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://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 980
-SubTopic 340
-Section 25
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=6499739&loc=d3e43603-110378

Regulatory Matters - Additional Information (Detail) $ in Millions									1 Months Ended	3 Months Ended		12 Months Ended
	Apr. 24, 2017 USD ($)	Apr. 20, 2017 USD ($)	Mar. 31, 2017 USD ($) State	Oct. 31, 2016 USD ($)	Aug. 19, 2016 MW	Jun. 24, 2016 USD ($)	Apr. 29, 2016 USD ($)	Apr. 15, 2015 USD ($)	Feb. 28, 2017 USD ($) Customer	Jun. 30, 2017 USD ($) MW	Mar. 31, 2017 USD ($) State Customer	Dec. 31, 2015 USD ($)	Jun. 30, 2027 MW	Dec. 31, 2022 MWh	Dec. 31, 2016 USD ($)	Mar. 31, 2016 USD ($)
Regulatory Assets [Line Items]
Current Portion of Regulatory Liabilities			$ 12.8								$ 12.8				$ 10.4	$ 18.8
Regulatory Liabilities - noncurrent			0.7								$ 0.7				$ 2.6	$ 3.3
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
Permanent rate increase		$ 4.1
Percentage of capital expenditure adjustments rate		80.00%
Unitil Energy Systems Inc \| New Hampshire \| First Rate Step Adjustments
Regulatory Assets [Line Items]
Permanent rate increase		$ 0.9
Fitchburg Gas and Electric Light Company
Regulatory Assets [Line Items]
Amendment effective date				May 01, 2017
Annual revenue adjustment			$ 0.6
Recovery amount				$ 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								3
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 \| 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 \| Minimum \| Scenario Forecast \| Offshore Wind Energy
Regulatory Assets [Line Items]
Power generation facility \| MW										400
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
Annual revenue adjustment											$ 0.3
Fitchburg Gas and Electric Light Company \| Electric base \| Minimum
Regulatory Assets [Line Items]
Annual revenue adjustment												$ 0.2
Fitchburg Gas and Electric Light Company \| Electric base \| Maximum
Regulatory Assets [Line Items]
Annual revenue adjustment												$ 0.3
Fitchburg Gas and Electric Light Company \| Gas base
Regulatory Assets [Line Items]
Amendment effective date							May 01, 2016
Increase in annual revenue							$ 1.6
Northern Utilities Inc
Regulatory Assets [Line Items]
Amendment effective date											May 01, 2014
Increase in annual revenue											$ 1.4
Period within which rate case filed											60 days
TIRA initial term											Four years
Amendment approved date											Apr. 25, 2017
Pipeline refund received								$ 22.0
Current Portion of Regulatory Liabilities			$ 3.6								$ 3.6
Regulatory Liabilities - noncurrent			$ 0.7								0.7
Northern Utilities Inc \| First Annual Adjustment
Regulatory Assets [Line Items]
Annual TIRA Adjustment											$ 1.1
Annual TIRA Adjustment, effective date											May 01, 2017
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 \| Subsequent Event
Regulatory Assets [Line Items]
Period within which rate case filed	60 days
Northern Utilities Inc \| New Hampshire \| Natural Gas Distribution
Regulatory Assets [Line Items]
Increase in annual revenue											$ 4.6
Northern Utilities Inc \| New Hampshire \| Natural Gas Distribution \| Subsequent Event
Regulatory Assets [Line Items]
Amount of increase gas base distribution rates	$ 5.0
Northern Utilities Inc \| Maine
Regulatory Assets [Line Items]
Period within which rate case filed			60 days
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
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 \| Maine \| Natural Gas Distribution
Regulatory Assets [Line Items]
Amount of increase gas base distribution rates			$ 5.5
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
Granite State Gas Transmission Inc \| Gas Transportation and Storage \| Minimum \| Scenario Forecast
Regulatory Assets [Line Items]
Increase in annual revenue										$ 0.3
Granite State Gas Transmission Inc \| Gas Transportation and Storage \| Maximum \| Scenario Forecast
Regulatory Assets [Line Items]
Increase in annual revenue										$ 0.4