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Investment Securities (Investment Holding Common Equities/ Other Investments) (Details) - USD ($) $ in Millions	9 Months Ended	12 Months Ended
	Sep. 30, 2018	Dec. 31, 2017
Common Stock
Debt and Equity Securities, FV-NI [Line Items]
Cost or amortized cost	$ 873.4	$ 739.7
Trading Securities, Unrealized Holding Gain	171.1	129.4
Trading Securities, Unrealized Holding Loss	(1.0)	(3.0)
Net foreign currency gains (losses)	0.0	0.0
Fixed maturity investments, at fair value	1,043.5	866.1
Other long-term investments
Debt and Equity Securities, FV-NI [Line Items]
Cost or amortized cost	303.8	246.6
Trading Securities, Unrealized Holding Gain	34.8	6.8
Trading Securities, Unrealized Holding Loss	(47.2)	(39.7)
Net foreign currency gains (losses)	(1.7)	(4.9)
Fixed maturity investments, at fair value	$ 289.7	$ 208.8

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Amount of investment in debt security measured at fair value with change in fair value recognized in net income (trading).

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 45
-Paragraph 1
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=109979856&loc=d3e26610-111562

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-Topic 320
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-Section 45
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=109979856&loc=d3e26610-111562

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Amount of unrealized holding gain before loss recognized in the income statement for investments in trading securities.

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