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Regulatory Liabilities (Detail) - USD ($) $ in Millions	Sep. 30, 2021	Dec. 31, 2020	Sep. 30, 2020	Dec. 31, 2017
Regulatory Liabilities [Line Items]
Regulatory Liabilities	$ 56.0	$ 49.8	$ 55.3
Less: Current Portion of Regulatory Liabilities	13.1	5.5	10.6
Regulatory Liabilities – noncurrent	42.9	44.3	44.7
Income Taxes
Regulatory Liabilities [Line Items]
Regulatory Liabilities	44.6	45.5	45.8	$ 48.9
Rate Adjustment Mechanisms
Regulatory Liabilities [Line Items]
Regulatory Liabilities	11.3	4.1	9.2
Other
Regulatory Liabilities [Line Items]
Regulatory Liabilities	$ 0.1	$ 0.2	$ 0.3

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
-Publisher FASB
-Name Accounting Standards Codification
-Topic 980
-SubTopic 405
-Section 25
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=6500807&loc=d3e48068-110394

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- 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.

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- 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
-Publisher FASB
-Name Accounting Standards Codification
-Topic 980
-SubTopic 405
-Section 25
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=6500807&loc=d3e48068-110394

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- 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
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02.24)
-URI http://asc.fasb.org/extlink&oid=120391452&loc=d3e13212-122682

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