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Investment Securities (Investment Holding Common Equities/ Other Investments) (Details) - USD ($) $ in Millions	9 Months Ended	12 Months Ended
	Sep. 30, 2017	Dec. 31, 2016
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fixed maturity investments, at fair value	$ 1,490.0	$ 2,081.1
Common Stock
Schedule of Trading Securities and Other Trading Assets [Line Items]
Cost or amortized cost	693.8	258.6
Trading Securities, Unrealized Holding Gain	82.6	29.0
Trading Securities, Unrealized Holding Loss	(2.0)	(2.0)
Net foreign currency gains (losses)	0.0	0.0
Fixed maturity investments, at fair value	774.4	285.6
Other long-term investments.
Schedule of Trading Securities and Other Trading Assets [Line Items]
Cost or amortized cost	254.5	194.0
Trading Securities, Unrealized Holding Gain	13.1	7.9
Trading Securities, Unrealized Holding Loss	(21.2)	(25.2)
Net foreign currency gains (losses)	(16.8)	(3.9)
Fixed maturity investments, at fair value	$ 229.6	$ 172.8

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-Publisher FASB
-Name Accounting Standards Codification
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-SubTopic 210
-Section S99
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-URI http://asc.fasb.org/extlink&oid=6526789

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