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Summary of Significant Accounting Policies - Additional Information (Detail) $ in Millions	1 Months Ended	3 Months Ended
	Aug. 31, 2022	Mar. 31, 2023 USD ($) Subsidiary mi	Dec. 31, 2022 USD ($)	Mar. 31, 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		$ 120.0	$ 116.1	$ 109.8
Investments in trading securities		5.7	5.8	5.5
Allowance for doubtful accounts		3.1	2.6	3.4
Regulatory assets		118.0	114.3	153.3
Other Deferred Charges [Member]
Significant Accounting Policies [Line Items]
Regulatory assets		11.2	11.1	15.1
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		$ 3.0	2.5	3.3
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
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		$ 5.8	5.8	7.9
Environmental and Rate Case Costs and Other Expenditures | Recovered over the next seven years
Significant Accounting Policies [Line Items]
Regulatory assets		6.1
ISO-NE Obligations
Significant Accounting Policies [Line Items]
Cash Deposits		$ 4.2	6.0	2.9
Maximum
Significant Accounting Policies [Line Items]
Lease term		12 months
Deferred Compensation Plan [Member]
Significant Accounting Policies [Line Items]
Investments in trading securities		$ 0.9	$ 0.6	$ 0.8

X

- Definition

Amount of allowance for credit loss on accounts receivable.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
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X

- Definition

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.

+ References

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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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- Definition

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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Hardship accounts in regulatory assets.

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Length of natural gas transmission pipeline.

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- Definition

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- Definition

Percentage Of Sales Volume

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No definition available.

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