Investment Securities (Investment Holding Common Equities/ Other Investments) (Details) (USD $) In Millions, unless otherwise specified	6 Months Ended	12 Months Ended
	Jun. 30, 2014	Dec. 31, 2013
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fixed maturity investments, at fair value	$ 5,008.0	$ 5,030.5
Common shareholders' equity.
Schedule of Trading Securities and Other Trading Assets [Line Items]
Cost or amortized cost	913.5	890.2
Trading Securities, Unrealized Holding Gain	296.2	271.0
Trading Securities, Unrealized Holding Loss	(4.7)	(3.6)
Net foreign currency gains (losses)	1.1	(0.8)
Fixed maturity investments, at fair value	1,206.1	1,156.8
Convertible Debt Securities
Schedule of Trading Securities and Other Trading Assets [Line Items]
Cost or amortized cost	63.2	71.7
Trading Securities, Unrealized Holding Gain	7.7	9.9
Trading Securities, Unrealized Holding Loss	(0.4)	(0.9)
Net foreign currency gains (losses)	0.2	(0.2)
Fixed maturity investments, at fair value	70.7	80.5
Other long-term investments.
Schedule of Trading Securities and Other Trading Assets [Line Items]
Cost or amortized cost	242.4	238.3
Trading Securities, Unrealized Holding Gain	89.5	79.6
Trading Securities, Unrealized Holding Loss	(18.2)	(26.6)
Net foreign currency gains (losses)	(0.7)	(2.4)
Fixed maturity investments, at fair value	$ 313.0	$ 288.9