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