Regulatory Matters - Additional Information (Detail)											1 Months Ended	9 Months Ended
	May 01, 2019 USD ($)	May 07, 2018 USD ($)	May 02, 2018 USD ($)	May 01, 2018 USD ($)	Apr. 30, 2018 USD ($)	Feb. 28, 2018 USD ($)	Jan. 01, 2018 USD ($)	Aug. 25, 2017 MW	Aug. 01, 2017 USD ($)	Apr. 20, 2017 USD ($)	Dec. 31, 2017	Sep. 30, 2018 USD ($) State Customer MW	Jun. 30, 2027 MW	Dec. 31, 2025 MWh	Dec. 31, 2022 MWh	Jun. 01, 2018 USD ($)	May 10, 2018 USD ($)	Jun. 30, 2017 MW	Jun. 23, 2017 kW	Nov. 09, 2016 MW
Regulatory Assets [Line Items]
Spending cap																	$ 4,400,000
Annual revenue adjustment				$ 2,300,000
Power capacity facility for small customers | kW																			100
Tax Year 2018
Regulatory Assets [Line Items]
Corporate income tax rate											21.00%	21.00%
Scenario Forecast
Regulatory Assets [Line Items]
Annual revenue adjustment	$ 2,200,000
Unitil Energy Systems Inc | New Hampshire
Regulatory Assets [Line Items]
Increase (decrease) in annual revenue					$ 300,000
Permanent rate increase										$ 4,100,000
Reduction of federal tax decrease					2,200,000
Reconciliation adjustment of recouped difference between temporary rates and final rates					1,400,000
Unitil Energy Systems Inc | New Hampshire | First Rate Step Adjustments
Regulatory Assets [Line Items]
Permanent rate increase										$ 900,000
Unitil Energy Systems Inc | New Hampshire | Second rate Step Adjustments [Member]
Regulatory Assets [Line Items]
Increase (decrease) in annual revenue					$ 3,300,000
Fitchburg Gas and Electric Light Company
Regulatory Assets [Line Items]
Increase (decrease) in annual revenue				$ 900,000								$ (0.008)
Description of percentage of sales in legislation												The legislation contains numerous provisions, including: a requirement that increases the pace at which the Class 1 Renewable Portfolio Standard requirement increases, from the current pace of an additional 1 percent of sales each year to an additional 2 percent of sales each year during the period from January 1, 2020 through December 31, 2029.
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
Recovery amount																$ 300,000
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 | Maximum
Regulatory Assets [Line Items]
Power generation facility | MW												1,600
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														1,000	9,450,000
Fitchburg Gas and Electric Light Company | Scenario Forecast | Offshore Wind Energy
Regulatory Assets [Line Items]
Power generation facility | MW													1,600
Northern Utilities Inc
Regulatory Assets [Line Items]
Increase (decrease) in annual revenue			$ 2,600,000				$ 3,200,000		$ 2,600,000
Increase in annual base rate		2.40%
Annual TIRA Adjustment		$ 1,100,000
TIRA initial term												Four years
Northern Utilities Inc | Revenue Offset [Member]
Regulatory Assets [Line Items]
Increase (decrease) in annual revenue			$ (1,700,000)
Northern Utilities Inc | Maine
Regulatory Assets [Line Items]
Increase (decrease) in annual revenue						$ 2,100,000
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
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