Fair Value (Fair Values of Financial Instruments) (Details) - USD ($) $ in Millions	Sep. 30, 2026	Dec. 31, 2025
Carrying Value [Member]
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Cash and cash equivalents	$ 506.3	$ 1,122.7
Total financial instruments	10,839.7	11,207.7
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	9,835.7	9,536.5
Carrying Value [Member] \| Income Approach Valuation Technique [Member] \| Fair Value Through Net Income [Member] \| Location, Statement of Financial Position, Balance [Axis]: us-gaap:OtherInvestments
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Fair value of investments	78.5	80.9
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	197.1	184.8
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	506.3	1,122.7
Debt	844.2	1,218.3
Carrying Value [Member] \| Cost Approach Valuation Technique [Member] \| Amortized Cost/Cost [Member] \| Location, Statement of Financial Position, Balance [Axis]: us-gaap:OtherInvestments
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Fair value of investments	222.1	282.8
Estimated Fair Value [Member]
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Cash and cash equivalents	506.3	1,122.7
Total financial instruments	10,828.5	11,199.4
Debt	788.4	1,206.6
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	9,835.7	9,536.5
Estimated Fair Value [Member] \| Income Approach Valuation Technique [Member] \| Fair Value Through Net Income [Member] \| Location, Statement of Financial Position, Balance [Axis]: us-gaap:OtherInvestments
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Fair value of investments	78.5	80.9
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	197.1	184.8
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	506.3	1,122.7
Debt	788.4	1,206.6
Estimated Fair Value [Member] \| Cost Approach Valuation Technique [Member] \| Amortized Cost/Cost [Member] \| Location, Statement of Financial Position, Balance [Axis]: us-gaap:OtherInvestments
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Fair value of investments	$ 210.9	$ 274.5

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

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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.

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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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