Fair Value (Fair Value and Cost of Non-Derivative Instruments) (Details) - USD ($) $ in Millions	Jun. 30, 2024	Dec. 31, 2023
Long-term Debt	$ 1,838.8	$ 1,837.6
Subsidiaries [Member]
Long-term Debt	1,012.7	1,012.5
Money Market Funds [Member] \| Fair Value [Member]
Investments, Fair Value Disclosure	0.3	0.6
Money Market Funds [Member] \| Fair Value [Member] \| Subsidiaries [Member]
Investments, Fair Value Disclosure	$ 0.3	$ 0.6

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 3: http://fasb.org/us-gaap/role/ref/legacyRef
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Reference 4: http://www.xbrl.org/2003/role/exampleRef
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Reference 6: http://www.xbrl.org/2003/role/disclosureRef
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Reference 7: http://www.xbrl.org/2009/role/commonPracticeRef
-Topic 944
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Name:	us-gaap_LongTermDebt
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant