Carrying value as of the balance sheet date of liabilities incurred (and for which invoices have typically been received) and payable to vendors for goods and services received that are used in an entity's business. Used to reflect the current portion of the liabilities (due within one year or within the normal operating cycle if longer).

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Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

Debt and Financing Arrangements - Additional Information (Detail) - USD ($)		1 Months Ended	3 Months Ended
	Jul. 25, 2018	Nov. 30, 2018	Mar. 31, 2019	Mar. 31, 2018	Dec. 31, 2018	Oct. 30, 2015
Line of Credit Facility [Line Items]
Weighted average interest rate on short term borrowings			3.70%	2.90%	3.30%
Gain on divestiture of business, net			$ 9,800,000
Issuance of long-term debt		$ 500,000
Capital lease obligation, total capitalized cost			4,967,000	$ 8,000,000	$ 5,800,000
Capital lease obligation, current			3,000,000	3,100,000	3,100,000
Capital lease obligation, noncurrent			1,900,000	4,900,000	2,700,000
Accounts Payable			33,000,000	30,100,000	42,600,000
Guarantee outstanding			4,300,000
Total rental expense under operating leases			400,000	500,000
Operating lease obligations			400,000
Net Utility Plant			1,037,800,000	972,400,000	1,036,800,000
Current operating lease obligation			1,100,000
Non-current operating lease obligation			$ 2,800,000
Operating lease, weighted average remaining lease term			3 years 10 months 24 days
Operating lease, weighted average discount rate percentage			5.30%
Financing Arrangements
Line of Credit Facility [Line Items]
Capital lease obligation, current			$ 2,800,000
Capital lease obligation, noncurrent			1,600,000
Assets under Capital Leases
Line of Credit Facility [Line Items]
Net Utility Plant			14,900,000	15,000,000	15,000,000
Net Utility Plant, accumulated amortization			$ 1,800,000	1,100,000	1,700,000
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			$ 2,800,000
Capital lease obligation, noncurrent			1,600,000
Northern Utilities Inc
Line of Credit Facility [Line Items]
Natural gas storage inventory			2,200,000	1,000,000	8,400,000
Accounts Payable			2,100,000	1,000,000	900,000
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			75,700,000
Repayments of lines of credit			$ 92,700,000
Credit Facility \| 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 \| 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 March 31, 2018, March 31, 2017 and December 31, 2017, the Company was in compliance with the covenants contained in the Credit Facility in effect on that date.
4.18% First Mortgage Bonds, Due November 30, 2048 \| Unitil Energy
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