Fair Value (Fair Values of Financial Instruments) (Details) - USD ($) $ in Millions	Dec. 31, 2023	Dec. 31, 2022
Carrying Value [Member]
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Cash and cash equivalents	$ 316.1	$ 305.0
Total financial instruments	8,963.8	8,592.8
Carrying Value [Member] \| Income Approach Valuation Technique [Member] \| Fair Value Through AOCI [Member] \| Debt Securities [Member]
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Fair value of investments	7,985.3	7,481.8
Carrying Value [Member] \| Income Approach Valuation Technique [Member] \| Fair Value Through Net Income [Member] \| Other Investments [Member]
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Fair value of investments	118.4	131.8
Carrying Value [Member] \| Income Approach Valuation Technique [Member] \| Fair Value Through Net Income [Member] \| Equity Securities [Member]
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Fair value of investments	130.9	241.9
Carrying Value [Member] \| Cost Approach Valuation Technique [Member] \| Amortized Cost/Cost [Member]
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Cash and cash equivalents	316.1	305.0
Debt	783.2	782.4
Carrying Value [Member] \| Cost Approach Valuation Technique [Member] \| Amortized Cost/Cost [Member] \| Other Investments [Member]
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Fair value of investments	413.1	432.3
Estimated Fair Value [Member]
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Cash and cash equivalents	316.1	305.0
Total financial instruments	8,944.2	8,561.5
Debt	727.5	713.9
Estimated Fair Value [Member] \| Income Approach Valuation Technique [Member] \| Fair Value Through AOCI [Member] \| Debt Securities [Member]
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Fair value of investments	7,985.3	7,481.8
Estimated Fair Value [Member] \| Income Approach Valuation Technique [Member] \| Fair Value Through Net Income [Member] \| Other Investments [Member]
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Fair value of investments	118.4	131.8
Estimated Fair Value [Member] \| Income Approach Valuation Technique [Member] \| Fair Value Through Net Income [Member] \| Equity Securities [Member]
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Fair value of investments	130.9	241.9
Estimated Fair Value [Member] \| Cost Approach Valuation Technique [Member] \| Amortized Cost/Cost [Member]
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Cash and cash equivalents	316.1	305.0
Debt	727.5	713.9
Estimated Fair Value [Member] \| Cost Approach Valuation Technique [Member] \| Amortized Cost/Cost [Member] \| Other Investments [Member]
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Fair value of investments	$ 393.5	$ 401.0

Fair value portion of probable future economic benefits obtained or controlled by an entity as a result of past transactions or events.

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Name Accounting Standards Codification
-Topic 820
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (a)
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147482106/820-10-50-2

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Fair value portion of currency on hand as well as demand deposits with banks or financial institutions. Includes other kinds of accounts that have the general characteristics of demand deposits. Also includes short-term, highly liquid investments that are both readily convertible to known amounts of cash and so near their maturity that they present insignificant risk of changes in value because of changes in interest rates.

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Fair value portion of investment securities, including, but not limited to, marketable securities, derivative financial instruments, and investments accounted for under the equity method.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Topic 820
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 2
-Subparagraph (a)
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147482106/820-10-50-2

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The fair value amount of long-term debt whether such amount is presented as a separate caption or as a parenthetical disclosure. Additionally, this element may be used in connection with the fair value disclosures required in the footnote disclosures to the financial statements. The element may be used in both the balance sheet and disclosure in the same submission.

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