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Commitments and Contingencies - Additional Information (Detail) $ in Millions |  |  |  |  |  |  | 12 Months Ended |  |  |  |
--- | --- | --- | --- | --- | --- | --- | --- | --- | --- | --- | ---
Oct. 31, 2016 USD ($) | Aug. 19, 2016 MW | Jun. 28, 2016 USD ($) | Jun. 24, 2016 USD ($) | Apr. 29, 2016 USD ($) | Apr. 15, 2015 USD ($) | Dec. 31, 2016 USD ($) State Customer | Dec. 31, 2014 USD ($) | Jun. 30, 2027 MW | Dec. 31, 2022 MWh | Jun. 30, 2017 MW | Dec. 31, 2015 USD ($)
Regulatory Assets [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Regulatory Assets, Remaining Balance |  |  |  |  |  |  | $ 1.6 |  |  |  |  |
Accrued Revenue |  |  |  |  |  |  | 49.5 |  |  |  |  | $ 38.4
Regulatory Assets |  |  |  |  |  |  | 104.1 |  |  |  |  | 99.6
Current Portion of Regulatory Liabilities |  |  |  |  |  |  | 10.4 |  |  |  |  | 15.6
Regulatory Liabilities - noncurrent |  |  |  |  |  |  | 2.6 |  |  |  |  | $ 8.1
Other Restructuring |  |  |  |  |  |  |  |  |  |  |  |
Regulatory Assets [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Accrued Revenue |  |  |  |  |  |  | 0.3 |  |  |  |  |
Regulatory Assets |  |  |  |  |  |  | $ 1.3 |  |  |  |  |
Cost recovery period, years |  |  |  |  |  |  | 5 years |  |  |  |  |
Unitil Energy Systems Inc | New Hampshire |  |  |  |  |  |  |  |  |  |  |  |
Regulatory Assets [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Increase in base rates, amount |  |  |  |  | $ 6.3 |  |  |  |  |  |  |
Percent of increase in total revenue |  |  |  |  | 3.60% |  |  |  |  |  |  |
Temporary rate increase |  |  | $ 2.4 |  |  |  |  |  |  |  |  |
Amendment effective date |  |  |  |  |  |  | Jul. 01, 2016 |  |  |  |  |
Fitchburg Gas and Electric Light Company |  |  |  |  |  |  |  |  |  |  |  |
Regulatory Assets [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Amendment effective date | May 01, 2017 |  |  |  | May 01, 2016 |  |  |  |  |  |  |
Recovery amount | $ 0.9 |  |  |  | $ 0.9 |  |  |  |  |  |  |
Percentage of waiver sought on annual changes in the revenue requirement eligible for recovery | 1.50% |  |  |  |  |  |  |  |  |  |  |
Number of states with electric distribution companies | State |  |  |  |  |  |  | 3 |  |  |  |  |
Fitchburg Gas and Electric Light Company | Scenario Forecast |  |  |  |  |  |  |  |  |  |  |  |
Regulatory Assets [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Power generation capacity | MWh |  |  |  |  |  |  |  |  |  | 9,450,000 |  |
Fitchburg Gas and Electric Light Company | Electric base |  |  |  |  |  |  |  |  |  |  |  |
Regulatory Assets [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Amendment effective date |  |  |  |  | May 01, 2016 |  | Jan. 01, 2017 |  |  |  |  |
Increase in annual revenue |  |  |  |  | $ 2.1 |  |  |  |  |  |  |
Recovery amount |  |  |  |  |  |  | $ 0.5 |  |  |  |  |
Fitchburg Gas and Electric Light Company | Gas base |  |  |  |  |  |  |  |  |  |  |  |
Regulatory Assets [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Amendment effective date |  |  |  |  | May 01, 2016 |  |  |  |  |  |  |
Increase in annual revenue |  |  |  |  | $ 1.6 |  |  |  |  |  |  |
Fitchburg Gas and Electric Light Company | Solar Generation |  |  |  |  |  |  |  |  |  |  |  |
Regulatory Assets [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Power generation facility | MW |  | 1.3 |  |  |  |  |  |  |  |  |  |
Expected completion date of facility |  | 2017-11 |  |  |  |  |  |  |  |  |  |
Fitchburg Gas and Electric Light Company | Offshore Wind Energy | Scenario Forecast |  |  |  |  |  |  |  |  |  |  |  |
Regulatory Assets [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Power generation facility | MW |  |  |  |  |  |  |  |  | 1,600 |  |  |
Fitchburg Gas and Electric Light Company | Offshore Wind Energy | Scenario Forecast | Minimum |  |  |  |  |  |  |  |  |  |  |  |
Regulatory Assets [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Power generation facility | MW |  |  |  |  |  |  |  |  |  |  | 400 |
Northern Utilities Inc |  |  |  |  |  |  |  |  |  |  |  |
Regulatory Assets [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Amendment effective date |  |  |  |  |  |  | May 01, 2014 |  |  |  |  |
Increase in annual revenue |  |  |  |  |  |  | $ 1.4 |  |  |  |  |
TIRA initial term |  |  |  |  |  |  | Four years |  |  |  |  |
Amendment approved date |  |  |  |  |  |  | Apr. 28, 2016 |  |  |  |  |
Pipeline refund received |  |  |  |  |  | $ 22.0 |  |  |  |  |  |
Current Portion of Regulatory Liabilities |  |  |  |  |  |  | $ 4.4 |  |  |  |  |
Regulatory Liabilities - noncurrent |  |  |  |  |  |  | $ 2.4 |  |  |  |  |
Northern Utilities Inc | First Annual Adjustment |  |  |  |  |  |  |  |  |  |  |  |
Regulatory Assets [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Annual TIRA Adjustment |  |  |  |  |  |  |  | $ 1.5 |  |  |  |
Annual TIRA Adjustment, effective date |  |  |  |  |  |  | May 01, 2016 |  |  |  |  |
Northern Utilities Inc | New Hampshire |  |  |  |  |  |  |  |  |  |  |  |
Regulatory Assets [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Amendment effective date |  |  |  |  |  |  | May 01, 2015 |  |  |  |  |
Increase in annual revenue |  |  |  |  |  |  | $ 1.8 |  |  |  |  |
Grace period to receive refund |  |  |  |  |  |  | 3 years |  |  |  |  |
Northern Utilities Inc | New Hampshire | Natural Gas Distribution |  |  |  |  |  |  |  |  |  |  |  |
Regulatory Assets [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Increase in annual revenue |  |  |  |  |  |  | $ 4.6 |  |  |  |  |
Northern Utilities Inc | Maine |  |  |  |  |  |  |  |  |  |  |  |
Regulatory Assets [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Construction completion period |  |  |  |  |  |  | 3 years |  |  |  |  |
Number of new customers | Customer |  |  |  |  |  |  | 1,000 |  |  |  |  |
Potential regulated operating revenue |  |  |  |  |  |  | $ 1.0 |  |  |  |  |
Grace period to receive refund |  |  |  |  |  |  | 3 years |  |  |  |  |
Date to receive a refund as one-time lump sum payment |  |  |  |  |  |  | Oct. 05, 2015 |  |  |  |  |
Granite State Gas Transmission Inc | Gas Transportation and Storage |  |  |  |  |  |  |  |  |  |  |  |
Regulatory Assets [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Amendment effective date |  |  |  | Aug. 01, 2016 |  |  |  |  |  |  |  |
Increase in annual revenue |  |  |  | $ 0.3 |  |  |  |  |  |  |  |
Amendment approved date |  |  |  | Jul. 13, 2016 |  |  |  |  |  |  |  |

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