Investment Securities (Net Realized 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)	$ 25.3	$ 51.2
Trading securities, tax on realized holding gain (loss) on investments	(4.6)	(11.8)
Trading securities realized holding gain (loss) on investments after tax	20.7	39.4
Foreign Currency Transaction Gain (Loss), Realized	(3.6)	(14.7)
Trading securities, realized foreign currency transaction gain (loss) t	0.9	3.3
Trading securities, realized foreign currency gain (loss) net of tax	(2.7)	(11.4)
Trading Securities, Realized Gain (Loss), Total	21.7	36.5
Tax on Net Realized Gain (Loss) and Foreign Currency Gain (Loss) on Trading Securities	(3.7)	(8.5)
Net Realized Gain (Loss) and Foreign Currency Gain (Loss) on Trading Securities Net of Tax	18.0	28.0
Fixed maturity investments.
Investment securities
Trading Securities, Realized Gain (Loss)	4.4	22.6
Foreign Currency Transaction Gain (Loss), Realized	(3.5)	(14.6)
Trading Securities, Realized Gain (Loss), Total	0.9	8.0
Short-term investments
Investment securities
Trading Securities, Realized Gain (Loss)	0	0
Foreign Currency Transaction Gain (Loss), Realized	0	0.1
Trading Securities, Realized Gain (Loss), Total	0	0.1
Common equity securities
Investment securities
Trading Securities, Realized Gain (Loss)	18.9	22.6
Foreign Currency Transaction Gain (Loss), Realized	(0.1)	(0.2)
Trading Securities, Realized Gain (Loss), Total	18.8	22.4
Convertible fixed maturity investments
Investment securities
Trading Securities, Realized Gain (Loss)	2.4	2.7
Foreign Currency Transaction Gain (Loss), Realized	0	0
Trading Securities, Realized Gain (Loss), Total	2.4	2.7
Other long-term investments
Investment securities
Trading Securities, Realized Gain (Loss)	(0.3)	2.9
Foreign Currency Transaction Gain (Loss), Realized	0	0
Trading Securities, Realized Gain (Loss), Total	(0.3)	2.9
Forward Contracts
Investment securities
Trading Securities, Realized Gain (Loss)	(0.1)	0.4
Foreign Currency Transaction Gain (Loss), Realized	0	0
Trading Securities, Realized Gain (Loss), Total	$ (0.1)	$ 0.4