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Commitments and Contingencies - Additional Information (Detail)											1 Months Ended	12 Months Ended
	Dec. 17, 2019 USD ($)	Oct. 29, 2019 USD ($)	Jun. 28, 2019 USD ($)	Jun. 25, 2019	May 01, 2019 USD ($)	Apr. 17, 2019 USD ($)	Nov. 01, 2018 USD ($)	May 07, 2018 USD ($)	May 02, 2018 USD ($)	Apr. 20, 2017 USD ($)	Dec. 31, 2017	Dec. 31, 2019 USD ($)	Dec. 31, 2018 USD ($)	Dec. 31, 2017	May 23, 2019 MWh	Apr. 30, 2019 USD ($)	Jul. 31, 2018 MW MWh	Jun. 30, 2017 MW MWh	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)
Regulatory Assets [Line Items]
Regulatory Assets, Remaining Balance												$ 600,000
Accrued Revenue												50,000,000.0	$ 54,700,000
Regulatory Assets												$ 112,000,000.0	$ 99,000,000.0
Corporate income tax rate												21.00%	21.00%	34.00%
Spending cap												$ 599,000,000.0
Accrued Estimated Costs For Remediation												$ 700,000
Tax Year 2018
Regulatory Assets [Line Items]
Corporate income tax rate											21.00%	21.00%	21.00%	21.00%
Other Restructuring
Regulatory Assets [Line Items]
Accrued Revenue												$ 300,000
Regulatory Assets												$ 300,000
Cost recovery period, years												2 years
New Hampshire
Regulatory Assets [Line Items]
Increase (decrease) in annual revenue									$ 3,200,000
Northern Utilities Inc
Regulatory Assets [Line Items]
Increase (decrease) in annual revenue			$ 7,000,000.0						$ 1,400,000
Increase in annual base rate						2.10%		2.40%
Annual TIRA adjustment						$ 1,000,000.0		$ 1,100,000
Percentage of increase in test year operating revenue			7.00%
Fitchburg Gas and Electric Light Company
Regulatory Assets [Line Items]
Increase (decrease) in annual revenue	$ 2,700,000	$ 1,100,000			$ 1,000,000.0		$ 900,000
Spending cap																			$ 400,000	$ 400,000
Recovery amount					600,000											$ 1,600,000
Percentage of change in revenue over previous year	4.10%
Power generation capacity | MWh																		9,450,000
Fitchburg Gas and Electric Light Company | Offshore Wind Energy
Regulatory Assets [Line Items]
Power generation facility | MW																	400	1,600
Remuneration Percentage				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 | MWh															800
Fitchburg Gas and Electric Light Company | Qualified Clean Energy
Regulatory Assets [Line Items]
Power generation capacity | MWh																	9,554,940
Remuneration Percentage				2.75%
Unitil Energy Systems Inc | New Hampshire
Regulatory Assets [Line Items]
Permanent rate increase										$ 4,100,000
Unitil Energy Systems Inc | New Hampshire | Second Rate Step Adjustments
Regulatory Assets [Line Items]
Increase (decrease) in annual revenue					$ 340,000
Fitchburg Gas Company
Regulatory Assets [Line Items]
Increase (decrease) in annual revenue	$ 7,300,000
Percentage of change in revenue over previous year	20.80%

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