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Fair Value (Fair Values of Financial Instruments) (Details) - USD ($) $ in Millions | Sep. 30, 2024 | Dec. 31, 2023
--- | --- | ---
Carrying Value [Member] |  |
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items] |  |
Cash and cash equivalents | $ 427.1 | $ 316.1
Total financial instruments | 9,672.7 | 8,963.8
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 | 8,620.0 | 7,985.3
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 | 101.2 | 118.4
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 | 150.1 | 130.9
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 | 427.1 | 316.1
Debt | 783.9 | 783.2
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 | 374.3 | 413.1
Estimated Fair Value [Member] |  |
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items] |  |
Cash and cash equivalents | 427.1 | 316.1
Total financial instruments | 9,658.4 | 8,944.2
Debt | 752.5 | 727.5
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 | 8,620.0 | 7,985.3
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 | 101.2 | 118.4
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 | 150.1 | 130.9
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 | 427.1 | 316.1
Debt | 752.5 | 727.5
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 | $ 360.0 | $ 393.5

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- Definition Fair value portion of asset recognized for present right to economic benefit. + References Reference 1: http://www.xbrl.org/2003/role/exampleRef -Topic 820 -SubTopic 10 -Name Accounting Standards Codification -Section 55 -Paragraph 100 -Publisher FASB -URI https://asc.fasb.org/1943274/2147482078/820-10-55-100 Reference 2: http://www.xbrl.org/2003/role/disclosureRef -Topic 820 -SubTopic 10 -Name Accounting Standards Codification -Section 50 -Paragraph 2 -Subparagraph (b) -Publisher FASB -URI https://asc.fasb.org/1943274/2147482106/820-10-50-2 Reference 3: 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 + Details
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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 -Name Accounting Standards Codification -Section 50 -Paragraph 2 -Subparagraph (b) -Publisher FASB -URI https://asc.fasb.org/1943274/2147482106/820-10-50-2 Reference 2: http://www.xbrl.org/2009/role/commonPracticeRef -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 + Details
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- Definition 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. + References No definition available. + Details
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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 -SubTopic 10 -Name Accounting Standards Codification -Section 50 -Paragraph 2E -Publisher FASB -URI https://asc.fasb.org/1943274/2147482106/820-10-50-2E Reference 2: http://www.xbrl.org/2009/role/commonPracticeRef -Topic 820 -SubTopic 10 -Name Accounting Standards Codification -Section 50 -Paragraph 2 -Subparagraph (b) -Publisher FASB -URI https://asc.fasb.org/1943274/2147482106/820-10-50-2 Reference 3: http://www.xbrl.org/2009/role/commonPracticeRef -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 + Details
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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 -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 Reference 2: http://www.xbrl.org/2009/role/commonPracticeRef -Topic 820 -SubTopic 10 -Name Accounting Standards Codification -Section 50 -Paragraph 2 -Subparagraph (bbb)(2)(i) -Publisher FASB -URI https://asc.fasb.org/1943274/2147482106/820-10-50-2 Reference 3: http://www.xbrl.org/2009/role/commonPracticeRef -Topic 820 -SubTopic 10 -Name Accounting Standards Codification -Section 50 -Paragraph 2 -Subparagraph (bbb)(2) -Publisher FASB -URI https://asc.fasb.org/1943274/2147482106/820-10-50-2 Reference 4: http://www.xbrl.org/2009/role/commonPracticeRef -Topic 820 -SubTopic 10 -Name Accounting Standards Codification -Section 50 -Paragraph 2 -Subparagraph (bbb)(1) -Publisher FASB -URI https://asc.fasb.org/1943274/2147482106/820-10-50-2 + Details
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