Summary of Significant Accounting Policies - Additional Information (Detail) $ in Millions	3 Months Ended
	Mar. 31, 2024 USD ($) Subsidiary mi	Dec. 31, 2023 USD ($)	Mar. 31, 2023 USD ($)
Significant Accounting Policies [Line Items]
Number of Subsidiaries | Entity | Subsidiary	3
Length of Pipeline | mi	85
Cost of removal obligation	$ 130.6	$ 126.3	$ 120.0
Investments in trading securities	5.9	6.0	5.7
Allowance for doubtful accounts	2.8	2.4	3.1
Regulatory assets	117.2	109.6	118.0
Other Deferred Charges [Member]
Significant Accounting Policies [Line Items]
Regulatory assets	12.6	11.0	11.2
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	$ 2.7	2.3	3.0
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	$ 6.9	6.0	5.8
Environmental and Rate Case Costs and Other Expenditures | Recovered over the next seven years
Significant Accounting Policies [Line Items]
Regulatory assets	6.9
ISO-NE Obligations
Significant Accounting Policies [Line Items]
Cash Deposits	$ 3.1	3.3	4.2
Maximum
Significant Accounting Policies [Line Items]
Lease term	12 months
Deferred Compensation Plan [Member]
Significant Accounting Policies [Line Items]
Investments in trading securities	$ 1.8	$ 1.3	$ 0.9