Fair Value (Fair Value and Cost of Non-Derivative Instruments) (Details) - USD ($) $ in Millions	Mar. 31, 2025	Dec. 31, 2024
Long-term Debt	$ 1,840.7	$ 1,840.0
Subsidiaries [Member]
Long-term Debt	1,013.0	1,012.9
Money Market Funds [Member] \| Fair Value [Member]
Investments, Fair Value Disclosure	0.6	0.4
Money Market Funds [Member] \| Fair Value [Member] \| Subsidiaries [Member]
Investments, Fair Value Disclosure	$ 0.6	$ 0.4

Amount, after deduction of unamortized premium (discount) and debt issuance cost, of long-term debt. Excludes lease obligation.

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
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Reference 2: http://www.xbrl.org/2003/role/exampleRef
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Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef
-Topic 942
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-Name Accounting Standards Codification
-Section S99
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-Subparagraph (SX 210.9-03(16))
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Reference 4: http://www.xbrl.org/2003/role/exampleRef
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Reference 5: http://www.xbrl.org/2003/role/exampleRef
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Reference 6: http://www.xbrl.org/2003/role/disclosureRef
-Topic 470
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Reference 7: http://www.xbrl.org/2009/role/commonPracticeRef
-Topic 944
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-Name Accounting Standards Codification
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-Topic 470
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-URI https://asc.fasb.org/1943274/2147481139/470-20-50-4

Name:	us-gaap_LongTermDebt
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant