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Debt and Financing Arrangements - Additional Information (Detail) - USD ($) $ in Millions |  | 12 Months Ended |
--- | --- | --- | ---
Sep. 29, 2022 | Dec. 31, 2023 | Dec. 31, 2022 | Dec. 31, 2021 | Jul. 06, 2023
Line of Credit Facility [Line Items] |  |  |  |  |
Issuance of long-term debt |  |  |  |  | $ 0.2
Weighted average interest rate on short term borrowings |  | 6.40% | 3.30% | 1.20% |
Total rental expense under operating leases |  | $ 2.1 | $ 1.8 | $ 1.9 |
Accounts Payable |  | 47.7 | 68.6 |  |
Operating lease obligations |  | 2.1 | 1.8 |  |
Net Utility Plant |  | 1,420.9 | 1,331.7 |  |
Operating Lease Obligations (current portion) |  | 1.9 | 1.5 |  |
Operating Lease Obligations (long-term portion) |  | $ 3.7 | $ 2.8 |  |
Operating lease, weighted average remaining lease term |  | 3 years 8 months 12 days | 3 years 4 months 24 days |  |
Operating lease, weighted average discount rate percentage |  | 4.40% | 3.90% |  |
Restriction on retained earnings for dividend payments |  | Therefore, there were no restrictions on the Company’s Retained Earnings at December 31, 2023 for the payment of dividends. |  |  |
Amount available for the payment of dividends |  | $ 204.3 |  |  |
Retained Earnings |  | 151.5 | $ 132.5 |  |
Long term debt repayments |  | 6.9 | 10.4 | 25.8 |
Other Current Liabilities (current portion) |  | 0.1 | 0.1 |  |
Finance Lease Liability Noncurrent |  | 0.4 | 0.1 |  |
Subsidiaries [Member] |  |  |  |  |
Line of Credit Facility [Line Items] |  |  |  |  |
Amount available for the payment of dividends |  | 409.9 |  |  |
Credit Facility [Member] |  |  |  |  |
Line of Credit Facility [Line Items] |  |  |  |  |
Revolving credit facility | $ 75.0 |  |  |  |
Credit Facility [Member] | Third Amendment Credit Facility Member [Member] |  |  |  |  |
Line of Credit Facility [Line Items] |  |  |  |  |
Revolving credit facility | 200.0 |  |  |  |
Sublimit for the issuance of standby letters of credit | $ 25.0 |  |  |  |
Bonds [Member] |  |  |  |  |
Line of Credit Facility [Line Items] |  |  |  |  |
Long term debt repayments |  | 6.9 | 10.4 | $ 25.8 |
Debt repayment, 2024 |  | 4.9 |  |  |
Debt repayment, 2025 |  | 4.9 |  |  |
Debt repayment, 2026 |  | 37.9 |  |  |
Debt repayment, 2027 |  | 55.7 |  |  |
Debt repayment, 2028 |  | 10.7 |  |  |
Debt repayment, Thereafter |  | 403.1 |  |  |
Revolving Credit Facility |  |  |  |  |
Line of Credit Facility [Line Items] |  |  |  |  |
Revolving credit facility |  | 200.0 | 200.0 |  |
Proceeds from lines of credit |  | 327.2 | 295.9 |  |
Repayments of lines of credit |  | $ 281.2 | 244.0 |  |
Revolving Credit Facility | Credit Facility [Member] |  |  |  |  |
Line of Credit Facility [Line Items] |  |  |  |  |
Percentage of capitalization |  | The affirmative and negative covenants under the Credit Facility shall apply to Unitil until the Credit Facility terminates and all amounts borrowed under the 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. |  |  |
Natural gas storage inventory |  | $ 10.9 |  |  |
Northern Utilities Inc |  |  |  |  |
Line of Credit Facility [Line Items] |  |  |  |  |
Accounts Payable |  | $ 2.2 |  |  |
Total funded indebtedness as percentage of capitalization |  | 65.00% |  |  |
Granite State Gas Transmission Inc |  |  |  |  |
Line of Credit Facility [Line Items] |  |  |  |  |
Total funded indebtedness as percentage of capitalization |  | 65.00% |  |  |
Fitchburg Gas and Electric Light Company | 5.70 % Senior Note Due to July 2, 2033 |  |  |  |  |
Line of Credit Facility [Line Items] |  |  |  |  |
Long-term debt, aggregate principal amount |  |  |  |  | $ 12.0
Long-term debt, stated interest rate |  | 5.70% |  |  | 5.70%
Fitchburg Gas and Electric Light Company | 5.96% Senior Note Due to July 2, 2053 |  |  |  |  |
Line of Credit Facility [Line Items] |  |  |  |  |
Long-term debt, aggregate principal amount |  |  |  |  | $ 13.0
Long-term debt, stated interest rate |  | 5.96% |  |  | 5.96%
Unitil Corporation | Maximum [Member] |  |  |  |  |
Line of Credit Facility [Line Items] |  |  |  |  |
Total funded indebtedness as percentage of capitalization |  | 70.00% |  |  |
Secured Overnight Financing Rate [Member] | Revolving Credit Facility | Credit Facility [Member] | Third Amendment Credit Facility Member [Member] |  |  |  |  |
Line of Credit Facility [Line Items] |  |  |  |  |
Credit facility, daily fluctuating rate of interest | 0.10% |  |  |  |
Secured Overnight Financing Rate [Member] | Revolving Credit Facility | Credit Facility [Member] | Third Amendment Credit Facility Member [Member] | Maximum [Member] |  |  |  |  |
Line of Credit Facility [Line Items] |  |  |  |  |
Debt instrument variable interest rate additional spread | 1.375% |  |  |  |
Secured Overnight Financing Rate [Member] | Revolving Credit Facility | Credit Facility [Member] | Third Amendment Credit Facility Member [Member] | Minimum |  |  |  |  |
Line of Credit Facility [Line Items] |  |  |  |  |
Debt instrument variable interest rate additional spread | 1.125% |  |  |  |
Assets under Capital Leases [Member] |  |  |  |  |
Line of Credit Facility [Line Items] |  |  |  |  |
Net Utility Plant |  | $ 0.7 | 0.6 |  |
Net Utility Plant, accumulated amortization |  | $ 0.2 | $ 0.4 |  |

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