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Debt and Financing Arrangements - Additional Information (Detail) - USD ($) |  |  | 3 Months Ended |  |
--- | --- | --- | --- | --- | ---
Jul. 24, 2015 | Oct. 04, 2013 | Mar. 31, 2016 | Mar. 31, 2015 | Dec. 31, 2015 | Oct. 30, 2015
Line of Credit Facility [Line Items] |  |  |  |  |  |
Weighted average interest rate on short term borrowings |  |  | 1.70% | 1.60% | 1.50% |
Capital lease obligation, current fact already tagged |  |  | $ 3,100,000 | $ 500,000 | $ 3,100,000 |
Capital lease obligation, noncurrent fact already tagged |  |  | 10,400,000 | 8,200,000 | 11,000,000 |
Accounts Payable |  |  | 27,000,000 | 34,800,000 | 33,300,000 |
Guarantee outstanding |  |  | 18,600,000 |  |  |
Northern Utilities Inc |  |  |  |  |  |
Line of Credit Facility [Line Items] |  |  |  |  |  |
Natural gas storage inventory |  |  | 6,700,000 | 5,600,000 | 10,800,000 |
Accounts Payable |  |  | $ 400,000 | 2,000,000 | 600,000 |
Unitil Service Corp. |  |  |  |  |  |
Line of Credit Facility [Line Items] |  |  |  |  |  |
Capital lease obligation, total capitalized cost |  |  |  |  |  | $ 13,400,000
Capital lease obligation, maturity period |  |  | Sep. 30, 2020 |  |  |
Capital lease obligation, current fact already tagged |  |  | $ 2,600,000 |  |  |
Capital lease obligation, noncurrent fact already tagged |  |  | 9,800,000 |  |  |
Revolving Credit Facility |  |  |  |  |  |
Line of Credit Facility [Line Items] |  |  |  |  |  |
Revolving credit facility |  |  | 120,000,000 | 120,000,000 | 120,000,000 |
Proceeds from lines of credit |  |  | 55,900,000 | 70,800,000 |  |
Repayments of lines of credit |  |  | $ 50,000,000 | 68,100,000 |  |
Credit Facility | Revolving Credit Facility |  |  |  |  |  |
Line of Credit Facility [Line Items] |  |  |  |  |  |
Revolving credit facility expiration date |  | Oct. 04, 2018 |  |  |  |
Revolving credit facility |  | $ 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 March 31, 2016, March 31, 2015 and December 31, 2015, the Company was in compliance with the covenants contained in the Credit Facility in effect on that date. (See also “Credit Arrangements” in Note 5.) |  |  |
Credit Facility | Revolving Credit Facility | London Interbank Offered Rate (LIBOR) |  |  |  |  |  |
Line of Credit Facility [Line Items] |  |  |  |  |  |
Credit facility, daily fluctuating rate of interest |  | 1.375% |  |  |  |
Credit Facility | Amended Credit Facility |  |  |  |  |  |
Line of Credit Facility [Line Items] |  |  |  |  |  |
Revolving credit facility expiration date | Oct. 04, 2020 |  |  |  |  |
Credit Facility | Amended Credit Facility | London Interbank Offered Rate (LIBOR) |  |  |  |  |  |
Line of Credit Facility [Line Items] |  |  |  |  |  |
Credit facility, daily fluctuating rate of interest | 1.25% |  |  |  |  |
8.00% Senior Secured Notes, Due August 1, 2017 | Unitil Realty Corp |  |  |  |  |  |
Line of Credit Facility [Line Items] |  |  |  |  |  |
Senior Secured Notes |  |  | $ 900,000 | 1,500,000 | 1,100,000 |
7.15% Senior Notes, Due December 15, 2018 | Granite State Gas Transmission Inc |  |  |  |  |  |
Line of Credit Facility [Line Items] |  |  |  |  |  |
Senior Notes |  |  | $ 10,000,000 | $ 10,000,000 | $ 10,000,000 |

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