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Debt and Financing Arrangements - Additional Information (Detail) - USD ($) |  |  | 12 Months Ended |  |
--- | --- | --- | --- | --- | ---
Jul. 24, 2015 | Oct. 04, 2013 | Dec. 31, 2015 | Dec. 31, 2014 | Dec. 31, 2013 | Oct. 30, 2015 | Oct. 15, 2014
Line of Credit Facility [Line Items] |  |  |  |  |  |  |
Amount available for the payment of dividends |  |  | $ 194,600,000 |  |  |  |
Retained earnings |  |  | $ 45,100,000 | $ 38,400,000 |  |  |
Restriction on retained earnings for dividend payments |  |  | There were no restrictions on the Company's Retained Earnings at December 31, 2015 for the payment of dividends |  |  |  |
Long term debt repayments |  |  | $ 7,400,000 | $ 4,400,000 | $ 500,000 |  |
Weighted average interest rate on short term borrowings |  |  | 1.50% | 1.60% | 1.80% |  |
Capital lease obligation, total capitalized cost |  |  |  |  |  | $ 13,400,000 |
Capital lease obligation, maturity period |  |  | Sep. 30, 2020 |  |  |  |
Accounts Payable |  |  | $ 33,300,000 | $ 44,200,000 |  |  |
Total rental expense under operating leases |  |  | 1,700,000 | 1,300,000 | $ 1,200,000 |  |
Net Utility Plant |  |  | 808,900,000 | 733,700,000 |  |  |
Capital lease obligations, current |  |  | 3,100,000 | 500,000 |  |  |
Capital lease obligations, Noncurrent |  |  | 11,000,000 | 7,500,000 |  |  |
Guarantee outstanding |  |  | 19,300,000 |  |  |  |
Assets under Capital Leases |  |  |  |  |  |  |
Line of Credit Facility [Line Items] |  |  |  |  |  |  |
Net Utility Plant |  |  | 15,300,000 | 9,700,000 |  |  |
Net Utility Plant, accumulated amortization |  |  | $ 800,000 | 800,000 |  |  |
Unitil Corporation | Maximum |  |  |  |  |  |  |
Line of Credit Facility [Line Items] |  |  |  |  |  |  |
Total funded indebtedness as percentage of capitalization |  |  | 70.00% |  |  |  |
Fitchburg Gas and Electric Light Company |  |  |  |  |  |  |
Line of Credit Facility [Line Items] |  |  |  |  |  |  |
Total funded indebtedness as percentage of capitalization |  |  | 65.00% |  |  |  |
Northern Utilities Inc |  |  |  |  |  |  |
Line of Credit Facility [Line Items] |  |  |  |  |  |  |
Total funded indebtedness as percentage of capitalization |  |  | 65.00% |  |  |  |
Natural gas storage inventory |  |  | $ 10,800,000 | 15,100,000 |  |  |
Accounts Payable |  |  | $ 600,000 | 1,000,000 |  |  |
Granite State Gas Transmission Inc |  |  |  |  |  |  |
Line of Credit Facility [Line Items] |  |  |  |  |  |  |
Total funded indebtedness as percentage of capitalization |  |  | 65.00% |  |  |  |
Unitil Energy, Fitchburg, Northern Utilities and Granite State |  |  |  |  |  |  |
Line of Credit Facility [Line Items] |  |  |  |  |  |  |
Amount available for the payment of dividends |  |  | $ 163,100,000 |  |  |  |
Financing Arrangements |  |  |  |  |  |  |
Line of Credit Facility [Line Items] |  |  |  |  |  |  |
Capital lease obligations, current |  |  | 2,600,000 |  |  |  |
Capital lease obligations, Noncurrent |  |  | 10,400,000 |  |  |  |
Revolving Credit Facility |  |  |  |  |  |  |
Line of Credit Facility [Line Items] |  |  |  |  |  |  |
Revolving credit facility |  |  | 120,000,000 | 120,000,000 |  |  |
Proceeds from lines of credit |  |  | 140,300,000 | 179,400,000 |  |  |
Repayments of lines of credit |  |  | $ 127,600,000 | $ 210,300,000 |  |  |
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% |  |  |
Long-term debt, maturity date |  |  | Aug. 01, 2017 | Aug. 01, 2017 |  |  |
Senior Secured Notes |  |  | $ 1,100,000 | $ 1,700,000 |  |  |
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% |  |  |
Long-term debt, maturity date |  |  | Dec. 15, 2018 | Dec. 15, 2018 |  |  |
Senior Notes |  |  | $ 10,000,000 | $ 10,000,000 |  |  |
Bonds |  |  |  |  |  |  |
Line of Credit Facility [Line Items] |  |  |  |  |  |  |
Long term debt repayments |  |  | 7,400,000 | $ 4,400,000 | $ 500,000 |  |
Debt repayment, 2016 |  |  | 17,400,000 |  |  |  |
Debt repayment, 2017 |  |  | 17,200,000 |  |  |  |
Debt repayment, 2018 |  |  | 30,100,000 |  |  |  |
Debt repayment, 2019 |  |  | 18,800,000 |  |  |  |
Debt repayment, 2020 |  |  | 19,800,000 |  |  |  |
Debt repayment, Thereafter |  |  | $ 222,200,000 |  |  |  |
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 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. |  |  |  |
Credit Facility | Revolving Credit Facility | London Interbank Offered Rate |  |  |  |  |  |  |
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 |  |  |  |  |  |  |
Line of Credit Facility [Line Items] |  |  |  |  |  |  |
Credit facility, daily fluctuating rate of interest | 1.25% |  |  |  |  |  |

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