Debt and Financing Arrangements - Additional Information (Detail) - USD ($)	1 Months Ended		3 Months Ended		6 Months Ended		12 Months Ended
	Nov. 30, 2018	Jul. 25, 2018	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018	Dec. 31, 2018	Oct. 30, 2015
Line of Credit Facility [Line Items]
Weighted average interest rate on short term borrowings			3.60%	3.30%	3.70%	3.10%	3.30%
Gain on divestiture of business, net					$ 9,800,000
Capital lease obligation, total capitalized cost			$ 458,000	$ 7,200,000	458,000	$ 7,200,000	$ 5,800,000
Capital lease obligation, current			200,000	3,100,000	200,000	3,100,000	3,100,000
Capital lease obligation, noncurrent			200,000	4,100,000	200,000	4,100,000	2,700,000
Accounts Payable			21,900,000	24,700,000	21,900,000	24,700,000	42,600,000
Guarantee outstanding				4,300,000		4,300,000
Total rental expense under operating leases			300,000	600,000	700,000	1,100,000
Operating lease obligations					700,000
Net Utility Plant			$ 1,053,500,000	989,800,000	$ 1,053,500,000	989,800,000	1,036,800,000
Operating lease, weighted average remaining lease term			3 years 8 months 12 days		3 years 8 months 12 days
Operating lease, weighted average discount rate percentage			5.30%		5.30%
Credit Facility [Member] | 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 [Member] | London Interbank Offered Rate | Amended Credit Facility [Member]
Line of Credit Facility [Line Items]
Credit facility, daily fluctuating rate of interest		1.125%
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					131,400,000
Repayments of lines of credit					$ 149,400,000
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 June 30, 2019, June 30, 2018 and December 31, 2018, 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	900,000
Natural gas storage inventory			5,000,000.0	5,300,000	5,000,000.0	5,300,000	8,400,000
Unitil Energy [Member] | 4.18% First Mortgage Bonds, Due November 30, 2048
Line of Credit Facility [Line Items]
Long-term debt, aggregate principal amount	$ 30,000,000
Long-term debt, stated interest rate	4.18%
Long-term debt, maturity date	Nov. 30, 2048
Issuance of long-term debt	$ 500,000
Assets under Capital Leases [Member]
Line of Credit Facility [Line Items]
Net Utility Plant			1,500,000	15,000,000.0	1,500,000	15,000,000.0	15,000,000.0
Net Utility Plant, accumulated amortization			$ 1,000,000.0	$ 1,400,000	$ 1,000,000.0	$ 1,400,000	$ 1,700,000