Fair Value (Estimated Fair Values of Financial Instruments Not Carried at Fair Value, Including Assets Held-for-Sale) (Details) - USD ($) $ in Millions	Sep. 30, 2018	Dec. 31, 2017
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Equity securities	$ 574.8	$ 576.2
Level 3 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Equity securities	1.1	1.1
Other investments	3.6	3.6
Carrying Value [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents	251.3	376.4
Estimated Fair Value [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents	251.3	376.4
Total financial instruments, including held-for-sale	655.0	753.7
Total financial instruments, excluding held-for-sale	521.6	675.2
Equity securities		8.4
Other investments	403.7	368.9
Debt	823.9	865.7
Estimated Fair Value [Member] \| Level 1 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents	251.3	376.4
Total financial instruments, including held-for-sale	251.3	376.4
Total financial instruments, excluding held-for-sale	117.9	297.9
Estimated Fair Value [Member] \| Level 2 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total financial instruments, including held-for-sale	8.7	8.4
Total financial instruments, excluding held-for-sale	8.7	8.4
Equity securities		8.4
Other investments	8.7
Debt	823.9	865.7
Estimated Fair Value [Member] \| Level 3 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total financial instruments, including held-for-sale	395.0	368.9
Total financial instruments, excluding held-for-sale	395.0	368.9
Other investments	395.0	368.9
Estimated Fair Value [Member] \| Chaucer [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Financial instruments, held-for-sale (Chaucer)	133.4	78.5
Estimated Fair Value [Member] \| Chaucer [Member] \| Level 1 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Financial instruments, held-for-sale (Chaucer)	$ 133.4	$ 78.5

Financial assets, excluding financial assets held-for-sale

No definition available.

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- Definition

Financial instrument held for sale that were not carried at fair value.

+ References

No definition available.

+ Details

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- Definition

Financial instrumen, that were not carried at fair values including held-for-sale.

+ References

No definition available.

+ Details

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- Definition

Fair value of investment other than investment in equity security, investment in debt security and equity method investment. Includes, but is not limited to, investment in certain entities that calculate net asset value per share. Example includes, but is not limited to, investment in hedge fund, venture capital fund, private equity fund, and real estate partnership or fund.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 820
-SubTopic 10
-Section 50
-Paragraph 6A
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=99377789&loc=SL6742756-110258

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 820
-SubTopic 10
-Section 35
-Paragraph 54B
-URI http://asc.fasb.org/extlink&oid=99392525&loc=SL7495116-110257

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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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- Definition

Amount of investment in equity security measured at fair value with change in fair value recognized in net income (FV-NI).

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 825
-SubTopic 10
-Section 45
-Paragraph 1A
-URI http://asc.fasb.org/extlink&oid=99377092&loc=SL75136599-209740

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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.

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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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