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Commitments and Contingencies - Additional Information (Detail) - USD ($) |  |  |  |  |  |  |  |  | 3 Months Ended | 6 Months Ended | 12 Months Ended |
--- | --- | --- | --- | --- | --- | --- | --- | --- | --- | --- | --- | ---
Jun. 16, 2015 | Apr. 15, 2015 | Nov. 26, 2014 | May. 30, 2014 | Apr. 30, 2014 | Apr. 21, 2014 | Dec. 27, 2013 | Apr. 25, 2013 | Dec. 31, 2015 | Jun. 30, 2015 | Dec. 31, 2015 | Dec. 31, 2014
Regulatory Assets [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Regulatory Assets, Remaining Balance |  |  |  |  |  |  |  |  | $ 1,900,000 |  | $ 1,900,000 |
Accrued Revenue |  |  |  |  |  |  |  |  | 38,400,000 |  | 38,400,000 | $ 48,500,000
Regulatory Assets |  |  |  |  |  |  |  |  | 99,600,000 |  | 99,600,000 | 107,600,000
Current Portion of Regulatory Liabilities |  |  |  |  |  |  |  |  | 15,600,000 |  | 15,600,000 | 8,700,000
Regulatory Liabilities - noncurrent |  |  |  |  |  |  |  |  | 8,100,000 |  | $ 8,100,000 | $ 100,000
Maine | Environmental Restoration Costs |  |  |  |  |  |  |  |  |  |  |  |
Regulatory Assets [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Amortization period for environmental costs |  |  |  |  |  |  |  |  |  |  | 5 years |
New Hampshire | Environmental Restoration Costs |  |  |  |  |  |  |  |  |  |  |  |
Regulatory Assets [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Amortization period for environmental costs |  |  |  |  |  |  |  |  |  |  | 7 years |
Other Restructuring |  |  |  |  |  |  |  |  |  |  |  |
Regulatory Assets [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Accrued Revenue |  |  |  |  |  |  |  |  | 300,000 |  | $ 300,000 |
Regulatory Assets |  |  |  |  |  |  |  |  | 1,600,000 |  | $ 1,600,000 |
Cost recovery period, years |  |  |  |  |  |  |  |  |  |  | 6 years |
Environmental Matters |  |  |  |  |  |  |  |  |  |  |  |
Regulatory Assets [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Income (loss) incurred in excess of recorded amount |  |  |  |  |  |  |  |  |  |  | $ 0 |
Northern Utilities Inc |  |  |  |  |  |  |  |  |  |  |  |
Regulatory Assets [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Increase in annual revenue |  |  |  |  |  | $ 1,400,000 |  |  |  |  |  |
Amendment effective date |  |  |  |  |  |  |  |  |  |  | May 01, 2014 |
TIRA initial term |  |  |  |  |  |  |  |  |  |  | Four years |
First annual TIRA Adjustment, effective date |  |  |  |  |  |  | May 01, 2014 |  |  |  |  |
Pipeline refund received |  | $ 22,000,000 |  |  |  |  |  |  |  |  |  |
Current Portion of Regulatory Liabilities |  |  |  |  |  |  |  |  | 7,600,000 |  | $ 7,600,000 |
Regulatory Liabilities - noncurrent |  |  |  |  |  |  |  |  | 8,100,000 |  | $ 8,100,000 |
Northern Utilities Inc | Maine |  |  |  |  |  |  |  |  |  |  |  |
Regulatory Assets [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Increase in annual revenue |  |  |  |  |  |  | $ 3,800,000 |  |  |  |  |
Amendment effective date |  |  |  |  |  |  |  |  |  |  | Jan. 01, 2014 |
Grace period to receive refund |  |  |  |  |  |  |  |  |  |  | 3 years |
Date to receive a refund as one-time lump sum payment |  |  |  |  |  |  |  |  |  |  | Oct. 05, 2015 |
Net gain on sale of property |  |  |  |  |  |  |  |  | 900,000 |  |  |
Northern Utilities Inc | New Hampshire |  |  |  |  |  |  |  |  |  |  |  |
Regulatory Assets [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Increase in annual revenue |  |  |  |  |  | 1,800,000 |  |  |  |  |  |
Amendment effective date |  |  |  |  |  |  |  |  |  |  | May 01, 2015 |
Grace period to receive refund |  |  |  |  |  |  |  |  |  |  | 3 years |
Northern Utilities Inc | New Hampshire | Natural Gas Distribution |  |  |  |  |  |  |  |  |  |  |  |
Regulatory Assets [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Increase in annual revenue |  |  |  |  |  | $ 4,600,000 |  |  |  |  |  |
Unitil Energy Systems Inc |  |  |  |  |  |  |  |  |  |  |  |
Regulatory Assets [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Amendment effective date |  |  |  |  |  |  |  |  |  |  | May 01, 2014 |
Date to receive a refund as one-time lump sum payment |  |  |  |  | May 01, 2016 |  |  |  |  |  |  |
Adjustment to recover the increased spending for vegetation management program and reliability enhancement program and a proposed storm resiliency program |  |  |  |  | $ 1,500,000 |  |  |  |  |  |  |
Interest rate allowed by regulators on the unrecovered amount of storm costs which have been approved for recovery through rate adjustments |  |  |  |  |  |  |  | 4.52% |  |  |  |
Construction expenditures |  |  | $ 400,000 |  |  |  |  |  |  |  |  |
Recovery of damage of electrical system |  |  | 1,700,000 |  |  |  |  |  |  |  |  |
Unitil Energy Systems Inc | Storm Costs |  |  |  |  |  |  |  |  |  |  |  |
Regulatory Assets [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Cost recovery period, years |  |  |  |  |  |  |  | 5 years |  |  |  |
Recovery amount |  |  |  |  |  |  |  | $ 2,300,000 |  |  |  |
Storm expenditures |  |  | 2,100,000 |  |  |  |  |  |  |  |  |
Granite State Gas Transmission Inc | Gas Transportation and Storage |  |  |  |  |  |  |  |  |  |  |  |
Regulatory Assets [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Increase in annual revenue |  |  |  |  |  |  |  |  |  | $ 400,000 |  |
Amendment effective date |  |  |  |  |  |  |  |  |  | Aug. 01, 2015 |  |
Fitchburg Gas and Electric Light Company |  |  |  |  |  |  |  |  |  |  |  |
Regulatory Assets [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Cost recovery period, years |  |  |  | 5 years |  |  |  |  |  |  |  |
Increase in annual revenue |  |  |  | $ 5,600,000 |  |  |  |  |  |  |  |
Amendment effective date |  |  |  |  |  |  |  |  |  |  | May 01, 2016 |
Percentage of approved return on equity |  |  |  | 9.70% |  |  |  |  |  |  |  |
Percentage of approved common equity ratio |  |  |  | 48.00% |  |  |  |  |  |  |  |
Annual funding amount approved |  |  |  | $ 500,000 |  |  |  |  |  |  |  |
Recovery amount |  |  |  | $ 900,000 |  |  |  |  |  |  |  |
Storm expenditures |  |  | 200,000 |  |  |  |  |  |  |  |  |
Construction expenditures |  |  | 100,000 |  |  |  |  |  |  |  |  |
Fitchburg Gas and Electric Light Company | Amendment Agreement [Member] |  |  |  |  |  |  |  |  |  |  |  |
Regulatory Assets [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Amendment effective date |  |  |  |  |  |  |  |  |  |  | Jun. 01, 2014 |
Fitchburg Gas and Electric Light Company | Cumulative capital investments for April 30, 2015 |  |  |  |  |  |  |  |  |  |  |  |
Regulatory Assets [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Recovery amount |  |  |  |  |  |  |  |  | 300,000 |  | $ 300,000 |
Fitchburg Gas and Electric Light Company | Cumulative capital investments for 2015 and 2016 |  |  |  |  |  |  |  |  |  |  |  |
Regulatory Assets [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Recovery amount |  |  |  |  |  |  |  |  | $ 900,000 |  | $ 900,000 |
Fitchburg Gas and Electric Light Company | Electric base |  |  |  |  |  |  |  |  |  |  |  |
Regulatory Assets [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Increase in annual revenue | $ 3,800,000 |  |  |  |  |  |  |  |  |  |  |
Percentage of approved return on equity | 5.60% |  |  |  |  |  |  |  |  |  |  |
Fitchburg Gas and Electric Light Company | Gas base |  |  |  |  |  |  |  |  |  |  |  |
Regulatory Assets [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Increase in annual revenue | $ 3,000,000 |  |  |  |  |  |  |  |  |  |  |
Percentage of approved return on equity | 8.30% |  |  |  |  |  |  |  |  |  |  |
Fitchburg Gas and Electric Light Company | Storm Costs |  |  |  |  |  |  |  |  |  |  |  |
Regulatory Assets [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Cost recovery period, years |  |  |  | 3 years |  |  |  |  |  |  |  |
Deferred emergency storm repair costs incurred |  |  |  | $ 5,000,000 |  |  |  |  |  |  |  |
Storm expenditures |  |  | $ 300,000 | $ 5,000,000 |  |  |  |  |  |  |  |

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