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%