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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 |

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