Fair Value (Fair Values of Financial Instruments) (Details) - USD ($) $ in Millions	Mar. 31, 2025	Dec. 31, 2024
Carrying Value [Member]
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Cash and cash equivalents	$ 315.1	$ 435.5
Total financial instruments	9,713.4	9,566.0
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	8,808.3	8,542.2
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	97.0	86.9
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	161.0	157.7
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	315.1	435.5
Debt	784.3	784.1
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	332.0	343.7
Estimated Fair Value [Member]
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Cash and cash equivalents	315.1	435.5
Total financial instruments	9,697.4	9,546.5
Debt	751.9	744.4
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	8,808.3	8,542.2
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	97.0	86.9
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	161.0	157.7
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	315.1	435.5
Debt	751.9	744.4
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	$ 316.0	$ 324.2

Fair value portion of asset recognized for present right to economic benefit.

Reference 1: http://www.xbrl.org/2003/role/exampleRef
-Topic 820
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-Section 55
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-URI https://asc.fasb.org/1943274/2147482078/820-10-55-100

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Reference 3: http://www.xbrl.org/2003/role/disclosureRef
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- Definition

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.

+ References

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Topic 820
-SubTopic 10
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-Section 50
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Reference 2: http://www.xbrl.org/2009/role/commonPracticeRef
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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.

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No definition available.

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- Definition

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/2009/role/commonPracticeRef
-Topic 820
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-URI https://asc.fasb.org/1943274/2147482106/820-10-50-2E

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- Definition

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.

+ References

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Topic 820
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