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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Maximum borrowing capacity under the credit facility without consideration of any current restrictions on the amount that could be borrowed or the amounts currently outstanding under the facility.

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Long Term Debt, Credit Arrangements and Guarantees - Additional Information (Detail) (USD $) In Millions, unless otherwise specified							9 Months Ended
	Sep. 30, 2012	Dec. 31, 2011	Sep. 30, 2011	Sep. 30, 2012 Northern Utilities Inc	Dec. 31, 2011 Northern Utilities Inc	Sep. 30, 2011 Northern Utilities Inc	Sep. 30, 2012 Revolving Facility	Dec. 31, 2011 Revolving Facility	Sep. 30, 2011 Revolving Facility	Jul. 24, 2012 Revolving Facility Scenario, Forecast	Sep. 30, 2012 Senior Secured Notes Eight Point Zero Zero Percent Due Through August One Twenty Seventeen Unitil Realty Corp	Sep. 30, 2012 Senior Notes Seven Point One Five Percent Due December Fifteen Twenty Eighteen Granite State Gas Transmission Inc
Line of Credit Facility [Line Items]
Revolving credit facility expiration date							Oct. 08, 2013
Revolving credit facility							$ 60.0	$ 115.0	$ 80.0	$ 115.0
Reduced Borrowing Limit										60
Percentage of capitalization							The revolving credit agreement also contains a covenant restricting the Company's ability to permit funded debt to exceed 65% of capitalization at the end of each fiscal quarter. As of September 30,2012, September 30, 2011 and December 31, 2011, the Company was in compliance with the financial covenants contained in the revolving credit agreement.
Outstanding Inventory				10.6	14.9	13.0
Accounts Payable	17.1	26.4	16.0	0	2.5	0
Guarantee outstanding	20.6
Guarantee end date	Feb. 28, 2014
Principal amount of notes outstanding											$ 3.0	$ 10.0