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DEBT AND FINANCING ARRANGEMENTS - Additional Information (Detail) - USD ($) shares in Millions, $ in Millions |  |  | 3 Months Ended | 12 Months Ended
--- | --- | --- | --- | ---
Jul. 06, 2023 | Sep. 29, 2022 | Mar. 31, 2024 | Mar. 31, 2023 | Dec. 31, 2023
Line of Credit Facility [Line Items] |  |  |  |  |
Weighted average interest rate on short term borrowings |  |  | 6.70% | 5.90% | 6.40%
Capital lease obligation, current |  |  | $ 0.1 | $ 0.1 | $ 0.1
Capital lease obligation, noncurrent |  |  | 0.3 | 0.3 | 0.4
Accounts Payable |  |  | 37.9 | 46.3 | 47.7
Total rental expense under operating leases |  |  | 0.5 | 0.5 |
Net Utility Plant |  |  | 1,429.8 | 1,341.3 | 1,420.9
Operating lease obligations |  |  | 0.5 | 0.5 |
Other current operating lease obligation |  |  | 1.8 | 1.8 | 1.9
Other noncurrent operating lease obligation |  |  | $ 3.8 | $ 3.9 | 3.7
Operating lease, weighted average remaining lease term |  |  | 3 years 8 months 12 days | 3 years 9 months 18 days |
Operating lease, weighted average discount rate percentage |  |  | 4.60% | 4.50% |
Assets under Capital Leases [Member] |  |  |  |  |
Line of Credit Facility [Line Items] |  |  |  |  |
Net Utility Plant |  |  | $ 0.7 | $ 0.9 | 0.7
Net Utility Plant, accumulated amortization |  |  | $ 0.2 | 0.4 | 0.2
Northern Utilities Inc |  |  |  |  |
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 |  |  | $ 200.0 | $ 200.0 | $ 200.0
Proceeds from lines of credit |  |  | 85.9 |  |
Repayments of lines of credit |  |  | 78.0 |  |
Accounts Payable |  |  | $ 1.4 |  |
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] |  |  |  |  |
Subordinated Borrowing, Due Date | Jul. 02, 2053 |  |  |  |
cost of issue |  |  | $ 0.2 |  |
Debt Instrument, Interest Rate, Effective Percentage | 5.96% |  |  |  |
Debt Conversion, Converted Instrument, Warrants or Options Issued | 13.0 |  |  |  |
Natural gas storage inventory |  |  | $ 3.3 |  |
Credit Facility | Revolving Credit Facility | Fitchburg [Member] |  |  |  |  |
Line of Credit Facility [Line Items] |  |  |  |  |
Debt Conversion, Converted Instrument, Shares Issued | 12.0 |  |  |  |
Subordinated Borrowing, Due Date | Jul. 02, 2033 |  |  |  |
Debt Instrument, Interest Rate, Effective Percentage | 5.70% |  |  |  |
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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