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

Regulatory Matters - Additional Information (Detail)													1 Months Ended	6 Months Ended
Regulatory Matters - Additional Information (Detail)	May 01, 2019 USD ($)	May 07, 2018 USD ($)	May 02, 2018 USD ($)	May 01, 2018 USD ($)	Apr. 30, 2018 USD ($)	Feb. 28, 2018 USD ($)	Dec. 20, 2017 USD ($)	Oct. 31, 2017 USD ($)	Aug. 25, 2017 MW	Aug. 01, 2017 USD ($)	Apr. 20, 2017 USD ($)	Apr. 29, 2016 USD ($)	Dec. 31, 2017	Jun. 30, 2018 USD ($) State Customer	Jun. 30, 2027 MW	Dec. 31, 2022 MWh	Jun. 01, 2018 USD ($)	May 10, 2018 USD ($)	Jun. 30, 2017 MW	Jun. 23, 2017 kW	Aug. 19, 2016 MW
Regulatory Assets [Line Items]
Annual revenue adjustment				$ 2.3
Spending cap																		$ 4,400,000
Power capacity facility for small customers \| kW																				100
Scenario Forecast
Regulatory Assets [Line Items]
Annual revenue adjustment	$ 2.2
Tax Year 2018
Regulatory Assets [Line Items]
Corporate income tax rate													21.00%	21.00%
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								$ 400,000						$ (800,000)
Recovery amount								$ 900,000
Percentage of waiver approved on annual changes in the revenue requirement eligible for recovery					1.50%
Amendment effective date								May 01, 2018
Revenues sought approval to collect, deferred for recovery in future periods								$ 900,000
Percentage of waiver sought on annual changes in the revenue requirement eligible for recovery					1.50%			3.00%
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 \| 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 facility \| MW															1,600
Fitchburg Gas and Electric Light Company \| Gas base
Regulatory Assets [Line Items]
Increase (decrease) in annual revenue												$ 1,600,000
Amendment effective date												May 01, 2016
Fitchburg Gas and Electric Light Company \| Electric base
Regulatory Assets [Line Items]
Increase (decrease) in annual revenue												$ 2,100,000
Amendment effective date							Jan. 01, 2018					May 01, 2016		Jan. 01, 2017
Annual revenue adjustment							$ 400,000							$ 500,000
Northern Utilities Inc
Regulatory Assets [Line Items]
Increase (decrease) in annual revenue			$ 2.6							$ 2.6
Increase in annual base rate		$ 0.024
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			$ (1.7)
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