Regulatory Matters - Additional Information (Detail)		1 Months Ended								6 Months Ended
	Nov. 01, 2017 USD ($)	Jun. 28, 2019 USD ($)	May 01, 2019 USD ($)	Feb. 28, 2019 USD ($)	May 07, 2018 USD ($)	May 02, 2018 USD ($)	Apr. 30, 2018 USD ($)	Dec. 31, 2017	Apr. 20, 2017 USD ($)	Jun. 30, 2019	Jun. 30, 2017 MW	Jun. 30, 2027 MW	Dec. 31, 2022 MWh	Jun. 25, 2019	Apr. 30, 2019 USD ($)	Jul. 31, 2018	Jul. 31, 2018 MWh	Jul. 31, 2018 MW
Regulatory Assets [Line Items]
Annual revenue adjustment	$ 1,400,000
Power generation facility | MW												1,600
Pro rata percentage share of contracts											1
Tax Year 2018
Regulatory Assets [Line Items]
Corporate income tax rate								21.00%		21.00%
Unitil Energy Systems Inc | New Hampshire
Regulatory Assets [Line Items]
Permanent rate increase									$ 4,100,000
Increase (decrease) in annual revenue				$ 340,000
Fitchburg Gas and Electric Light Company
Regulatory Assets [Line Items]
Increase (decrease) in annual revenue	900,000		$ 1,000,000.0				$ 1,000,000.0
Recovery amount			$ 600,000												$ 1,600,000
Annual revenue adjustment	$ 400,000
Amendment effective date	Nov. 01, 2018
Fitchburg Gas and Electric Light Company | Offshore Wind Energy
Regulatory Assets [Line Items]
Power generation facility | MWh																	400
Remuneration Percentage														2.75		2.75
Fitchburg Gas and Electric Light Company | Offshore Wind Energy | First Solicitation [Member]
Regulatory Assets [Line Items]
Power generation facility | MWh																	800
Fitchburg Gas and Electric Light Company | Offshore Wind Energy | Second Solicitation [Member]
Regulatory Assets [Line Items]
Power generation facility																	800	400
Fitchburg Gas and Electric Light Company | Qualified Clean Energy
Regulatory Assets [Line Items]
Power Generation Capacity | MWh																	9,554,940
Fitchburg Gas and Electric Light Company | Minimum [Member] | 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
Northern Utilities Inc
Regulatory Assets [Line Items]
Increase (decrease) in annual revenue		$ 7,000,000.0				$ 3,200,000
Increase In Annual Base Rate				2.1	2.4
Annual TIRA Adjustment				$ 1,000,000.0	$ 1,100,000
TIRA initial term										four years
Percentage Of Increase In Test Year Operating Revenue		7.00%
Northern Utilities Inc | Maine
Regulatory Assets [Line Items]
TIRA extended term										8 years