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Debt and Financing Arrangements - Additional Information (Detail) - USD ($) |  |  |  | 1 Months Ended | 3 Months Ended | 9 Months Ended | 12 Months Ended |
--- | --- | --- | --- | --- | --- | --- | --- | ---
Sep. 15, 2020 | Dec. 18, 2019 | Sep. 12, 2019 | Jul. 25, 2018 | Sep. 30, 2020 | Sep. 30, 2019 | Mar. 31, 2019 | Sep. 30, 2020 | Sep. 30, 2019 | Dec. 31, 2019 | Oct. 30, 2015
Line of Credit Facility [Line Items] |  |  |  |  |  |  |  |  |  |  |
Weighted average interest rate on short term borrowings |  |  |  |  | 1.30% | 3.40% |  | 1.30% | 3.40% | 3.40% |
Gain on divestiture of business, net |  |  |  |  |  |  | $ 9,800,000 |  |  |  |
Capital lease obligation, total capitalized cost |  |  |  |  | $ 485,000 | $ 600,000 |  | $ 485,000 | $ 600,000 | $ 500,000 |
Capital lease obligation, current |  |  |  |  | 200,000 | 300,000 |  | 200,000 | 300,000 | 200,000 |
Capital lease obligation, noncurrent |  |  |  |  | 300,000 | 300,000 |  | 300,000 | 300,000 | 300,000 |
Accounts Payable |  |  |  |  | 25,100,000 | 22,900,000 |  | 25,100,000 | 22,900,000 | 37,600,000 |
Guarantee outstanding |  |  |  |  | 6,200,000 |  |  | 6,200,000 |  |  |
Total rental expense under operating leases |  |  |  |  | 500,000 | 400,000 |  | 1,400,000 | 1,100,000 |  |
Operating lease obligations |  |  |  |  |  |  |  | 1,400,000 |  |  |
Net Utility Plant |  |  |  |  | 1,185,300,000 | 1,083,900,000 |  | 1,185,300,000 | 1,083,900,000 | 1,111,500,000 |
Other current operating lease obligation |  |  |  |  | 1,600,000 | 1,200,000 |  | 1,600,000 | 1,200,000 | 1,200,000 |
Other noncurrent operating lease obligation |  |  |  |  | $ 4,000,000.0 | $ 3,000,000.0 |  | $ 4,000,000.0 | $ 3,000,000.0 | $ 2,800,000 |
Issuance of long-term debt | $ 500,000 | $ 200,000 | $ 200,000 |  |  |  |  |  |  |  |
Operating lease, weighted average remaining lease term |  |  |  |  | 4 years |  |  | 4 years |  |  |
Operating lease, weighted average discount rate percentage |  |  |  |  | 4.40% |  |  | 4.40% |  |  |
Credit Facility [Member] | 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 [Member] | London Interbank Offered Rate | Second Amended Credit Facility |  |  |  |  |  |  |  |  |  |  |
Line of Credit Facility [Line Items] |  |  |  |  |  |  |  |  |  |  |
Credit facility, daily fluctuating rate of interest |  |  |  | 1.125% |  |  |  |  |  |  |
3.43% Senior Notes, Due December 18, 2029 |  |  |  |  |  |  |  |  |  |  |
Line of Credit Facility [Line Items] |  |  |  |  |  |  |  |  |  |  |
Long-term debt, stated interest rate |  |  |  |  | 3.43% | 3.43% |  | 3.43% | 3.43% | 3.43% |
Long-term debt, maturity date |  |  |  |  |  |  |  | Dec. 18, 2029 | Dec. 18, 2029 | Dec. 18, 2029 |
4.04% Senior Notes, Due September 12, 2049 |  |  |  |  |  |  |  |  |  |  |
Line of Credit Facility [Line Items] |  |  |  |  |  |  |  |  |  |  |
Long-term debt, stated interest rate |  |  |  |  | 4.04% | 4.04% |  | 4.04% | 4.04% | 4.04% |
Long-term debt, maturity date |  |  |  |  |  |  |  | Sep. 12, 2049 | Sep. 12, 2049 | Sep. 12, 2049 |
Revolving Credit Facility [Member] |  |  |  |  |  |  |  |  |  |  |
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 |  |  |  |  |  |  |  | 186,000,000.0 |  |  |
Repayments of lines of credit |  |  |  |  |  |  |  | $ 227,300,000 |  |  |
Revolving Credit Facility [Member] | Credit Facility [Member] |  |  |  |  |  |  |  |  |  |  |
Line of Credit Facility [Line Items] |  |  |  |  |  |  |  |  |  |  |
Percentage of capitalization |  |  |  |  |  |  |  | The affirmative and negative covenants under the Credit Facility shall 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, 2020, September 30, 2019 and December 31, 2019, the Company was in compliance with the covenants contained in the Credit Facility in effect on that date. |  |  |
Unitil Service Corp [Member] |  |  |  |  |  |  |  |  |  |  |
Line of Credit Facility [Line Items] |  |  |  |  |  |  |  |  |  |  |
Capital lease obligation, total capitalized cost |  |  |  |  |  |  |  |  |  |  | $ 13,400,000
Northern Utilities Inc |  |  |  |  |  |  |  |  |  |  |
Line of Credit Facility [Line Items] |  |  |  |  |  |  |  |  |  |  |
Accounts Payable |  |  |  |  | 100,000 | 100,000 |  | $ 100,000 | 100,000 | 1,000,000.0 |
Natural gas storage inventory |  |  |  |  | 6,000,000.0 | $ 8,100,000 |  | $ 6,000,000.0 | $ 8,100,000 | 6,500,000 |
Total funded indebtedness as percentage of capitalization |  |  |  |  |  |  |  | 65.00% |  |  |
Northern Utilities Inc | 4.04% Senior Notes, Due September 12, 2049 |  |  |  |  |  |  |  |  |  |  |
Line of Credit Facility [Line Items] |  |  |  |  |  |  |  |  |  |  |
Long-term debt, aggregate principal amount |  |  | $ 40,000,000 |  |  |  |  |  |  |  |
Long-term debt, stated interest rate |  |  | 4.04% |  |  | 4.04% |  |  | 4.04% |  |
Long-term debt, maturity date |  |  | Sep. 12, 2049 |  |  |  |  |  | Sep. 12, 2049 |  |
Northern Utilities Inc | 3.78% Senior Notes, Due September 12, 2040 |  |  |  |  |  |  |  |  |  |  |
Line of Credit Facility [Line Items] |  |  |  |  |  |  |  |  |  |  |
Long-term debt, aggregate principal amount | $ 40,000,000 |  |  |  |  |  |  |  |  |  |
Long-term debt, maturity date | Sep. 15, 2040 |  |  |  |  |  |  |  |  |  |
Unitil Energy [Member] | 3.58% Mortgage Bonds, Due September 12, 2040 |  |  |  |  |  |  |  |  |  |  |
Line of Credit Facility [Line Items] |  |  |  |  |  |  |  |  |  |  |
Long-term debt, aggregate principal amount | $ 27,500,000 |  |  |  |  |  |  |  |  |  |
Long-term debt, stated interest rate | 3.58% |  |  |  |  |  |  |  |  |  |
Long-term debt, maturity date | Sep. 15, 2040 |  |  |  |  |  |  |  |  |  |
Fitchburg Gas and Electric Light Company | 3.78% Senior Notes, Due September 12, 2040 |  |  |  |  |  |  |  |  |  |  |
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 |  |  |  |  |  |  |  |  |  |
Unitil Corporation | 3.43% Senior Notes, Due December 18, 2029 |  |  |  |  |  |  |  |  |  |  |
Line of Credit Facility [Line Items] |  |  |  |  |  |  |  |  |  |  |
Long-term debt, aggregate principal amount |  | $ 30,000,000 |  |  |  |  |  |  |  |  |
Long-term debt, stated interest rate |  | 3.43% |  |  |  |  |  |  |  |  |
Long-term debt, maturity date |  | Dec. 18, 2029 |  |  |  |  |  |  |  |  |
Northern Utilities And Fitchburg | 3.78% Senior Notes, Due September 12, 2040 |  |  |  |  |  |  |  |  |  |  |
Line of Credit Facility [Line Items] |  |  |  |  |  |  |  |  |  |  |
Long-term debt, stated interest rate | 3.78% |  |  |  |  |  |  |  |  |  |
Assets under Capital Leases [Member] |  |  |  |  |  |  |  |  |  |  |
Line of Credit Facility [Line Items] |  |  |  |  |  |  |  |  |  |  |
Net Utility Plant |  |  |  |  | 1,100,000 | $ 1,300,000 |  | $ 1,100,000 | $ 1,300,000 | 1,200,000 |
Net Utility Plant, accumulated amortization |  |  |  |  | $ 600,000 | $ 600,000 |  | $ 600,000 | $ 600,000 | $ 600,000 |

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