Debt and Financing Arrangements - Additional Information (Detail) - USD ($)		1 Months Ended		3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 12, 2019	Nov. 30, 2018	Jul. 25, 2018	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018	Dec. 31, 2018	Oct. 30, 2015
Line of Credit Facility [Line Items]
Weighted average interest rate on short term borrowings				3.40%	3.30%	3.60%	3.20%	3.30%
Gain on divestiture of business, net						$ 9,800,000
Capital lease obligation, total capitalized cost				$ 620,000	$ 6,500,000	620,000	$ 6,500,000	$ 5,800,000
Capital lease obligation, current				300,000	3,100,000	300,000	3,100,000	3,100,000
Capital lease obligation, noncurrent				300,000	3,400,000	300,000	3,400,000	2,700,000
Accounts Payable				22,900,000	27,400,000	22,900,000	27,400,000	42,600,000
Guarantee outstanding				4,300,000		4,300,000
Total rental expense under operating leases				400,000	400,000	1,100,000	1,400,000
Operating lease obligations						1,100,000
Net Utility Plant				1,083,900,000	1,022,300,000	1,083,900,000	1,022,300,000	1,036,800,000
Other current operating lease obligation				1,200,000		1,200,000
Other noncurrent operating lease obligation				$ 3,000,000.0		$ 3,000,000.0
Long-term debt, maturity date	Sep. 12, 2049
Operating lease, weighted average remaining lease term				4 years		4 years
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						187,100,000
Repayments of lines of credit						$ 220,700,000
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, 2019, September 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				$ 8,100,000	$ 9,600,000	$ 8,100,000	$ 9,600,000	$ 8,400,000
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%	4.04%	4.04%	4.04%
Long-term debt, maturity date						Sep. 12, 2049	Sep. 12, 2049	Sep. 12, 2049
Issuance of long-term debt	$ 200,000
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				$ 1,300,000	$ 15,000,000.0	$ 1,300,000	$ 15,000,000.0	$ 15,000,000.0
Net Utility Plant, accumulated amortization				$ 600,000	$ 2,100,000	$ 600,000	$ 2,100,000	$ 1,700,000