Investment Securities (Net Realized and Unrealized Investment Gains (Losses) (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended
	Mar. 31, 2014	Mar. 31, 2013
Investment securities
Trading Securities, Realized Gain (Loss), Total	$ 21.7	$ 36.5
Net Unrealized Gain (Loss) and Foreign Currency Gain (Loss) on Trading Securities	42.1	38.7
Net Realized and Unrealized Gain (Loss) on Trading Securities	63.8	75.2
Tax on net realized and unrealized gains (loss), trading securities	15.7	13.7
Realized and unrealized gain	48.1	61.5
Fixed maturity investments.
Investment securities
Trading Securities, Realized Gain (Loss), Total	0.9	8.0
Net Unrealized Gain (Loss) and Foreign Currency Gain (Loss) on Trading Securities	31.6	(26.0)
Short-term investments
Investment securities
Trading Securities, Realized Gain (Loss), Total	0	0.1
Net Unrealized Gain (Loss) and Foreign Currency Gain (Loss) on Trading Securities	0	0
Common equity securities
Investment securities
Trading Securities, Realized Gain (Loss), Total	18.8	22.4
Net Unrealized Gain (Loss) and Foreign Currency Gain (Loss) on Trading Securities	4.5	61.9
Convertible fixed maturity investments
Investment securities
Trading Securities, Realized Gain (Loss), Total	2.4	2.7
Net Unrealized Gain (Loss) and Foreign Currency Gain (Loss) on Trading Securities	0.7	(1.3)
Other long-term investments
Investment securities
Trading Securities, Realized Gain (Loss), Total	(0.3)	2.9
Net Unrealized Gain (Loss) and Foreign Currency Gain (Loss) on Trading Securities	5.3	4.1
Forward Contracts
Investment securities
Trading Securities, Realized Gain (Loss), Total	(0.1)	0.4
Net Unrealized Gain (Loss) and Foreign Currency Gain (Loss) on Trading Securities	$ 0	$ 0