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 SEC

-Name Regulation S-X (SX)

-Number 210

-Section 02

-Paragraph 19

-Subparagraph a

-Article 5

Reference 2: 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 $) In Millions, unless otherwise specified	0 Months Ended	9 Months Ended						0 Months Ended		0 Months Ended	9 Months Ended				0 Months Ended									0 Months Ended
	Sep. 30, 2014	Sep. 30, 2014	Sep. 30, 2013	Dec. 31, 2013	Sep. 30, 2014 Northern Utilities Inc	Dec. 31, 2013 Northern Utilities Inc	Sep. 30, 2013 Northern Utilities Inc	Oct. 15, 2014 Northern Utilities Inc Subsequent Event	Apr. 30, 2014 Capital Lease Obligations	Oct. 04, 2013 Revolving Credit Facility	Sep. 30, 2014 Revolving Credit Facility	Dec. 31, 2013 Revolving Credit Facility	Oct. 04, 2013 Revolving Credit Facility	Sep. 30, 2013 Revolving Credit Facility	Oct. 04, 2013 Revolving Credit Facility London Interbank Offered Rate	Sep. 30, 2014 8.00% Senior Secured Notes, Due Through August 1, 2017 Unitil Realty Corp	Dec. 31, 2013 8.00% Senior Secured Notes, Due Through August 1, 2017 Unitil Realty Corp	Sep. 30, 2013 8.00% Senior Secured Notes, Due Through August 1, 2017 Unitil Realty Corp	Sep. 30, 2014 8.00% Senior Secured Notes, Due Through August 1, 2017 Unitil Realty Corp Debt Outstanding, Principal Amount	Sep. 30, 2014 7.15% Senior Notes, Due December 15, 2018 Granite State Gas Transmission Inc	Dec. 31, 2013 7.15% Senior Notes, Due December 15, 2018 Granite State Gas Transmission Inc	Sep. 30, 2013 7.15% Senior Notes, Due December 15, 2018 Granite State Gas Transmission Inc	Sep. 30, 2014 7.15% Senior Notes, Due December 15, 2018 Granite State Gas Transmission Inc Debt Outstanding, Principal Amount	Oct. 15, 2014 4.42% Senior Unsecured Notes due October 15, 2044 Northern Utilities Inc Subsequent Event	Oct. 15, 2014 4.42% Senior Unsecured Notes due October 15, 2044 Northern Utilities Inc Subsequent Event
Line of Credit Facility [Line Items]
Long-term debt, aggregate principal amount																									$ 50
Long-term debt, stated interest rate																8.00%	8.00%	8.00%		7.15%	7.15%	7.15%			4.42%
Long-term debt, maturity date																								Oct. 15, 2044
Repayment of short-term debt		16.5	5.8					42.3
Reclassification of short-term debt to long-term debt		42.3
Revolving credit facility expiration date										Oct. 04, 2018
Line of credit facility, maximum borrowing capacity									15.0		120.0	120.0	120.0	60.0
Sublimit for the issuance of standby letters of credit													25
Credit facility, daily fluctuating rate of interest															1.375%
Credit Facility by an aggregate additional amount													30
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, 2014, the Company was in compliance with the covenants contained in the Credit Facility in effect on that date.
Proceeds from capital lease obligation	6.0
Natural gas storage inventory					13.6	12.5	11.8
Accounts Payable		19.3	21.5	38.1	0.2	2.7	0
Guarantee outstanding		29.7
Senior Secured Notes																1.9	2.3	2.5	1.9
Senior Notes																				$ 10.0	$ 10.0	$ 10.0	$ 10.0