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Debt and Financing Arrangements - Additional Information (Detail) - USD ($) |  |  | 1 Months Ended | 3 Months Ended | 9 Months Ended | 12 Months Ended
--- | --- | --- | --- | --- | --- | ---
Dec. 18, 2020 | Sep. 15, 2020 | Jul. 25, 2018 | Sep. 30, 2021 | Sep. 30, 2020 | Sep. 30, 2021 | Sep. 30, 2020 | Dec. 31, 2020
Line of Credit Facility [Line Items] |  |  |  |  |  |  |  |
Issuance of long-term debt |  | $ 500,000 |  |  |  |  |  |
Weighted average interest rate on short term borrowings |  |  |  | 1.20% | 1.30% | 1.20% | 1.80% | 1.70%
Capital lease obligation, current |  |  |  | $ 200,000 |  | $ 200,000 |  |
Capital lease obligation, noncurrent |  |  |  | 200,000 |  | 200,000 |  |
Accounts Payable |  |  |  | 29,500,000 | $ 25,100,000 | 29,500,000 | $ 25,100,000 | $ 33,200,000
Total rental expense under operating leases |  |  |  | 500,000 | 500,000 | 1,400,000 | 1,400,000 |
Net Utility Plant |  |  |  | 1,237,300,000 | $ 1,185,300,000 | 1,237,300,000 | 1,185,300,000 | 1,193,200,000
Guarantee outstanding |  |  |  | 700,000 |  | 700,000 |  |
Operating lease obligations |  |  |  |  |  | 1,400,000 | $ 1,400,000 |
Other current operating lease obligation |  |  |  | 1,600,000 |  | 1,600,000 |  |
Other noncurrent operating lease obligation |  |  |  | $ 3,500,000 |  | $ 3,500,000 |  |
Operating lease, weighted average remaining lease term |  |  |  | 3 years 8 months 12 days | 4 years | 3 years 8 months 12 days | 4 years |
Operating lease, weighted average discount rate percentage |  |  |  | 4.00% | 4.40% | 4.00% | 4.40% |
Long-term Debt, Gross |  |  |  | $ 515,100,000 | $ 539,100,000 | $ 515,100,000 | $ 539,100,000 | 535,400,000
Payments of Debt Issuance Costs |  |  |  |  |  |  | 500,000 |
Guarantee Term Period |  |  |  |  |  | 1 year |  |
Assets under Capital Leases [Member] |  |  |  |  |  |  |  |
Line of Credit Facility [Line Items] |  |  |  |  |  |  |  |
Net Utility Plant |  |  |  | 700,000 | 1,100,000 | $ 700,000 | 1,100,000 | 1,000,000.0
Net Utility Plant, accumulated amortization |  |  |  | $ 300,000 | 600,000 | $ 300,000 | 600,000 | 500,000
Northern Utilities Inc |  |  |  |  |  |  |  |
Line of Credit Facility [Line Items] |  |  |  |  |  |  |  |
Total funded indebtedness as percentage of capitalization |  |  |  | 65.00% |  | 65.00% |  |
Natural gas storage inventory |  |  |  | $ 9,700,000 | 6,000,000.0 | $ 9,700,000 | 6,000,000.0 | 5,400,000
Accounts Payable |  |  |  | 100,000 | 100,000 | 100,000 | 100,000 | 1,000,000.0
Unitil Reality Corp [Member] |  |  |  |  |  |  |  |
Line of Credit Facility [Line Items] |  |  |  |  |  |  |  |
Long-term debt, stated interest rate | 2.64% |  |  |  |  |  |  |
Long-term debt, maturity date | Dec. 18, 2020 |  |  |  |  |  |  |
Long-term Debt, Gross | $ 4,700,000 |  |  |  |  |  |  |
Payments of Debt Issuance Costs | $ 100,000 |  |  |  |  |  |  |
Revolving Credit Facility |  |  |  |  |  |  |  |
Line of Credit Facility [Line Items] |  |  |  |  |  |  |  |
Revolving credit facility |  |  |  | $ 120,000,000.0 | $ 120,000,000.0 | 120,000,000.0 | $ 120,000,000.0 | $ 120,000,000.0
Proceeds from lines of credit |  |  |  |  |  | 170,700,000 |  |
Repayments of lines of credit |  |  |  |  |  | $ 194,900,000 |  |
Credit Facility | Second Amended Credit Facility |  |  |  |  |  |  |  |
Line of Credit Facility [Line Items] |  |  |  |  |  |  |  |
Revolving credit facility |  |  | $ 120,000,000 |  |  |  |  |
Sublimit for the issuance of standby letters of credit |  |  | $ 25,000,000 |  |  |  |  |
Revolving credit facility termination date |  |  | Jul. 25, 2023 |  |  |  |  |
Increase in borrowing limit |  |  | $ 50,000,000 |  |  |  |  |
Credit Facility | Second Amended Credit Facility | London Interbank Offered Rate |  |  |  |  |  |  |  |
Line of Credit Facility [Line Items] |  |  |  |  |  |  |  |
Credit facility, daily fluctuating rate of interest |  |  | 1.125% |  |  |  |  |
Credit Facility | Revolving Credit Facility |  |  |  |  |  |  |  |
Line of Credit Facility [Line Items] |  |  |  |  |  |  |  |
Percentage of capitalization |  |  |  |  |  | The affirmative and negative covenants under the Credit Facility apply 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 Funded Debt to Capitalization (as each term is defined in the Credit Facility) cannot exceed 65%, tested on a quarterly basis. At September 30, 2021, September 30, 2020 and December 31, 2020, the Company was in compliance with the covenants contained in the Credit Facility in effect on those dates. |  |
Three Point Five Eight Percentage Mortage BondsDue September Tweleve twenty Fourty [Member] | Unitil Energy |  |  |  |  |  |  |  |
Line of Credit Facility [Line Items] |  |  |  |  |  |  |  |
Long-term debt, maturity date |  | Sep. 15, 2040 |  |  |  |  |  |
Three Point Five Eight Percentage Mortage BondsDue September Tweleve twenty Fourty [Member] | Unitil Energy Systems Inc [Member] |  |  |  |  |  |  |  |
Line of Credit Facility [Line Items] |  |  |  |  |  |  |  |
Long-term debt, aggregate principal amount |  | $ 27,500,000 |  |  |  |  |  |
Long-term debt, stated interest rate |  | 3.58% |  |  |  |  |  |
3.78% Senior Notes, Due September 15, 2040 | Northern Utilities Inc |  |  |  |  |  |  |  |
Line of Credit Facility [Line Items] |  |  |  |  |  |  |  |
Long-term debt, aggregate principal amount |  | $ 40,000,000 |  |  |  |  |  |
Long-term debt, stated interest rate |  | 3.78% |  |  |  |  |  |
Long-term debt, maturity date |  | Sep. 15, 2040 |  |  |  |  |  |
3.78% Senior Notes, Due September 15, 2040 | Fitchburg Gas and Electric Light Company |  |  |  |  |  |  |  |
Line of Credit Facility [Line Items] |  |  |  |  |  |  |  |
Long-term debt, aggregate principal amount |  | $ 27,500,000 |  |  |  |  |  |
Long-term debt, stated interest rate |  | 3.78% |  |  |  |  |  |
Long-term debt, maturity date |  | Sep. 15, 2040 |  |  |  |  |  |

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