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Debt and Financing Arrangements - Additional Information (Detail) - USD ($) |  | 9 Months Ended |  |
--- | --- | --- | --- | ---
Oct. 04, 2013 | Sep. 30, 2016 | Sep. 30, 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 |  | 9,000,000 | 12,100,000 | 11,000,000 |
Accounts Payable |  | 23,100,000 | 21,200,000 | 33,300,000 |
Guarantee outstanding |  | $ 21,100,000 |  |  |
Unitil Service Corp. |  |  |  |  |
Line of Credit Facility [Line Items] |  |  |  |  |
Capital lease obligation, total capitalized cost |  |  |  |  | $ 13,400,000
Capital lease obligation, maturity period |  | Oct. 30, 2015 |  |  |
Capital lease obligation, current fact already tagged |  | $ 2,600,000 |  |  |
Capital lease obligation, noncurrent fact already tagged |  | 8,400,000 |  |  |
Northern Utilities Inc |  |  |  |  |
Line of Credit Facility [Line Items] |  |  |  |  |
Natural gas storage inventory |  | 9,300,000 | 9,900,000 | 10,800,000 |
Accounts Payable |  | 100,000 | 100,000 | 600,000 |
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 |  | 148,300,000 | 77,100,000 |  |
Repayments of lines of credit |  | 153,400,000 | 102,300,000 |  |
Letters of Credit Outstanding |  | $ 1,100,000 |  |  |
Credit Facility | Revolving Credit Facility |  |  |  |  |
Line of Credit Facility [Line Items] |  |  |  |  |
Revolving credit facility termination date | Oct. 04, 2020 |  |  |  |
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 September 30, 2016, September 30, 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 | 1.25% |  |  |  |
8.00% Senior Secured Notes, Due August 1, 2017 | Unitil Realty Corp |  |  |  |  |
Line of Credit Facility [Line Items] |  |  |  |  |
Senior Secured Notes |  | $ 600,000 | 1,300,000 | 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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- Definition Amount equal to the present value (the principal) at the beginning of the lease term of minimum lease payments during the lease term (excluding that portion of the payments representing executory costs such as insurance, maintenance, and taxes to be paid by the lessor, together with any profit thereon) net of payments or other amounts applied to the principal through the balance sheet date. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 840 -SubTopic 30 -Section 50 -Paragraph 1 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=6455398&loc=d3e45280-112737 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.5-02(22)) -URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 210 -Section S99 -Paragraph 1 -Subparagraph (SX 210.7-03(a)(16)) -URI http://asc.fasb.org/extlink&oid=6879938&loc=d3e572229-122910 + Details
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