Investment Securities (Investment Holding Common Equities/ Other Investments) (Details) - USD ($) $ in Millions	6 Months Ended	12 Months Ended
	Jun. 30, 2016	Dec. 31, 2015
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fixed maturity investments, at fair value	$ 4,362.8	$ 2,630.2
Common Stock
Schedule of Trading Securities and Other Trading Assets [Line Items]
Cost or amortized cost	374.1	822.5
Trading Securities, Unrealized Holding Gain	43.5	302.8
Trading Securities, Unrealized Holding Loss	(10.1)	(11.4)
Net foreign currency gains (losses)	0.0	0.0
Fixed maturity investments, at fair value	407.5	1,113.9
Other long-term investments.
Schedule of Trading Securities and Other Trading Assets [Line Items]
Cost or amortized cost	305.0	304.5
Trading Securities, Unrealized Holding Gain	36.7	32.0
Trading Securities, Unrealized Holding Loss	(10.3)	(18.4)
Net foreign currency gains (losses)	(2.2)	(2.3)
Fixed maturity investments, at fair value	$ 329.2	$ 315.8