Summary of Significant Accounting Policies - Additional Information (Detail) $ in Millions			1 Months Ended	3 Months Ended
	Oct. 31, 2025 USD ($)	Jan. 31, 2025 USD ($)	Dec. 31, 2024 Megawatts	Mar. 31, 2026 USD ($) Subsidiary Customer MW mi	Mar. 31, 2025 USD ($)	Apr. 30, 2026 USD ($)	Dec. 31, 2025 USD ($)
Significant Accounting Policies [Line Items]
Number of Subsidiaries \| Entity \| Subsidiary				5
Length of Pipeline \| mi				85
Cost of removal obligation				$ 156.5	$ 143.9		$ 153.0
Investments in trading securities				6.5	6.1		6.7
Debt Instrument, Face Amount						$ 40.0
Goodwill				4.3	0.0		4.3
Deductible portion of goodwill	$ 1.9			$ 0.4
Amortization period	15 years			15 years
Business Acquisition Liabilities Assumed				$ 30.0
Cash And Cash Equivalents Acquired				70.0
Acquisition, Net of Cash Acquired				0.0	71.2
Asset Acquisition Consideration Transferred				100.0
Net Utility Plant				1,812.8	1,618.9		1,798.5
Allowance for doubtful accounts				2.8	2.9		2.2
Regulatory assets				109.7	100.9		116.9
Regulatory Liabilities				88.4	76.5		80.8
Profit Loss				33.2	27.5
Transition Services Agreement Cost				0.3
Electric And Gas Division [Member]
Significant Accounting Policies [Line Items]
Business Combination Price Of Acquisition Expected		$ 71.4
Other Deferred Charges [Member]
Significant Accounting Policies [Line Items]
Regulatory assets				16.1	14.3		12.8
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	2.8		2.1
Unitil Service; Unitil Realty; and Unitil Resources
Significant Accounting Policies [Line Items]
Number of Subsidiaries \| Entity \| Subsidiary				4
Fitchburg Gas And Electric Light Company [Member]
Significant Accounting Policies [Line Items]
Energy Storage Systems 1			5,000	1,268
Mid Duration Storage \| Megawatts			1,500
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				$ 9.5	8.2		8.9
Bangor Natural Gas
Significant Accounting Policies [Line Items]
Goodwill		1.6		$ 1.6
Net Utility Plant		66.7
Working Capital		3.8
Noncurrent assets		0.2
Regulatory Liabilities		$ 0.9
Maine Natural Gas Corporation
Significant Accounting Policies [Line Items]
Goodwill	$ 2.7
Acquisition, Net of Cash Acquired	91.1
Net Utility Plant	82.1
Working Capital	8.4
Noncurrent assets	1.4
Regulatory Liabilities	$ 3.5
Number of Customer \| Customer				6,300
Environmental and Rate Case Costs and Other Expenditures \| Recovered over the next seven years
Significant Accounting Policies [Line Items]
Regulatory assets				$ 8.6
ISO-NE Obligations
Significant Accounting Policies [Line Items]
Cash Deposits				$ 8.7	6.6		8.5
Maximum
Significant Accounting Policies [Line Items]
Lease term				12 months
Deferred Compensation Plan [Member]
Significant Accounting Policies [Line Items]
Investments in trading securities				$ 3.9	$ 2.5		$ 3.5

UNITIL CORP 10-Q