Investment Securities (Investment Holding Common Equities/ Other Investments) (Details) (USD $) In Millions, unless otherwise specified	9 Months Ended	12 Months Ended
	Sep. 30, 2014	Dec. 31, 2013
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fixed maturity investments, at fair value	$ 4,785.5	$ 5,030.5
Common shareholders' equity.
Schedule of Trading Securities and Other Trading Assets [Line Items]
Cost or amortized cost	766.7	890.2
Trading Securities, Unrealized Holding Gain	229.9	271.0
Trading Securities, Unrealized Holding Loss	(9.1)	(3.6)
Net foreign currency gains (losses)	(1.4)	(0.8)
Fixed maturity investments, at fair value	986.1	1,156.8
Convertible Debt Securities
Schedule of Trading Securities and Other Trading Assets [Line Items]
Cost or amortized cost	45.0	71.7
Trading Securities, Unrealized Holding Gain	4.4	9.9
Trading Securities, Unrealized Holding Loss	(1.1)	(0.9)
Net foreign currency gains (losses)	0.6	(0.2)
Fixed maturity investments, at fair value	48.9	80.5
Other long-term investments.
Schedule of Trading Securities and Other Trading Assets [Line Items]
Cost or amortized cost	256.1	238.3
Trading Securities, Unrealized Holding Gain	85.5	79.6
Trading Securities, Unrealized Holding Loss	(21.6)	(26.6)
Net foreign currency gains (losses)	0.9	(2.4)
Fixed maturity investments, at fair value	$ 320.9	$ 288.9