Investment Securities (Investment Holding Common Equities/ Other Investments) (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended	12 Months Ended
	Mar. 31, 2015	Dec. 31, 2014
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fixed maturity investments, at fair value	$ 4,771.9	$ 4,784.3
Common Stock
Schedule of Trading Securities and Other Trading Assets [Line Items]
Cost or amortized cost	623.3	633.6
Trading Securities, Unrealized Holding Gain	163.4	175.1
Trading Securities, Unrealized Holding Loss	(8.8)	(5.2)
Net foreign currency gains (losses)	9.0	(1.9)
Fixed maturity investments, at fair value	786.9	801.6
Convertible Debt Securities
Schedule of Trading Securities and Other Trading Assets [Line Items]
Cost or amortized cost	13.1	19.1
Trading Securities, Unrealized Holding Gain	1.8	0.9
Trading Securities, Unrealized Holding Loss	(0.3)	(0.2)
Net foreign currency gains (losses)	1.2	0.7
Fixed maturity investments, at fair value	15.8	20.5
Other long-term investments.
Schedule of Trading Securities and Other Trading Assets [Line Items]
Cost or amortized cost	339.8	343.3
Trading Securities, Unrealized Holding Gain	74.7	73.0
Trading Securities, Unrealized Holding Loss	(13.1)	(10.9)
Net foreign currency gains (losses)	1.2	1.6
Fixed maturity investments, at fair value	$ 402.6	$ 407.0