12.0 |  |  |  |  |  |
Credit Facility | Revolving Credit Facility | Northern Utilities Inc |  |  |  |  |  |  |  |
Line of Credit Facility [Line Items] |  |  |  |  |  |  |  |
Note issue percentage | 5.54% |  |  |  |  |  |  |
Note issued | 25.0 |  |  |  |  |  |  |
Credit Facility | Revolving Credit Facility | Northern Utilities Inc One |  |  |  |  |  |  |  |
Line of Credit Facility [Line Items] |  |  |  |  |  |  |  |
Note issue percentage | 5.74% |  |  |  |  |  |  |
Note issued | 15.0 |  |  |  |  |  |  |
Credit Facility | Revolving Credit Facility | Unitil Corporation |  |  |  |  |  |  |  |
Line of Credit Facility [Line Items] |  |  |  |  |  |  |  |
Note issue percentage | 5.99% |  |  |  |  |  |  |
Note issued | 20.0 |  |  |  |  |  |  |
Credit Facility | Revolving Credit Facility | Unitil Energy |  |  |  |  |  |  |  |
Line of Credit Facility [Line Items] |  |  |  |  |  |  |  |
Note issue percentage | 0.69% |  |  |  |  |  |  |
Note issued | 40.0 |  |  |  |  |  |  |
Credit Facility | Revolving Credit Facility | Granite State |  |  |  |  |  |  |  |
Line of Credit Facility [Line Items] |  |  |  |  |  |  |  |
Note issue percentage | 5.74% |  |  |  |  |  |  |
Note issued | 10.0 |  |  |  |  |  |  |
Credit Facility | Revolving Credit Facility | Third Amendment Credit Facility | Secured Overnight Financing Rate [Member] |  |  |  |  |  |  |  |
Line of Credit Facility [Line Items] |  |  |  |  |  |  |  |
Credit facility, daily fluctuating rate of interest |  |  | 0.10% |  |  |  |  |
Credit Facility | Revolving Credit Facility | Third Amendment Credit Facility | Secured Overnight Financing Rate [Member] | 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 | Secured Overnight Financing Rate [Member] | Minimum [Member] |  |  |  |  |  |  |  |
Line of Credit Facility [Line Items] |  |  |  |  |  |  |  |
Debt instrument variable interest rate additional spread |  |  | 1.125% |  |  |  |  |

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