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Regulatory Matters - Additional Information (Detail) $ in Millions												9 Months Ended	12 Months Ended
	Aug. 18, 2017 USD ($)	Aug. 01, 2017 USD ($)	Jun. 21, 2017 USD ($)	Jun. 05, 2017 USD ($)	May 31, 2017 USD ($)	Apr. 25, 2017 USD ($)	Apr. 20, 2017 USD ($)	Oct. 31, 2016 USD ($)	Aug. 19, 2016 MW	Apr. 29, 2016 USD ($)	Apr. 15, 2015 USD ($)	Sep. 30, 2017 USD ($) State Customer	Dec. 31, 2015 USD ($)	Jun. 30, 2027 MW	Dec. 31, 2022 MWh	Aug. 25, 2017 MW	Jun. 30, 2017 MW	Jun. 23, 2017 kW	Apr. 28, 2017	Dec. 31, 2016 USD ($)	Sep. 30, 2016 USD ($)
Regulatory Assets [Line Items]
Current Portion of Regulatory Liabilities												$ 15.0								$ 10.4	$ 13.2
Power capacity facility for small customers | kW																		100
Northern Utilities Inc
Regulatory Assets [Line Items]
Increase in annual revenue	$ 0.7				$ 6.0
Amendment effective date						May 01, 2017
Annual TIRA Adjustment						$ 1.1
TIRA initial term												Four years
Pipeline refund received											$ 22.0
Current Portion of Regulatory Liabilities												$ 3.4
Northern Utilities Inc | Maine
Regulatory Assets [Line Items]
Number of new customers | Customer												1,000
Potential regulated operating revenue												$ 1.0
Construction completion period												3 years
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												$ 2.0
Northern Utilities Inc | New Hampshire
Regulatory Assets [Line Items]
Increase in annual revenue		$ 1.6
Amendment effective date												Jul. 31, 2017
Deferred cost												$ 0.3
Grace period to receive refund												3 years
Northern Utilities Inc | New Hampshire | Natural Gas Distribution
Regulatory Assets [Line Items]
Increase in annual revenue				$ 4.7
Granite State Gas Transmission Inc | Gas Transportation and Storage
Regulatory Assets [Line Items]
Increase in annual revenue			$ 0.2
Amendment effective date			Aug. 01, 2017									Jul. 28, 2017
Unitil Energy Systems Inc | New Hampshire
Regulatory Assets [Line Items]
Permanent rate increase							$ 4.1
Unitil Energy Systems Inc | New Hampshire | Additional Rate Step Adjustments
Regulatory Assets [Line Items]
Permanent rate increase							$ 0.9
Fitchburg Gas and Electric Light Company
Regulatory Assets [Line Items]
Increase in annual revenue								$ 0.5
Recovery amount								0.9
Percentage of waiver approved on annual changes in the revenue requirement eligible for recovery																			1.50%
Revenues sought approval to collect, deferred for recovery in future periods								$ 0.4
Percentage of waiver sought on annual changes in the revenue requirement eligible for recovery								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 capacity | MW									1.3
Expected completion date of facility									2017-11
Fitchburg Gas and Electric Light Company | Minimum | Offshore Wind Energy
Regulatory Assets [Line Items]
Power generation capacity | 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 | Offshore Wind Energy
Regulatory Assets [Line Items]
Power generation capacity | MW														1,600
Fitchburg Gas and Electric Light Company | Electric base
Regulatory Assets [Line Items]
Increase in annual revenue										$ 2.1
Amendment effective date										May 01, 2016
Annual revenue adjustment													$ 0.5
Fitchburg Gas and Electric Light Company | Gas base
Regulatory Assets [Line Items]
Increase in annual revenue										$ 1.6
Amendment effective date										May 01, 2016

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- Definition

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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