Investment Securities (Net Realized and Unrealized Investment Gains (Losses) (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2014	Jun. 30, 2013	Jun. 30, 2014	Jun. 30, 2013
Schedule of Trading Securities and Other Trading Assets [Line Items]
Trading Securities, Realized Gain (Loss), Total	$ 30.5	$ 22.8	$ 52.2	$ 59.4
Net Unrealized Gain (Loss) and Foreign Currency Gain (Loss) on Trading Securities	83.4	(60.1)	125.5	(21.5)
Net Realized and Unrealized Gain (Loss) on Trading Securities	113.9	(37.3)	177.7	37.9
Tax on net realized and unrealized gains (loss), trading securities	(23.0)	12.5	(38.7)	(1.2)
Realized and unrealized gain	90.9	(24.8)	139.0	36.7
Fixed maturity investments.
Schedule of Trading Securities and Other Trading Assets [Line Items]
Trading Securities, Realized Gain (Loss), Total	8.7	2.8	9.6	10.8
Net Unrealized Gain (Loss) and Foreign Currency Gain (Loss) on Trading Securities	53.1	(44.3)	84.7	(70.3)
Short-term investments
Schedule of Trading Securities and Other Trading Assets [Line Items]
Trading Securities, Realized Gain (Loss), Total	0	0.7	0	0.8
Common equity securities
Schedule of Trading Securities and Other Trading Assets [Line Items]
Trading Securities, Realized Gain (Loss), Total	19.4	22.9	38.2	45.3
Net Unrealized Gain (Loss) and Foreign Currency Gain (Loss) on Trading Securities	22.4	(19.4)	26.9	42.5
Convertible fixed maturity investments
Schedule of Trading Securities and Other Trading Assets [Line Items]
Trading Securities, Realized Gain (Loss), Total	1.5	(3.4)	3.9	(0.6)
Net Unrealized Gain (Loss) and Foreign Currency Gain (Loss) on Trading Securities	(2.1)	(1.4)	(1.4)	(2.8)
Other long-term investments
Schedule of Trading Securities and Other Trading Assets [Line Items]
Trading Securities, Realized Gain (Loss), Total	0.9	0	0.6	2.9
Net Unrealized Gain (Loss) and Foreign Currency Gain (Loss) on Trading Securities	10.0	5.0	15.3	9.1
Forward Contracts
Schedule of Trading Securities and Other Trading Assets [Line Items]
Trading Securities, Realized Gain (Loss), Total	$ 0	$ (0.2)	$ (0.1)	$ 0.2