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Regulatory Liabilities (Detail) - USD ($) $ in Millions	Sep. 30, 2019	Dec. 31, 2018	Sep. 30, 2018	Dec. 31, 2017
Regulatory Liabilities [Line Items]
Regulatory Liabilities	$ 61.1	$ 58.5	$ 60.1
Less: Current Portion of Regulatory Liabilities	14.4	11.5	12.2
Regulatory Liabilities-noncurrent	46.7	47.0	47.9
Energy Supply & Other Rate Adjustment Mechanisms
Regulatory Liabilities [Line Items]
Regulatory Liabilities	13.5	11.5	12.2
Income Tax
Regulatory Liabilities [Line Items]
Regulatory Liabilities	$ 47.6	$ 47.0	$ 47.9	$ 48.9

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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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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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
-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

+ Details

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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
-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=6877327&loc=d3e13212-122682

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