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 ($)		9 Months Ended
	Oct. 04, 2013	Sep. 30, 2016	Sep. 30, 2015	Dec. 31, 2015	Oct. 30, 2015
Line of Credit Facility [Line Items]
Weighted average interest rate on short term borrowings		1.70%	1.60%	1.50%
Capital lease obligation, current fact already tagged		$ 3,100,000	$ 500,000	$ 3,100,000
Capital lease obligation, noncurrent fact already tagged		9,000,000	12,100,000	11,000,000
Accounts Payable		23,100,000	21,200,000	33,300,000
Guarantee outstanding		$ 21,100,000
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 fact already tagged		$ 2,600,000
Capital lease obligation, noncurrent fact already tagged		8,400,000
Northern Utilities Inc
Line of Credit Facility [Line Items]
Natural gas storage inventory		9,300,000	9,900,000	10,800,000
Accounts Payable		100,000	100,000	600,000
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		148,300,000	77,100,000
Repayments of lines of credit		153,400,000	102,300,000
Letters of Credit Outstanding		$ 1,100,000
Credit Facility \| Revolving Credit Facility
Line of Credit Facility [Line Items]
Revolving credit facility termination date	Oct. 04, 2020
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
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 September 30, 2016, September 30, 2015 and December 31, 2015, the Company was in compliance with the covenants contained in the Credit Facility in effect on that date. (See also “Credit Arrangements” in Note 5.)
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		$ 600,000	1,300,000	1,100,000
7.15% Senior Notes, Due December 15, 2018 \| Granite State Gas Transmission Inc
Line of Credit Facility [Line Items]
Senior Notes		$ 10,000,000	$ 10,000,000	$ 10,000,000