Investment Securities (Investment Holding Common Equities/ Other Investments) (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended	12 Months Ended
	Mar. 31, 2014	Dec. 31, 2013
Investment securities
Fixed maturity investments, at fair value	$ 4,988.7	$ 5,030.5
Common shareholders' equity.
Investment securities
Cost or amortized cost	905.6	890.2
Trading Securities, Unrealized Holding Gain	273.9	271.0
Trading Securities, Unrealized Holding Loss	(4.5)	(3.6)
Net foreign currency gains (losses)	(0.4)	(0.8)
Fixed maturity investments, at fair value	1,174.6	1,156.8
Convertible Debt Securities
Investment securities
Cost or amortized cost	64.3	71.7
Trading Securities, Unrealized Holding Gain	10.6	9.9
Trading Securities, Unrealized Holding Loss	(1.0)	(0.9)
Net foreign currency gains (losses)	(0.1)	(0.2)
Fixed maturity investments, at fair value	73.8	80.5
Other long-term investments.
Investment securities
Cost or amortized cost	246.4	238.3
Trading Securities, Unrealized Holding Gain	83.4	79.6
Trading Securities, Unrealized Holding Loss	(25.5)	(26.6)
Net foreign currency gains (losses)	(2.0)	(2.4)
Fixed maturity investments, at fair value	$ 302.3	$ 288.9