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Commitments and Contingencies - Additional Information (Detail) $ in Millions |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 12 Months Ended |  |  |  |
--- | --- | --- | --- | --- | --- | --- | --- | --- | --- | --- | --- | --- | --- | --- | --- | --- | --- | --- | --- | ---
Dec. 20, 2017 USD ($) | Oct. 31, 2017 | Aug. 25, 2017 MW | Aug. 18, 2017 USD ($) | Aug. 01, 2017 USD ($) | Jun. 21, 2017 USD ($) | Jun. 05, 2017 USD ($) | May 31, 2017 USD ($) | Apr. 28, 2017 | Apr. 25, 2017 USD ($) | Apr. 20, 2017 USD ($) | Oct. 31, 2016 USD ($) | Aug. 19, 2016 MW | Apr. 29, 2016 USD ($) | Apr. 15, 2015 USD ($) | Dec. 31, 2017 USD ($) State Customer | Dec. 31, 2016 USD ($) | Dec. 31, 2015 | Jun. 30, 2027 MW | Dec. 31, 2022 MWh | Jun. 30, 2017 MW | Jun. 23, 2017 kW
Regulatory Assets [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Regulatory Assets, Remaining Balance |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 1.2 |  |  |  |  |  |
Accrued Revenue |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 53.3 | $ 49.5 |  |  |  |  |
Regulatory Assets |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 109.6 | $ 104.1 |  |  |  |  |
Corporate income tax rate |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 34.00% | 34.00% | 34.00% |  |  |  |
Current Portion of Regulatory Liabilities |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 9.2 | $ 10.4 |  |  |  |  |
Power capacity facility for small customers | kW |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 100
Tax Year 2018 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Regulatory Assets [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Corporate income tax rate |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 21.00% |  |  |  |  |  |
Other Restructuring |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Regulatory Assets [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Accrued Revenue |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 0.3 |  |  |  |  |  |
Regulatory Assets |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 0.9 |  |  |  |  |  |
Cost recovery period, years |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 4 years |  |  |  |  |  |
Northern Utilities Inc |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Regulatory Assets [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Increase in annual revenue |  |  |  | $ 0.7 |  |  |  | $ 6.0 |  |  |  |  |  |  |  |  |  |  |  |  |  |
Amendment effective date |  |  |  |  |  |  |  |  |  | May 01, 2017 |  |  |  |  |  |  |  |  |  |  |  |
Annual TIRA Adjustment |  |  |  |  |  |  |  |  |  | $ 1.1 |  |  |  |  |  |  |  |  |  |  |  |
TIRA initial term |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | Four years |  |  |  |  |  |
Pipeline refund received |  |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 22.0 | $ 19.7 |  |  |  |  |  |
Current Portion of Regulatory Liabilities |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 2.3 |  |  |  |  |  |
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 |  |  |  |  |  |
Northern Utilities Inc | Maine | Second TAB program |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Regulatory Assets [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Number of new customers | Customer |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 2,000 |  |  |  |  |  |
Potential regulated operating revenue |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 2.0 |  |  |  |  |  |
Northern Utilities Inc | New Hampshire |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Regulatory Assets [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Increase in annual revenue |  |  |  |  | $ 1.6 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Amendment effective date |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | Jul. 31, 2017 |  |  |  |  |  |
Deferred cost |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 0.7 |  |  |  |  |  |
Grace period to receive refund |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 3 years |  |  |  |  |  |
Northern Utilities Inc | New Hampshire | Natural Gas Distribution |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Regulatory Assets [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Increase in annual revenue |  |  |  |  |  |  | $ 4.7 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Granite State Gas Transmission Inc | Gas Transportation and Storage |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Regulatory Assets [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Increase in annual revenue |  |  |  |  |  | $ 0.2 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Amendment effective date |  |  |  |  |  | Aug. 01, 2017 |  |  |  |  |  |  |  |  |  | Jul. 28, 2017 |  |  |  |  |  |
Unitil Energy Systems Inc | New Hampshire |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Regulatory Assets [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Permanent rate increase |  |  |  |  |  |  |  |  |  |  | $ 4.1 |  |  |  |  |  |  |  |  |  |  |
Unitil Energy Systems Inc | New Hampshire | Additional Rate Step Adjustments |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Regulatory Assets [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Permanent rate increase |  |  |  |  |  |  |  |  |  |  | $ 0.9 |  |  |  |  |  |  |  |  |  |  |
Fitchburg Gas and Electric Light Company |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Regulatory Assets [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Increase in annual revenue |  |  |  |  |  |  |  |  |  |  |  | $ 0.5 |  |  |  |  |  |  |  |  |  |
Recovery amount |  |  |  |  |  |  |  |  |  |  |  | 0.9 |  |  |  |  |  |  |  |  |  |
Percentage of waiver approved on annual changes in the revenue requirement eligible for recovery |  |  |  |  |  |  |  |  | 1.50% |  |  |  |  |  |  |  |  |  |  |  |  |
Revenues sought approval to collect, deferred for recovery in future periods |  |  |  |  |  |  |  |  |  |  |  | $ 0.4 |  |  |  |  |  |  |  |  |  |
Percentage of waiver sought on annual changes in the revenue requirement eligible for recovery |  | 3.00% |  |  |  |  |  |  | 1.50% |  |  |  |  |  |  |  |  |  |  |  |  |
Number of states with electric distribution companies | State |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 3 |  |  |  |  |  |
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 | Solar Generation | Solar Massachusetts Renewable Target Program |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Regulatory Assets [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Power generation capacity | MW |  |  | 1,600 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Fitchburg Gas and Electric Light Company | Minimum | Offshore Wind Energy |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Regulatory Assets [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Power generation facility | MW |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 400 |
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 | Scenario Forecast | Offshore Wind Energy |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Regulatory Assets [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Power generation facility | MW |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 1,600 |  |  |
Fitchburg Gas and Electric Light Company | Electric base |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Regulatory Assets [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Increase in annual revenue |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 2.1 |  |  |  |  |  |  |  |
Amendment effective date | Jan. 01, 2018 |  |  |  |  |  |  |  |  |  |  |  |  | May 01, 2016 |  | Jan. 01, 2017 |  |  |  |  |  |
Annual revenue adjustment | $ 0.4 |  |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 0.5 |  |  |  |  |  |
Fitchburg Gas and Electric Light Company | Gas base |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Regulatory Assets [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Increase in annual revenue |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 1.6 |  |  |  |  |  |  |  |
Amendment effective date |  |  |  |  |  |  |  |  |  |  |  |  |  | May 01, 2016 |  |  |  |  |  |  |  |

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