Regulatory Matters - Additional Information (Detail)																	1 Months Ended	3 Months Ended
	May 01, 2019 USD ($)	May 01, 2018 USD ($)	Apr. 06, 2018 USD ($)	Mar. 16, 2018 USD ($)	Feb. 28, 2018 USD ($)	Dec. 20, 2017 USD ($)	Oct. 31, 2017	Aug. 25, 2017 MW	Jul. 31, 2017 USD ($)	Jun. 21, 2017 USD ($)	Jun. 05, 2017 USD ($)	Apr. 28, 2017	Apr. 20, 2017 USD ($)	Oct. 31, 2016 USD ($)	Apr. 29, 2016 USD ($)	Apr. 15, 2015 USD ($)	May 31, 2017 USD ($)	Mar. 31, 2018 USD ($) State Customer	Jun. 30, 2027 MW	Dec. 31, 2022 MWh	Jun. 01, 2018 USD ($)	Dec. 31, 2017 USD ($)	Jun. 30, 2017 MW	Jun. 23, 2017 kW	Mar. 31, 2017 USD ($)	Aug. 19, 2016 MW
Regulatory Assets [Line Items]
Corporate income tax rate																		21.00%
Current Portion of Regulatory Liabilities																		$ 10,900,000				$ 9,200,000			$ 12,800,000
Power capacity facility for small customers | kW																								100
Northern Utilities Inc
Regulatory Assets [Line Items]
Increase (decrease) in annual revenue									$ 1,600,000								$ 6,000,000	$ 700,000
Amendment effective date									Aug. 01, 2017
TIRA initial term																		Four years
Deferred cost																		$ 1,100,000
Pipeline refund received																$ 22,000,000		21,400,000
Current Portion of Regulatory Liabilities																		600,000
Northern Utilities Inc | Subsequent Event
Regulatory Assets [Line Items]
Increase (decrease) in annual revenue			$ 2,600,000
Northern Utilities Inc | Revenue Offset [Member] | Subsequent Event
Regulatory Assets [Line Items]
Increase (decrease) in annual revenue			$ (1,700,000)
Northern Utilities Inc | Scenario Forecast
Regulatory Assets [Line Items]
Increase (decrease) in annual revenue		$ 0.9
Annual revenue adjustment	$ 2,200,000	$ 2,300,000
Northern Utilities Inc | Maine
Regulatory Assets [Line Items]
Increase (decrease) in annual revenue					$ 2,100,000													$ (100,000)
Percentage of approved return on equity					9.50%
Percentage of approved return on equity, reflecting on equity					50.00%
Percentage of approved return on equity, reflecting on debt					50.00%
Reduction of annual operating costs					$ (500,000)
TIRA extended term																		8 years
Construction completion period																		3 years
Number of new customers | Customer																		1,000
Potential regulated operating revenue																		$ 1,000,000
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																		$ 2,000,000
Northern Utilities Inc | Maine | Revenue Offset [Member]
Regulatory Assets [Line Items]
Increase (decrease) in annual revenue					$ (2,200,000)
Northern Utilities Inc | New Hampshire
Regulatory Assets [Line Items]
Grace period to receive refund																		3 years
Northern Utilities Inc | New Hampshire | Natural Gas Distribution
Regulatory Assets [Line Items]
Increase (decrease) in annual revenue											$ 4,700,000
Granite State Gas Transmission Inc | Gas Transportation and Storage
Regulatory Assets [Line Items]
Increase (decrease) in annual revenue										$ 200,000
Amendment effective date										Aug. 01, 2017								Jul. 28, 2017
Unitil Energy Systems Inc | New Hampshire
Regulatory Assets [Line Items]
Increase (decrease) in annual revenue				$ 400,000
Permanent rate increase													$ 4,100,000
Unitil Energy Systems Inc | New Hampshire | First Rate Step Adjustments
Regulatory Assets [Line Items]
Permanent rate increase													$ 900,000
Fitchburg Gas and Electric Light Company
Regulatory Assets [Line Items]
Increase (decrease) in annual revenue														$ 500,000				$ (800,000)
Recovery amount														$ 900,000
Percentage of waiver approved on annual changes in the revenue requirement eligible for recovery												1.50%
Amendment effective date														May 01, 2017
Revenues sought approval to collect, deferred for recovery in future periods														$ 400,000
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
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 | Solar Massachusetts Renewable Target Program | Solar Generation
Regulatory Assets [Line Items]
Power generation capacity | MW								1,600
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 | Solar Generation
Regulatory Assets [Line Items]
Recovery amount																					$ 300,000
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 (decrease) in annual revenue															$ 2,100,000
Amendment effective date						Jan. 01, 2018									May 01, 2016			Jan. 01, 2017
Annual revenue adjustment						$ 400,000												$ 500,000
Fitchburg Gas and Electric Light Company | Gas base
Regulatory Assets [Line Items]
Increase (decrease) in annual revenue															$ 1,600,000
Amendment effective date															May 01, 2016