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Debt and Financing Arrangements - Additional Information (Detail) - USD ($) |  |  | 9 Months Ended | 12 Months Ended |
--- | --- | --- | --- | --- | ---
Jul. 25, 2018 | Nov. 01, 2017 | Sep. 30, 2018 | Sep. 30, 2017 | Dec. 31, 2017 | Oct. 30, 2015
Line of Credit Facility [Line Items] |  |  |  |  |  |
Weighted average interest rate on short term borrowings |  |  | 3.20% | 2.30% | 2.40% |
Issuance of long-term debt |  | $ 700,000 |  |  |  |
Capital lease obligation, current |  |  | $ 3,100,000 | $ 3,100,000 | $ 3,100,000 |
Capital lease obligation, noncurrent |  |  | 3,400,000 | 6,400,000 | 5,700,000 |
Accounts Payable |  |  | 27,400,000 | 24,100,000 | 41,500,000 |
Guarantee outstanding |  |  | $ 5,600,000 |  |  |
Unitil Service Corp. |  |  |  |  |  |
Line of Credit Facility [Line Items] |  |  |  |  |  |
Capital lease obligation, total capitalized cost |  |  |  |  |  | $ 13,400,000
Capital lease obligation, maturity period |  |  | Sep. 30, 2020 |  |  |
Capital lease obligation, current |  |  | $ 2,700,000 |  |  |
Capital lease obligation, noncurrent |  |  | 3,000,000 |  |  |
Northern Utilities Inc |  |  |  |  |  |
Line of Credit Facility [Line Items] |  |  |  |  |  |
Natural gas storage inventory |  |  | 9,600,000 | 9,000,000 | 8,500,000 |
Accounts Payable |  |  | 100,000 | 100,000 | 3,100,000 |
Revolving Credit Facility |  |  |  |  |  |
Line of Credit Facility [Line Items] |  |  |  |  |  |
Revolving credit facility |  |  | 120,000,000 | $ 120,000,000 | $ 120,000,000 |
Proceeds from lines of credit |  |  | 202,500,000 |  |  |
Repayments of lines of credit |  |  | $ 171,700,000 |  |  |
Credit Facility | Second Amended Credit Facility [Member] |  |  |  |  |  |
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 [Member] | 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 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, 2018, September 30, 2017 and December 31, 2017, the Company was in compliance with the covenants contained in the Credit Facility in effect on that date. |  |  |
7.15% Senior Notes, Due December 15, 2018 | Granite State Gas Transmission Inc |  |  |  |  |  |
Line of Credit Facility [Line Items] |  |  |  |  |  |
Long-term debt, maturity date |  |  | Dec. 15, 2018 | Dec. 15, 2018 | Dec. 15, 2018 |
Long-term debt, stated interest rate |  |  | 7.15% | 7.15% | 7.15% |
Senior Notes |  |  | $ 3,300,000 |  |  |
3.52% Senior Notes, Due November 1, 2027 | Northern Utilities Inc |  |  |  |  |  |
Line of Credit Facility [Line Items] |  |  |  |  |  |
Long-term debt, aggregate principal amount |  | $ 20,000,000 |  |  |  |
Long-term debt, maturity date |  | Nov. 01, 2027 | Nov. 01, 2027 | Nov. 01, 2027 | Nov. 01, 2027 |
Long-term debt, stated interest rate |  | 3.52% | 3.52% | 3.52% | 3.52% |
3.52% Senior Notes, Due November 1, 2027 | Fitchburg Gas and Electric Light Company |  |  |  |  |  |
Line of Credit Facility [Line Items] |  |  |  |  |  |
Long-term debt, aggregate principal amount |  | $ 10,000,000 |  |  |  |
Long-term debt, maturity date |  | Nov. 01, 2027 | Nov. 01, 2027 | Nov. 01, 2027 | Nov. 01, 2027 |
Long-term debt, stated interest rate |  | 3.52% | 3.52% | 3.52% | 3.52% |
4.32% Senior Notes, Due November 1, 2047 | Northern Utilities Inc |  |  |  |  |  |
Line of Credit Facility [Line Items] |  |  |  |  |  |
Long-term debt, aggregate principal amount |  | $ 30,000,000 |  |  |  |
Long-term debt, maturity date |  | Nov. 01, 2047 | Nov. 01, 2047 | Nov. 01, 2047 | Nov. 01, 2047 |
Long-term debt, stated interest rate |  | 4.32% | 4.32% | 4.32% | 4.32% |
4.32% Senior Notes, Due November 1, 2047 | Fitchburg Gas and Electric Light Company |  |  |  |  |  |
Line of Credit Facility [Line Items] |  |  |  |  |  |
Long-term debt, aggregate principal amount |  | $ 15,000,000 |  |  |  |
Long-term debt, maturity date |  | Nov. 01, 2047 | Nov. 01, 2047 | Nov. 01, 2047 | Nov. 01, 2047 |
Long-term debt, stated interest rate |  | 4.32% | 4.32% | 4.32% | 4.32% |
3.72% Senior Notes, Due November 1, 2027 | Granite State Gas Transmission Inc |  |  |  |  |  |
Line of Credit Facility [Line Items] |  |  |  |  |  |
Long-term debt, aggregate principal amount |  | $ 15,000,000 |  |  |  |
Long-term debt, maturity date |  | Nov. 01, 2027 | Nov. 01, 2027 | Nov. 01, 2027 | Nov. 01, 2027 |
Long-term debt, stated interest rate |  | 3.72% | 3.72% | 3.72% | 3.72% |

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