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).

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 210

-SubTopic 10

-Section S99

-Paragraph 1

-Subparagraph (SX 210.5-02.19(a))

-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

Debt and Financing Arrangements - Additional Information (Detail) - USD ($)		6 Months Ended
	Oct. 04, 2013	Jun. 30, 2015	Dec. 31, 2014	Oct. 15, 2014	Jun. 30, 2014	Apr. 30, 2014
Line of Credit Facility [Line Items]
Proceeds from capital lease obligation		$ 9,100,000
Accounts Payable		19,000,000	$ 44,200,000		$ 20,500,000
Guarantee outstanding		20,100,000
Capital Lease Obligations
Line of Credit Facility [Line Items]
Revolving credit facility						$ 15,000,000
Northern Utilities Inc
Line of Credit Facility [Line Items]
Natural gas storage inventory		6,800,000	15,100,000		7,800,000
Accounts Payable		100,000	1,000,000		200,000
Revolving Credit Facility
Line of Credit Facility [Line Items]
Revolving credit facility expiration date	Oct. 04, 2018
Revolving credit facility	$ 120,000,000	$ 120,000,000	$ 120,000,000		$ 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 June 30, 2015, June 30, 2014 and December 31, 2014, the Company was in compliance with the covenants contained in the Credit Facility in effect on that date.
Revolving Credit Facility \| London Interbank Offered Rate
Line of Credit Facility [Line Items]
Credit facility, daily fluctuating rate of interest	1.375%
4.42% Senior Unsecured Notes due October 15, 2044 \| Northern Utilities Inc
Line of Credit Facility [Line Items]
Long-term debt, aggregate principal amount				$ 50,000,000
Long-term debt, stated interest rate				4.42%
Long-term debt, maturity date		Oct. 15, 2044
8.00% Senior Secured Notes, Due August 1, 2017 \| Unitil Realty Corp
Line of Credit Facility [Line Items]
Long-term debt, stated interest rate		8.00%	8.00%		8.00%
Senior Secured Notes		$ 1,500,000	$ 1,700,000		$ 2,100,000
7.15% Senior Notes, Due December 15, 2018 \| Granite State Gas Transmission Inc
Line of Credit Facility [Line Items]
Long-term debt, stated interest rate		7.15%	7.15%		7.15%
Senior Notes		$ 10,000,000	$ 10,000,000		$ 10,000,000