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Regulatory Matters - Additional Information (Detail) $ in Millions |  |  |  |  |  |  |  | 9 Months Ended | 12 Months Ended |  |  |  |  |
--- | --- | --- | --- | --- | --- | --- | --- | --- | --- | --- | --- | --- | --- | ---
Aug. 19, 2016 MW | Jun. 28, 2016 USD ($) | Jun. 24, 2016 USD ($) | Apr. 29, 2016 USD ($) | Jun. 16, 2015 USD ($) | May 01, 2015 USD ($) | Apr. 15, 2015 USD ($) | Sep. 30, 2016 USD ($) State Customer | Dec. 31, 2014 USD ($) | Jun. 30, 2027 MW | Dec. 31, 2022 MWh | Jun. 30, 2017 MW | Dec. 31, 2015 USD ($) | Sep. 30, 2015 USD ($)
Regulatory Assets [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Current Portion of Regulatory Liabilities |  |  |  |  |  |  |  | $ 13.2 |  |  |  |  | $ 15.6 | $ 28.1
Regulatory Liabilities - non current |  |  |  |  |  |  |  | $ 3.7 |  |  |  |  | $ 8.1 | $ 11.0
Scenario Forecast |  |  |  |  |  |  |  |  |  |  |  |  |  |
Regulatory Assets [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Power generation capacity | MWh |  |  |  |  |  |  |  |  |  |  | 9,450,000 |  |  |
Fitchburg-Solar Generation |  |  |  |  |  |  |  |  |  |  |  |  |  |
Regulatory Assets [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Power generation facility | MW | 1.3 |  |  |  |  |  |  |  |  |  |  |  |  |
Expected completion date of facility | 2017-11 |  |  |  |  |  |  |  |  |  |  |  |  |
Offshore Wind Energy | Scenario Forecast |  |  |  |  |  |  |  |  |  |  |  |  |  |
Regulatory Assets [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Power generation facility | MW |  |  |  |  |  |  |  |  |  | 1,600 |  | 400 |  |
Fitchburg Gas and Electric Light Company |  |  |  |  |  |  |  |  |  |  |  |  |  |
Regulatory Assets [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Amendment effective date |  |  |  | May 01, 2016 |  |  |  |  |  |  |  |  |  |
Revenue requirement reconciled rates effective date |  |  |  | Nov. 01, 2016 |  |  |  |  |  |  |  |  |  |
Number of states with electric distribution companies | State |  |  |  |  |  |  |  | 3 |  |  |  |  |  |
Fitchburg Gas and Electric Light Company | Cumulative capital investments for 2015 and 2016 |  |  |  |  |  |  |  |  |  |  |  |  |  |
Regulatory Assets [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Recovery amount |  |  |  | $ 0.9 |  |  |  |  |  |  |  |  |  |
Fitchburg Gas and Electric Light Company | Electric base |  |  |  |  |  |  |  |  |  |  |  |  |  |
Regulatory Assets [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Amendment effective date |  |  |  | May 01, 2016 |  |  |  |  |  |  |  |  |  |
Increase in annual revenue |  |  |  | $ 2.1 | $ 3.8 |  |  |  |  |  |  |  |  |
Percentage of approved return on equity |  |  |  |  | 5.60% |  |  |  |  |  |  |  |  |
Fitchburg Gas and Electric Light Company | Gas base |  |  |  |  |  |  |  |  |  |  |  |  |  |
Regulatory Assets [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Amendment effective date |  |  |  | May 01, 2016 |  |  |  |  |  |  |  |  |  |
Increase in annual revenue |  |  |  | $ 1.6 | $ 3.0 |  |  |  |  |  |  |  |  |
Percentage of approved return on equity |  |  |  |  | 8.30% |  |  |  |  |  |  |  |  |
Northern Utilities Inc |  |  |  |  |  |  |  |  |  |  |  |  |  |
Regulatory Assets [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |
TIRA initial term |  |  |  |  |  |  |  | Four years |  |  |  |  |  |
Amendment approved date |  |  |  |  |  |  |  | Apr. 28, 2016 |  |  |  |  |  |
Pipeline refund received |  |  |  |  |  |  | $ 22.0 |  |  |  |  |  |  |
Current Portion of Regulatory Liabilities |  |  |  |  |  |  |  | $ 5.0 |  |  |  |  |  |
Regulatory Liabilities - non current |  |  |  |  |  |  |  | $ 3.5 |  |  |  |  |  |
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 | 2014 Step Adjustments |  |  |  |  |  |  |  |  |  |  |  |  |  |
Regulatory Assets [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Amendment effective date |  |  |  |  |  |  |  | May 01, 2014 |  |  |  |  |  |
Increase in annual revenue |  |  |  |  |  |  |  | $ 1.4 |  |  |  |  |  |
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 |  |  |  |  |  |
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 |  |  |  |  |  |
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, 2017 |  |  |  |  |  |  |  |  |  |  |
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 |  |  |  |  |  |

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