Summary of Significant Accounting Policies - Additional Information (Detail) $ in Millions	3 Months Ended
Mar. 31, 2021 USD ($) Subsidiary mi	Dec. 31, 2020 USD ($)	Mar. 31, 2020 USD ($)
Significant Accounting Policies [Line Items]
Length Of Pipeline \| mi	86
Cost of removal obligation	$ 102.5	$ 105.2	$ 98.8
Power Supply Contract Divestitures			0.2
Investments in trading securities	5.6	5.7	5.5
Allowance for doubtful accounts	4.4	3.3	1.8
Regulatory assets	152.7	164.7	133.7
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	$ 4.3	3.1	1.7
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
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	$ 2.2	2.4	2.4
Maximum
Significant Accounting Policies [Line Items]
Lease term	12 months
Deferred Compensation Plan [Member]
Significant Accounting Policies [Line Items]
Investments in trading securities	$ 0.6	$ 0.5	$ 0.4

Amount of allowance for credit loss on accounts receivable.

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
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Reference 2: http://www.xbrl.org/2003/role/disclosureRef
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Reference 3: 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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Term of lessee's operating lease, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents reported fact of one year, five months, and thirteen days.

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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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Length of natural gas transmission pipeline.

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