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Summary of Significant Accounting Policies - Additional Information (Detail) $ in Millions	6 Months Ended
	Jun. 30, 2021 USD ($) Subsidiary mi	Mar. 31, 2021 USD ($)	Dec. 31, 2020 USD ($)	Jun. 30, 2020 USD ($)
Significant Accounting Policies [Line Items]
Length Of Pipeline | mi	86
Cost of removal obligation	$ 105.1		$ 105.2	$ 102.6
Power Supply Contract Divestitures				0.1
Investments in trading securities	5.5		5.7	5.6
Allowance for doubtful accounts	4.6		3.3	1.4
Regulatory assets	156.1		164.7	131.7
Other Deferred Charges
Significant Accounting Policies [Line Items]
Regulatory assets	17.8		14.2	12.2
Unbilled Revenues [member]
Significant Accounting Policies [Line Items]
Allowance for doubtful accounts	0.1		0.2	0.1
Accounts Receivable [member]
Significant Accounting Policies [Line Items]
Allowance for doubtful accounts	$ 4.5		3.1	1.3
Utilities
Significant Accounting Policies [Line Items]
Number of Subsidiaries | Entity | Subsidiary	3
Unitil Service; Unitil Realty; and Unitil Resources
Significant Accounting Policies [Line Items]
Number of Subsidiaries | Entity | Subsidiary	3
Power Supply Contract Divestitures		$ 0.2
Fitchburg Gas and Electric Light Company | Other Deferred Charges | Electric And Gas Division
Significant Accounting Policies [Line Items]
Hardship accounts in regulatory assets	$ 9.8		6.8	7.3
Environmental and Rate Case Costs and Other Expenditures | Recovered over the next seven years
Significant Accounting Policies [Line Items]
Regulatory assets	8.3
ISO-NE Obligations
Significant Accounting Policies [Line Items]
Cash Deposits	$ 1.5		2.4	1.4
Maximum
Significant Accounting Policies [Line Items]
Lease term	12 months
Deferred Compensation Plan [Member]
Significant Accounting Policies [Line Items]
Investments in trading securities	$ 0.7		$ 0.5	$ 0.4

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