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Summary of Significant Accounting Policies - Regulatory Assets (Detail) - USD ($) $ in Millions		Sep. 30, 2024	Dec. 31, 2023	Sep. 30, 2023
Regulatory Assets [Line Items]
Regulatory Assets		$ 110.8	$ 109.6	$ 104.4
Less: Current Portion of Regulatory Assets	[1]	61.3	56.5	53.5
Regulatory Assets – noncurrent		49.5	53.1	50.9
Environmental Matters
Regulatory Assets [Line Items]
Regulatory Assets		6.3	6.1	6.1
Other Deferred Charges
Regulatory Assets [Line Items]
Regulatory Assets		12.4	11.0	10.6
Retirement Benefits
Regulatory Assets [Line Items]
Regulatory Assets		26.2	29.8	26.5
Deferred Storm Charges
Regulatory Assets [Line Items]
Regulatory Assets		8.9	9.2	9.0
Income Taxes
Regulatory Assets [Line Items]
Regulatory Assets		0.5	1.1	1.2
Energy Supply and Other Rate Adjustment Mechanisms
Regulatory Assets [Line Items]
Regulatory Assets		$ 56.5	$ 52.4	$ 51.0
[1] Reflects amounts included in the Accrued Revenue on the Company’s Consolidated Balance Sheets.

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The amount for the individual regulatory asset as itemized in a table of regulatory assets as of the end of the period.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Topic 980
-SubTopic 340
-Name Accounting Standards Codification
-Section 50
-Paragraph 1
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147478742/980-340-50-1

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Carrying amount as of the balance sheet date of capitalized costs of regulated entities that are expected to be recovered through revenue sources within one year or the normal operating cycle, if longer. Such costs are capitalized if they meet both of the following criteria: a. It is probable that future revenue in an amount at least equal to the capitalized cost will result from inclusion of that cost in allowable costs for rate-making purposes. b. Based on available evidence, the future revenue will be provided to permit recovery of the previously incurred cost rather than to provide for expected levels of similar future costs. If the revenue will be provided through an automatic rate-adjustment clause, this criterion requires that the regulator's intent clearly be to permit recovery of the previously incurred cost.

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-Section 50
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Carrying amount as of the balance sheet date of capitalized costs of regulated entities that are not expected to be recovered through revenue sources within one year or the normal operating cycle if longer.

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