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Regulatory Liabilities (Detail) - USD ($) $ in Millions	Sep. 30, 2025	Dec. 31, 2024	Sep. 30, 2024	Dec. 31, 2017
Regulatory Liabilities [Line Items]
Regulatory Liabilities	$ 70.6	$ 64.0	$ 68.7
Less: Current Portion of Regulatory Liabilities	18.4	17.2	22.4
Regulatory Liabilities – noncurrent	52.2	46.8	46.3
Income Taxes
Regulatory Liabilities [Line Items]
Regulatory Liabilities	48.8	50.4	50.0	$ 48.9
Rate Adjustment Mechanisms
Regulatory Liabilities [Line Items]
Regulatory Liabilities	15.5	$ 13.6	$ 18.7
Retirement Benefits
Regulatory Liabilities [Line Items]
Regulatory Liabilities	$ 6.3

X

- Definition

The amount for the individual regulatory liability as itemized in a table of regulatory liabilities as of the end of the period.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Topic 980
-SubTopic 405
-Name Accounting Standards Codification
-Section 25
-Paragraph 1
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147477571/980-405-25-1

+ Details

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Balance Type:	credit
Period Type:	instant

X

- Definition

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

+ References

No definition available.

+ Details

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Period Type:	duration

X

- Definition

The amount for the individual regulatory current liability as itemized in a table of regulatory current liabilities as of the end of the period.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Topic 980
-SubTopic 405
-Name Accounting Standards Codification
-Section 25
-Paragraph 1
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147477571/980-405-25-1

+ Details

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Data Type:	xbrli:monetaryItemType
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X

- Definition

The amount for the individual regulatory noncurrent liability as itemized in a table of regulatory noncurrent liabilities as of the end of the period.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Topic 210
-SubTopic 10
-Name Accounting Standards Codification
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02(24))
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147480566/210-10-S99-1

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- Details

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