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Summary of Significant Accounting Policies - Additional Information (Detail) $ / shares in Units, $ in Millions	1 Months Ended	12 Months Ended
	Dec. 31, 2024 USD ($) Megawatts	Dec. 31, 2025 $ / shares	Dec. 31, 2024 USD ($) Megawatts Subsidiary mi $ / shares	Dec. 31, 2023 USD ($) $ / shares	Dec. 31, 2022 USD ($) $ / shares	Dec. 31, 2021 USD ($)
Significant Accounting Policies [Line Items]
Length Of Pipeline | mi			85
Cost of removal obligation	$ 139.2		$ 139.2	$ 126.3
Regulatory assets	112.0		112.0	109.6
Investments in trading securities	$ 6.3		$ 6.3	6.0
Long-Term Renewable Contracts			30.00%
Energy storage systems | Megawatts	5,000
Mid-Duration Storage | Megawatts	1,500
Number of Subsidiaries | Subsidiary			3
Allowance for doubtful accounts	$ 2.4		$ 2.4	$ 2.4	$ 2.6	$ 3.3
Depreciation rate based on average depreciable property balance			3.49%	3.33%	3.26%
Dividends per Common Share | $ / shares			$ 1.7	$ 1.62	$ 1.56
Increase in dividend declared amount per share | $ / shares			$ 1.7
Weighted average cost inventory amount	1.1		$ 1.1	$ 1.0
Average Interest Rate On Debt			6.22%	5.48%	2.50%
Other Deferred Charges
Significant Accounting Policies [Line Items]
Regulatory assets	14.1		$ 14.1	$ 11.0
Electric and Gas Division
Significant Accounting Policies [Line Items]
Business Combination, Price of Acquisition, Expected			70.9
Materials And Supplies
Significant Accounting Policies [Line Items]
Weighted average cost inventory amount	14.2		14.2	13.5
Subsequent Event
Significant Accounting Policies [Line Items]
Increase in dividend declared amount per share | $ / shares		$ 1.8
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		$ 2.3	2.3
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
Significant Accounting Policies [Line Items]
Energy storage systems | Megawatts			5,000
Mid-Duration Storage | Megawatts			1,500
Fitchburg Gas and Electric Light Company | Electric and Gas Division | Other Deferred Charges
Significant Accounting Policies [Line Items]
Hardship accounts in regulatory assets	7.9		$ 7.9	$ 6.0
Quarterly Payment
Significant Accounting Policies [Line Items]
Dividends per Common Share | $ / shares			$ 0.425	$ 0.405	$ 0.39
Quarterly Payment | Subsequent Event
Significant Accounting Policies [Line Items]
Common stock dividend paid per share | $ / shares		0.45
Increase in dividend declared amount per share | $ / shares		$ 0.025
Environmental and Rate Case Costs and Other Expenditures | Recovered over the next seven years
Significant Accounting Policies [Line Items]
Regulatory assets	7.1		$ 7.1
ISO-NE Obligations
Significant Accounting Policies [Line Items]
Cash Deposits	$ 5.0		$ 5.0	$ 3.3
Maximum
Significant Accounting Policies [Line Items]
Lease term	12 months		12 months
Deferred Compensation Plan
Significant Accounting Policies [Line Items]
Investments in trading securities	$ 2.2		$ 2.2	$ 1.3

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