Fair Value (Fair Value of Financial Instruments) (Details) - USD ($) $ in Millions	Mar. 31, 2019	Dec. 31, 2018
Carrying Value [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents	$ 168.5	$ 1,020.7
Total financial instruments, excluding held-for-sale	7,834.4	8,236.0
Total financial instruments	7,837.2	8,263.8
Carrying Value [Member] \| Chaucer [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Financial instruments, held-for-sale (Chaucer)	2.8	27.8
Estimated Fair Value [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents	168.5	1,020.7
Total financial instruments, excluding held-for-sale	7,838.6	8,240.5
Total financial instruments	7,841.4	8,268.3
Debt	694.2	825.0
Estimated Fair Value [Member] \| Chaucer [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Financial instruments, held-for-sale (Chaucer)	2.8	27.8
Income Approach Valuation Technique [Member] \| Carrying Value [Member] \| Fair Value Through AOCI [Member] \| Debt Securities [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair value of investments	6,559.4	6,161.5
Income Approach Valuation Technique [Member] \| Carrying Value [Member] \| Fair Value Through Net Income [Member] \| Other Investments [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair value of investments	180.3	175.0
Income Approach Valuation Technique [Member] \| Carrying Value [Member] \| Fair Value Through Net Income [Member] \| Equity Securities [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair value of investments	504.7	464.4
Income Approach Valuation Technique [Member] \| Estimated Fair Value [Member] \| Fair Value Through AOCI [Member] \| Debt Securities [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair value of investments	6,559.4	6,161.5
Income Approach Valuation Technique [Member] \| Estimated Fair Value [Member] \| Fair Value Through Net Income [Member] \| Other Investments [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair value of investments	180.3	175.0
Income Approach Valuation Technique [Member] \| Estimated Fair Value [Member] \| Fair Value Through Net Income [Member] \| Equity Securities [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair value of investments	504.7	464.4
Cost Approach Valuation Technique [Member] \| Carrying Value [Member] \| Amortized Cost/Cost [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents	168.5	1,020.7
Debt	653.0	777.9
Cost Approach Valuation Technique [Member] \| Carrying Value [Member] \| Amortized Cost/Cost [Member] \| Other Investments [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair value of investments	421.5	414.4
Cost Approach Valuation Technique [Member] \| Estimated Fair Value [Member] \| Amortized Cost/Cost [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents	168.5	1,020.7
Debt	694.2	825.0
Cost Approach Valuation Technique [Member] \| Estimated Fair Value [Member] \| Amortized Cost/Cost [Member] \| Other Investments [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair value of investments	$ 425.7	$ 418.9

Financial assets held for sale.

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- Definition

Financial instruments excluding held for sale.

+ References

No definition available.

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- Definition

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

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 820
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=99377789&loc=d3e19207-110258

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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

No definition available.

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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.

+ References

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/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 820
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=99377789&loc=d3e19207-110258

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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

No definition available.

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