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).

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

Debt and Financing Arrangements - Additional Information (Detail) - USD ($)		3 Months Ended
	Oct. 04, 2013	Mar. 31, 2017	Apr. 21, 2017	Dec. 31, 2016	Mar. 31, 2016	Oct. 30, 2015
Line of Credit Facility [Line Items]
Weighted average interest rate on short term borrowings		2.10%		1.80%	1.70%
Capital lease obligation, current		$ 3,000,000		$ 3,000,000	$ 3,100,000
Capital lease obligation, noncurrent		7,500,000		8,300,000	10,400,000
Accounts Payable		28,100,000		32,400,000	27,000,000
Guarantee outstanding		$ 23,900,000
Unitil Service Corp.
Line of Credit Facility [Line Items]
Capital lease obligation, total capitalized cost						$ 13,400,000
Capital lease obligation, maturity period		Oct. 30, 2015
Capital lease obligation, current		$ 2,600,000
Capital lease obligation, noncurrent		7,100,000
Northern Utilities Inc
Line of Credit Facility [Line Items]
Natural gas storage inventory		3,800,000		9,900,000	6,700,000
Accounts Payable		2,000,000		2,100,000	400,000
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		55,700,000
Repayments of lines of credit		$ 61,000,000
Credit Facility \| Revolving Credit Facility
Line of Credit Facility [Line Items]
Revolving credit facility	$ 120,000,000
Sublimit for the issuance of standby letters of credit	25,000,000
Credit Facility by an aggregate additional amount	$ 30,000,000
Revolving credit facility termination date	Oct. 04, 2020
Percentage of capitalization		The affirmative and negative covenants under the Credit Facility shall apply to Unitil 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 Unitil’s Funded Debt to Capitalization (as each term is defined in the Credit Facility) cannot exceed 65%, tested on a quarterly basis. At March 31, 2017, March 31, 2016 and December 31, 2016, the Company was in compliance with the covenants contained in the Credit Facility in effect on that date.
Credit Facility \| Revolving Credit Facility \| London Interbank Offered Rate (LIBOR)
Line of Credit Facility [Line Items]
Credit facility, daily fluctuating rate of interest	1.25%
8.00% Senior Secured Notes, Due August 1, 2017 \| Unitil Realty Corp
Line of Credit Facility [Line Items]
Senior Secured Notes		$ 200,000		400,000	900,000
7.15% Senior Notes, Due December 15, 2018 \| Granite State Gas Transmission Inc
Line of Credit Facility [Line Items]
Senior Notes		$ 6,700,000		$ 6,700,000	$ 10,000,000
Senior Unsecured Notes \| Private Placement \| Subsequent Event
Line of Credit Facility [Line Items]
Senior Notes			$ 90,000,000