Summary of Significant Accounting Policies - Additional Information (Detail) $ / shares in Units, $ in Millions	1 Months Ended	12 Months Ended
	Jan. 31, 2024 $ / shares	Dec. 31, 2023 USD ($) Subsidiary mi $ / shares	Dec. 31, 2022 USD ($) $ / shares	Dec. 31, 2021 USD ($) $ / shares	Dec. 31, 2020 USD ($)
Significant Accounting Policies [Line Items]
Length Of Pipeline | mi		85
Cost of removal obligation		$ 126.3	$ 116.1
Regulatory assets		109.6	114.3
Investments in trading securities		$ 6.0	5.8
Number of Subsidiaries | Subsidiary		3
Allowance for doubtful accounts		$ 2.4	$ 2.6	$ 3.3	$ 3.3
Depreciation rate based on average depreciable property balance		3.33%	3.26%	3.29%
Dividends per Common Share | $ / shares		$ 1.62	$ 1.56	$ 1.52
Increase in dividend declared amount per share | $ / shares		$ 1.62
Weighted average cost inventory amount		$ 1.0	$ 1.8
Average Interest Rate On Debt		5.48%	2.50%	1.71%
Other Deferred Charges
Significant Accounting Policies [Line Items]
Regulatory assets		$ 11.0	$ 11.1
Materials And Supplies
Significant Accounting Policies [Line Items]
Weighted average cost inventory amount		13.5	11.4
Subsequent Event
Significant Accounting Policies [Line Items]
Increase in dividend declared amount per share | $ / shares	$ 1.7
Unbilled Revenues
Significant Accounting Policies [Line Items]
Allowance for doubtful accounts		0.1	0.1	$ 0.1
Accounts Receivable
Significant Accounting Policies [Line Items]
Allowance for doubtful accounts		$ 2.3	2.5
Utilities
Significant Accounting Policies [Line Items]
Number of Subsidiaries | Subsidiary		3
Unitil Service; Unitil Realty; and Unitil Resources
Significant Accounting Policies [Line Items]
Number of Subsidiaries | Subsidiary		3
Fitchburg Gas and Electric Light Company | Electric and Gas Division | Other Deferred Charges
Significant Accounting Policies [Line Items]
Hardship accounts in regulatory assets		$ 6.0	$ 5.8
Quarterly Payment
Significant Accounting Policies [Line Items]
Dividends per Common Share | $ / shares		$ 0.405	$ 0.39	$ 0.38
Quarterly Payment | Subsequent Event
Significant Accounting Policies [Line Items]
Common stock dividend paid per share | $ / shares	0.425
Increase in dividend declared amount per share | $ / shares	$ 0.02
Environmental and Rate Case Costs and Other Expenditures | Recovered over the next seven years
Significant Accounting Policies [Line Items]
Regulatory assets		$ 7.2
ISO-NE Obligations
Significant Accounting Policies [Line Items]
Cash Deposits		$ 3.3	$ 6.0
Maximum
Significant Accounting Policies [Line Items]
Lease term		12 months
Deferred Compensation Plan [Member]
Significant Accounting Policies [Line Items]
Investments in trading securities		$ 1.3	$ 0.6