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Regulatory Liabilities (Detail) - USD ($) $ in Millions | Sep. 30, 2019 | Dec. 31, 2018 | Sep. 30, 2018 | Dec. 31, 2017
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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

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- 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 + Details
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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. + Details
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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 + Details
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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=6877327&loc=d3e13212-122682 + Details
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