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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 |
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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 |  |

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- Definition Corporate guarantees on energy and natural gas storage management contracts entered into by the Company's distribution utilities. + References No definition available. + Details
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