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Summary of Significant Accounting Policies - Additional Information (Detail) $ in Millions	3 Months Ended	9 Months Ended
	Mar. 31, 2019 USD ($)	Sep. 30, 2020 USD ($) Subsidiary mi	Sep. 30, 2019 USD ($)	Dec. 31, 2019 USD ($)
Significant Accounting Policies [Line Items]
Length Of Pipeline | mi		86
Gain on divestiture of business, pretax			$ 13.4
Gain on divestiture of business, net	$ 9.8
Provision for income taxes			3.6
Cost of removal obligation		$ 107.1	98.4	$ 96.0
Regulatory assets		136.1	123.6	147.8
Allowance for doubtful accounts		1.6	1.0	1.0
Power Supply Contract Divestitures		0.0	0.4	0.3
Unbilled Revenues
Significant Accounting Policies [Line Items]
Allowance for doubtful accounts		0.1
Accounts Receivable
Significant Accounting Policies [Line Items]
Allowance for doubtful accounts		$ 1.5	1.0	1.0
Utilities
Significant Accounting Policies [Line Items]
Number of Subsidiaries | Subsidiary		3
Unitil Service; Unitil Realty; and Unitil Resources
Significant Accounting Policies [Line Items]
Number of Subsidiaries | Subsidiary		3
Power Supply Contract Divestitures		$ 0.3
Annual Electric Sales Volume
Significant Accounting Policies [Line Items]
Percentage of total sales volumes revenue subject to RDM		27.00%
Annual Natural Gas Sales Volume
Significant Accounting Policies [Line Items]
Percentage of total sales volumes revenue subject to RDM		11.00%
Environmental and Rate Case Costs and Other Expenditures | Recovered over the next seven years
Significant Accounting Policies [Line Items]
Regulatory assets		$ 7.5
ISO-NE Obligations
Significant Accounting Policies [Line Items]
Cash Deposits		$ 4.7	$ 3.5	$ 1.9
Lease term		12 months

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