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Summary of Significant Accounting Policies - Additional Information (Detail) $ in Millions	1 Months Ended	6 Months Ended
	Aug. 31, 2022	Jun. 30, 2023 USD ($) Subsidiary mi	Jul. 06, 2023 USD ($)	Dec. 31, 2022 USD ($)	Jun. 30, 2022 USD ($)
Significant Accounting Policies [Line Items]
Number of Subsidiaries | Entity | Subsidiary		3
Length of Pipeline | mi		86
Percentage of total sales volumes revenue subject to RDM	43.00%
Cost of removal obligation		$ 123.2		$ 116.1	$ 112.4
Investments in trading securities		5.8		5.8	5.4
Allowance for doubtful accounts		2.2		2.6	2.7
Regulatory assets		108.3		114.3	148.0
Other Deferred Charges [Member]
Significant Accounting Policies [Line Items]
Regulatory assets		11.0		11.1	13.3
Unbilled Revenues [Member]
Significant Accounting Policies [Line Items]
Allowance for doubtful accounts		0.1		0.1	0.1
Accounts Receivable [Member]
Significant Accounting Policies [Line Items]
Allowance for doubtful accounts		$ 2.2		2.5	2.6
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
Fitchburg Gas And Electric Light Company [Member] | Other Deferred Charges [Member] | Electric and Gas Division [Member]
Significant Accounting Policies [Line Items]
Hardship accounts in regulatory assets		$ 6.0		5.8	7.6
Subsequent Event [Member]
Significant Accounting Policies [Line Items]
Debt issuance principal amount			$ 25.0
Environmental and Rate Case Costs and Other Expenditures | Recovered over the next seven years
Significant Accounting Policies [Line Items]
Regulatory assets		5.5
ISO-NE Obligations
Significant Accounting Policies [Line Items]
Cash Deposits		$ 3.3		6.0	1.7
Maximum
Significant Accounting Policies [Line Items]
Lease term		12 months
Deferred Compensation Plan [Member]
Significant Accounting Policies [Line Items]
Investments in trading securities		$ 1.1		$ 0.6	$ 0.8

X

- Definition

Amount of allowance for credit loss on accounts receivable.

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Noncurrent portion of the carrying amount of a liability for an asset retirement obligation. An asset retirement obligation is a legal obligation associated with the disposal or retirement of a tangible long-lived asset that results from the acquisition, construction or development, or the normal operations of a long-lived asset, except for certain obligations of lessees.

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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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