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Summary of Significant Accounting Policies - Additional Information (Detail) $ in Millions	9 Months Ended
	Sep. 30, 2021 USD ($) Subsidiary mi	Dec. 31, 2020 USD ($)	Sep. 30, 2020 USD ($)
Significant Accounting Policies [Line Items]
Length Of Pipeline | mi	86
Cost of removal obligation	$ 108.2	$ 105.2	$ 107.1
Investments in trading securities	5.6	5.7	5.7
Allowance for doubtful accounts	4.3	3.3	1.6
Regulatory assets	163.0	164.7	136.1
Other Deferred Charges
Significant Accounting Policies [Line Items]
Regulatory assets	17.7	14.2	12.1
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	$ 4.2	3.1	1.5
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	$ 0.1
Fitchburg Gas and Electric Light Company | Other Deferred Charges | Electric And Gas Division
Significant Accounting Policies [Line Items]
Hardship accounts in regulatory assets	9.5	6.8	7.1
Environmental and Rate Case Costs and Other Expenditures | Recovered over the next seven years
Significant Accounting Policies [Line Items]
Regulatory assets	8.5
ISO-NE Obligations
Significant Accounting Policies [Line Items]
Cash Deposits	$ 5.7	2.4	4.7
Maximum
Significant Accounting Policies [Line Items]
Lease term	12 months
Deferred Compensation Plan [Member]
Significant Accounting Policies [Line Items]
Investments in trading securities	$ 0.5	$ 0.5	$ 0.5

X

- Definition

Amount of allowance for credit loss on accounts receivable.

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Reference 1: http://www.xbrl.org/2003/role/disclosureRef
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Fair value of liability arising from energy marketing activities (trading activities), expected to be settled after one year or beyond the normal operating cycle, if longer. Examples of energy marketing activities include, but are not limited to, trading of electricity, coal, natural gas and oil using financial instruments, including, but not limited to, forward contracts, options and swaps.

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The amount for the individual regulatory asset as itemized in a table of regulatory assets as of the end of the period.

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Amount of investment in debt security measured at fair value with change in fair value recognized in net income (trading) and investment in equity security measured at fair value with change in fair value recognized in net income (FV-NI).

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Cash Deposits

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Hardship accounts in regulatory assets.

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Length of natural gas transmission pipeline.

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Significant Accounting Policies [Line Items]

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