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.

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 980
-SubTopic 340
-Section 25
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=77907324&loc=d3e43603-110378

Commitments and Contingencies - Additional Information (Detail) $ in Thousands											1 Months Ended	12 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 ($)	Oct. 31, 2018 USD ($)	Dec. 31, 2018 USD ($) State Customer MW	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)	Jun. 30, 2027 MW	Dec. 31, 2025 MWh	Dec. 31, 2022 MWh	Jun. 01, 2018 USD ($)	May 10, 2018 USD ($)	Jun. 30, 2017 MW	Jun. 23, 2017 kW	Nov. 09, 2016 MW
Regulatory Assets [Line Items]
Regulatory Assets, Remaining Balance												$ 900
Accrued Revenue												54,700	$ 53,300
Regulatory Assets												$ 99,000	$ 109,600
Corporate income tax rate												21.00%	34.00%	34.00%
Reduction of federal tax decrease												$ 0	$ 0	$ 0
Spending cap																			$ 4,400
Annual revenue adjustment				$ 2,300
Power capacity facility for small customers \| kW																					100
Electric Base
Regulatory Assets [Line Items]
Refund of excess ADIT												$ 10,100
Number of years refund amortized												15 years
Gas Base
Regulatory Assets [Line Items]
Refund of excess ADIT												$ 10,400
Number of years refund amortized												14 years
Tax Year 2018
Regulatory Assets [Line Items]
Corporate income tax rate													21.00%
Other Restructuring
Regulatory Assets [Line Items]
Accrued Revenue												$ 300
Regulatory Assets												$ 600
Cost recovery period, years												2 years
Scenario Forecast
Regulatory Assets [Line Items]
Annual revenue adjustment	$ 2,200
Maine \| Environmental Restoration Costs
Regulatory Assets [Line Items]
Amortization period for environmental costs												5 years
New Hampshire \| Environmental Restoration Costs
Regulatory Assets [Line Items]
Amortization period for environmental costs												7 years
Northern Utilities Inc
Regulatory Assets [Line Items]
Increase (decrease) in annual revenue			$ 2,600				$ 3,200		$ 2,600
Increase in annual base rate		2.40%
Annual TIRA Adjustment		$ 1,100
TIRA initial term												Four years
Northern Utilities Inc \| Revenue Offset [Member]
Regulatory Assets [Line Items]
Increase (decrease) in annual revenue			$ (1,700)
Northern Utilities Inc \| Maine
Regulatory Assets [Line Items]
Increase (decrease) in annual revenue						$ 2,100
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
TIRA extended term												8 years
Construction completion period												3 years
Number of new customers \| Customer												1,000
Potential regulated operating revenue												$ 1,000
Northern Utilities Inc \| Maine \| Revenue Offset [Member]
Regulatory Assets [Line Items]
Increase (decrease) in annual revenue						$ 2,200
Northern Utilities Inc \| Maine \| Second TAB program
Regulatory Assets [Line Items]
Number of new customers \| Customer												2,000
Potential regulated operating revenue												$ 2,000
Fitchburg Gas and Electric Light Company
Regulatory Assets [Line Items]
Increase (decrease) in annual revenue				$ 900								$ (800)
Annual revenue adjustment											$ 400
Amendment effective date											Nov. 01, 2018
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
Fitchburg Gas and Electric Light Company \| Solar Generation \| Solar Massachusetts Renewable Target Program
Regulatory Assets [Line Items]
Power generation capacity \| MW								1,600
Fitchburg Gas and Electric Light Company \| Maximum
Regulatory Assets [Line Items]
Power generation facility \| MW												1,600
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 \| Scenario Forecast
Regulatory Assets [Line Items]
Power generation capacity \| MWh																1,600	9,450,000
Fitchburg Gas and Electric Light Company \| Scenario Forecast \| Offshore Wind Energy
Regulatory Assets [Line Items]
Power generation facility \| MW															1,600
Unitil Energy Systems Inc \| New Hampshire
Regulatory Assets [Line Items]
Increase (decrease) in annual revenue					$ 300
Permanent rate increase										$ 4,100
Reduction of federal tax decrease					2,200
Reconciliation adjustment of recouped difference between temporary rates and final rates					1,400
Unitil Energy Systems Inc \| New Hampshire \| First Rate Step Adjustments
Regulatory Assets [Line Items]
Permanent rate increase										$ 900
Unitil Energy Systems Inc \| New Hampshire \| Second rate Step Adjustments [Member]
Regulatory Assets [Line Items]
Increase (decrease) in annual revenue					$ 3,300