Commitments and Contingencies - Additional Information (Detail) $ in Millions							12 Months Ended
	Oct. 31, 2016 USD ($)	Aug. 19, 2016 MW	Jun. 28, 2016 USD ($)	Jun. 24, 2016 USD ($)	Apr. 29, 2016 USD ($)	Apr. 15, 2015 USD ($)	Dec. 31, 2016 USD ($) State Customer	Dec. 31, 2014 USD ($)	Jun. 30, 2027 MW	Dec. 31, 2022 MWh	Jun. 30, 2017 MW	Dec. 31, 2015 USD ($)
Regulatory Assets [Line Items]
Regulatory Assets, Remaining Balance							$ 1.6
Accrued Revenue							49.5					$ 38.4
Regulatory Assets							104.1					99.6
Current Portion of Regulatory Liabilities							10.4					15.6
Regulatory Liabilities - noncurrent							2.6					$ 8.1
Other Restructuring
Regulatory Assets [Line Items]
Accrued Revenue							0.3
Regulatory Assets							$ 1.3
Cost recovery period, years							5 years
Unitil Energy Systems Inc | New Hampshire
Regulatory Assets [Line Items]
Increase in base rates, amount					$ 6.3
Percent of increase in total revenue					3.60%
Temporary rate increase			$ 2.4
Amendment effective date							Jul. 01, 2016
Fitchburg Gas and Electric Light Company
Regulatory Assets [Line Items]
Amendment effective date	May 01, 2017				May 01, 2016
Recovery amount	$ 0.9				$ 0.9
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 | Scenario Forecast
Regulatory Assets [Line Items]
Power generation capacity | MWh										9,450,000
Fitchburg Gas and Electric Light Company | Electric base
Regulatory Assets [Line Items]
Amendment effective date					May 01, 2016		Jan. 01, 2017
Increase in annual revenue					$ 2.1
Recovery amount							$ 0.5
Fitchburg Gas and Electric Light Company | Gas base
Regulatory Assets [Line Items]
Amendment effective date					May 01, 2016
Increase in annual revenue					$ 1.6
Fitchburg Gas and Electric Light Company | Solar Generation
Regulatory Assets [Line Items]
Power generation facility | MW		1.3
Expected completion date of facility		2017-11
Fitchburg Gas and Electric Light Company | Offshore Wind Energy | Scenario Forecast
Regulatory Assets [Line Items]
Power generation facility | MW									1,600
Fitchburg Gas and Electric Light Company | Offshore Wind Energy | Scenario Forecast | Minimum
Regulatory Assets [Line Items]
Power generation facility | MW											400
Northern Utilities Inc
Regulatory Assets [Line Items]
Amendment effective date							May 01, 2014
Increase in annual revenue							$ 1.4
TIRA initial term							Four years
Amendment approved date							Apr. 28, 2016
Pipeline refund received						$ 22.0
Current Portion of Regulatory Liabilities							$ 4.4
Regulatory Liabilities - noncurrent							$ 2.4
Northern Utilities Inc | First Annual Adjustment
Regulatory Assets [Line Items]
Annual TIRA Adjustment								$ 1.5
Annual TIRA Adjustment, effective date							May 01, 2016
Northern Utilities Inc | New Hampshire
Regulatory Assets [Line Items]
Amendment effective date							May 01, 2015
Increase in annual revenue							$ 1.8
Grace period to receive refund							3 years
Northern Utilities Inc | New Hampshire | Natural Gas Distribution
Regulatory Assets [Line Items]
Increase in annual revenue							$ 4.6
Northern Utilities Inc | Maine
Regulatory Assets [Line Items]
Construction completion period							3 years
Number of new customers | Customer							1,000
Potential regulated operating revenue							$ 1.0
Grace period to receive refund							3 years
Date to receive a refund as one-time lump sum payment							Oct. 05, 2015
Granite State Gas Transmission Inc | Gas Transportation and Storage
Regulatory Assets [Line Items]
Amendment effective date				Aug. 01, 2016
Increase in annual revenue				$ 0.3
Amendment approved date				Jul. 13, 2016