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Long Term Debt, Credit Arrangements and Guarantees - Additional Information (Detail) (USD $) In Millions, unless otherwise specified |  |  |  |  |  |  | 6 Months Ended |  |  |  |  |
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Jun. 30, 2012 | Dec. 31, 2011 | Jun. 30, 2011 | Jun. 30, 2012 Northern Utilities Inc | Dec. 31, 2011 Northern Utilities Inc | Jun. 30, 2011 Northern Utilities Inc | Jun. 30, 2012 Revolving Facility | Dec. 31, 2011 Revolving Facility | Dec. 30, 2011 Revolving Facility | Jun. 30, 2011 Revolving Facility | Jul. 24, 2012 Revolving Facility Scenario, Forecast | Jun. 30, 2012 Senior Secured Notes Eight Point Zero Zero Percent Due Through August One Twenty Seventeen Unitil Realty Corp | Jun. 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 |  |  |  |  |  |  | $ 115.0 |  | $ 115.0 | $ 80.0 | $ 115.0 |  |
Reduced Borrowing Limit |  |  |  |  |  |  |  |  |  |  | 60 |  |
Percentage of capitalization |  |  |  |  |  |  | 65.00% | 65.00% |  | 65.00% |  |  |
Outstanding Inventory |  |  |  | 8.0 | 14.9 | 6.6 |  |  |  |  |  |  |
Accounts Payable | 15.9 | 26.4 | 15.8 | 0 | 2.5 | 0 |  |  |  |  |  |  |
Guarantee outstanding | 20.6 |  |  |  |  |  |  |  |  |  |  |  |
Guarantee end date | Feb. 28, 2014 |  |  |  |  |  |  |  |  |  |  |  |
Principal amount of notes outstanding |  |  |  |  |  |  |  |  |  |  |  | $ 3.1 | $ 10.0

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