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Regulatory Liabilities (Detail) - USD ($) $ in Millions | Mar. 31, 2021 | Dec. 31, 2020 | Mar. 31, 2020 | Dec. 31, 2017
--- | --- | --- | --- | ---
Regulatory Liabilities [Line Items] |  |  |  |
Regulatory Liabilities | $ 58.2 | $ 49.8 | $ 55.6 |
Less: Current Portion of Regulatory Liabilities | 14.7 | 5.5 | 10.5 |
Regulatory Liabilities – noncurrent | 43.5 | 44.3 | 45.1 |
Income Taxes |  |  |  |
Regulatory Liabilities [Line Items] |  |  |  |
Regulatory Liabilities | 45.1 | 45.5 | 46.1 | $ 48.9
Rate Adjustment Mechanisms |  |  |  |
Regulatory Liabilities [Line Items] |  |  |  |
Regulatory Liabilities | 12.9 | 4.1 | 9.1 |
Other |  |  |  |
Regulatory Liabilities [Line Items] |  |  |  |
Regulatory Liabilities | $ 0.2 | $ 0.2 | $ 0.4 |

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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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Period Type: | instant

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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=120391452&loc=d3e13212-122682 + Details
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