Summary of Significant Accounting Policies - Additional Information (Detail) $ in Millions	9 Months Ended
	Sep. 30, 2022 USD ($) Subsidiary mi	Dec. 31, 2021 USD ($)	Sep. 30, 2021 USD ($)
Significant Accounting Policies [Line Items]
Number of Subsidiaries | Entity | Subsidiary	3
Length Of Pipeline | mi	86
Cost of removal obligation	$ 115.4	$ 107.5	$ 108.2
Investments in trading securities	5.6	5.7	5.6
Allowance for doubtful accounts	2.4	3.3	4.3
Regulatory assets	149.5	156.3	163.0
Other Deferred Charges [Member]
Significant Accounting Policies [Line Items]
Regulatory assets	12.0	15.3	17.7
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	$ 2.4	3.1	4.2
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	7.9	9.5
Environmental and Rate Case Costs and Other Expenditures | Recovered over the next seven years
Significant Accounting Policies [Line Items]
Regulatory assets	5.8
ISO-NE Obligations
Significant Accounting Policies [Line Items]
Cash Deposits	$ 4.8	2.7	5.7
Maximum
Significant Accounting Policies [Line Items]
Lease term	12 months
Deferred Compensation Plan [Member]
Significant Accounting Policies [Line Items]
Investments in trading securities	$ 0.6	$ 0.6	$ 0.5