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Regulatory Liabilities (Detail) - USD ($) $ in Millions | Jun. 30, 2025 | Dec. 31, 2024 | Jun. 30, 2024 | Dec. 31, 2017
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Regulatory Liabilities [Line Items] |  |  |  |
Regulatory Liabilities | $ 73.5 | $ 64.0 | $ 57.8 |
Less: Current Portion of Regulatory Liabilities | 21.0 | 17.2 | 24.5 |
Regulatory Liabilities – noncurrent | 52.5 | 46.8 | 33.3 |
Income Taxes |  |  |  |
Regulatory Liabilities [Line Items] |  |  |  |
Regulatory Liabilities | 49.7 | 50.4 | 37.2 | $ 48.9
Rate Adjustment Mechanisms |  |  |  |
Regulatory Liabilities [Line Items] |  |  |  |
Regulatory Liabilities | 17.8 | $ 13.6 | $ 20.6 |
Retirement Benefits |  |  |  |
Regulatory Liabilities [Line Items] |  |  |  |
Regulatory Liabilities | $ 6.0 |  |  |

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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 -Topic 980 -SubTopic 405 -Name Accounting Standards Codification -Section 25 -Paragraph 1 -Publisher FASB -URI https://asc.fasb.org/1943274/2147477571/980-405-25-1 + 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 -Topic 980 -SubTopic 405 -Name Accounting Standards Codification -Section 25 -Paragraph 1 -Publisher FASB -URI https://asc.fasb.org/1943274/2147477571/980-405-25-1 + 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 -Topic 210 -SubTopic 10 -Name Accounting Standards Codification -Section S99 -Paragraph 1 -Subparagraph (SX 210.5-02(24)) -Publisher FASB -URI https://asc.fasb.org/1943274/2147480566/210-10-S99-1 + Details
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