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Regulatory Liabilities (Detail) - USD ($) $ in Millions	Sep. 30, 2023	Dec. 31, 2022	Sep. 30, 2022
Regulatory Liabilities [Line Items]
Regulatory Liabilities	$ 51.7	$ 51.9	$ 58.7
Less: Current Portion of Regulatory Liabilities	16.9	15.0	20.8
Regulatory Liabilities – noncurrent	34.8	36.9	37.9
Income Taxes
Regulatory Liabilities [Line Items]
Regulatory Liabilities	38.9	41.0	42.1
Rate Adjustment Mechanisms
Regulatory Liabilities [Line Items]
Regulatory Liabilities	$ 12.8	$ 10.9	$ 16.6

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//1943274/2147481854/980-405-25-1

+ 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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Namespace Prefix:	us-gaap_
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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//1943274/2147481854/980-405-25-1

+ Details

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Namespace Prefix:	us-gaap_
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
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-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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