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Investment Securities (Investment Holding Common Equities/ Other Investments) (Details) - USD ($) $ in Millions	9 Months Ended	12 Months Ended
	Sep. 30, 2016	Dec. 31, 2015
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fixed maturity investments, at fair value	$ 4,131.5	$ 2,630.2
Common Stock
Schedule of Trading Securities and Other Trading Assets [Line Items]
Cost or amortized cost	378.9	822.5
Trading Securities, Unrealized Holding Gain	50.7	302.8
Trading Securities, Unrealized Holding Loss	(4.6)	(11.4)
Net foreign currency gains (losses)	0.0	0.0
Fixed maturity investments, at fair value	425.0	1,113.9
Other long-term investments.
Schedule of Trading Securities and Other Trading Assets [Line Items]
Cost or amortized cost	317.1	304.5
Trading Securities, Unrealized Holding Gain	42.7	32.0
Trading Securities, Unrealized Holding Loss	(6.4)	(18.4)
Net foreign currency gains (losses)	(2.0)	(2.3)
Fixed maturity investments, at fair value	$ 351.4	$ 315.8

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-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 210
-Section S99
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-Publisher FASB
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-Glossary Trading Securities
-URI http://asc.fasb.org/extlink&oid=6526789

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