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Investment Securities (Investment Holding Common Equities/ Other Investments) (Details) - USD ($) $ in Millions	6 Months Ended	12 Months Ended
	Jun. 30, 2018	Dec. 31, 2017
Common Stock
Schedule of Trading Securities and Other Trading Assets [Line Items]
Cost or amortized cost	$ 796.0	$ 739.7
Trading Securities, Unrealized Holding Gain	117.9	129.4
Trading Securities, Unrealized Holding Loss	(2.7)	(3.0)
Net foreign currency gains (losses)	0.0	0.0
Fixed maturity investments, at fair value	911.2	866.1
Other long-term investments.
Schedule of Trading Securities and Other Trading Assets [Line Items]
Cost or amortized cost	289.8	246.6
Trading Securities, Unrealized Holding Gain	17.6	6.8
Trading Securities, Unrealized Holding Loss	(50.2)	(39.7)
Net foreign currency gains (losses)	(1.7)	(4.9)
Fixed maturity investments, at fair value	$ 255.5	$ 208.8

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