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Regulatory Matters - Additional Information (Detail)		1 Months Ended							6 Months Ended	9 Months Ended
	Nov. 01, 2017 USD ($)	Jun. 28, 2019 USD ($)	May 01, 2019 USD ($)	Feb. 28, 2019 USD ($)	May 07, 2018 USD ($)	May 02, 2018 USD ($)	Dec. 31, 2017	Apr. 20, 2017 USD ($)	Jun. 30, 2017 MW	Sep. 30, 2019	Jun. 30, 2027 MW	Dec. 31, 2022 MWh	Jun. 25, 2019	Apr. 30, 2019 USD ($)	Jul. 31, 2018 MWh	Jul. 31, 2018	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
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
Percentage Of Increase In Test Year Operating Revenue		7.00%
Northern Utilities Inc | Maine
Regulatory Assets [Line Items]
TIRA extended term										8 years

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Carrying amount as of the balance sheet date of the portion of the reserved funds established by a utility from customer payments to cover the repair costs of damages from major storms, which repairs are expected to be incurred beyond one year or the normal operating cycle, if longer.

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