Investment Securities (Investment Holding Common Equities/ Other Investments) (Details) - USD ($) $ in Millions	6 Months Ended	12 Months Ended
	Jun. 30, 2015	Dec. 31, 2014
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fixed maturity investments, at fair value	$ 4,874.5	$ 4,784.3
Common Stock
Schedule of Trading Securities and Other Trading Assets [Line Items]
Cost or amortized cost	656.7	633.6
Trading Securities, Unrealized Holding Gain	69.1	175.1
Trading Securities, Unrealized Holding Loss	(12.2)	(5.2)
Net foreign currency gains (losses)	0.2	(1.9)
Fixed maturity investments, at fair value	713.8	801.6
Convertible Debt Securities
Schedule of Trading Securities and Other Trading Assets [Line Items]
Cost or amortized cost	3.1	19.1
Trading Securities, Unrealized Holding Gain	1.4	0.9
Trading Securities, Unrealized Holding Loss	0.0	(0.2)
Net foreign currency gains (losses)	0.0	0.7
Fixed maturity investments, at fair value	4.5	20.5
Other long-term investments.
Schedule of Trading Securities and Other Trading Assets [Line Items]
Cost or amortized cost	346.0	343.3
Trading Securities, Unrealized Holding Gain	54.0	73.0
Trading Securities, Unrealized Holding Loss	(11.9)	(10.9)
Net foreign currency gains (losses)	2.0	1.6
Fixed maturity investments, at fair value	$ 390.1	$ 407.0