Investment Securities (Net Unrealized Investment Gains (Losses)) (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012
Investment securities
Trading Securities, Change in Unrealized Holding Gain (Loss)	$ 50.4	$ 93.3	$ (2.5)	$ 122.3
Trading Securities, Tax on Unrealized Holding Gain (Loss) on Investments	(8.6)	(16.7)	13.1	(25.9)
Trading Securities Unrealized Holding Gain (Loss) on Investments after Tax	41.8	76.6	10.6	96.4
Foreign Currency Transaction Gain (Loss), Unrealized	(33.9)	(44.3)	8.4	(39.6)
Trading Securities, Unrealized Foreign Currency Transaction Gain (Loss) Tax	7.4	11.8	(2.9)	10.4
Trading Securities, Unrealized Foreign Currency Gain (Loss) Net of Tax	(26.5)	(32.5)	5.5	(29.2)
Net Unrealized Gain (Loss) and Foreign Currency Gain (Loss) on Trading Securities	16.5	49.0	5.9	82.7
Net Unrealized Gain (Loss) and Foreign Currency Gain (Loss) on Trading Securities Tax	(1.2)	(4.9)	10.2	(15.5)
Net Unrealized Gain (Loss) and Foreign Currency Gain (Loss) on Trading Securities Net of Tax	15.3	44.1	16.1	67.2
Fixed maturity investments.
Investment securities
Trading Securities, Change in Unrealized Holding Gain (Loss)	9.3	27.2	(94.8)	35.5
Foreign Currency Transaction Gain (Loss), Unrealized	(30.5)	(40.6)	6.2	(36.9)
Net Unrealized Gain (Loss) and Foreign Currency Gain (Loss) on Trading Securities	(21.2)	(13.4)	(88.6)	(1.4)
Short-term investments
Investment securities
Trading Securities, Change in Unrealized Holding Gain (Loss)	0	0	0	0
Foreign Currency Transaction Gain (Loss), Unrealized	0.1	0.1	0	0.1
Net Unrealized Gain (Loss) and Foreign Currency Gain (Loss) on Trading Securities	0.1	0.1	0	0.1
Common equity securities
Investment securities
Trading Securities, Change in Unrealized Holding Gain (Loss)	35.6	65.1	85.2	71.5
Foreign Currency Transaction Gain (Loss), Unrealized	(2.0)	(0.1)	(1.0)	(0.1)
Net Unrealized Gain (Loss) and Foreign Currency Gain (Loss) on Trading Securities	33.6	65.0	84.2	71.4
Convertible fixed maturity investments
Investment securities
Trading Securities, Change in Unrealized Holding Gain (Loss)	2.4	(0.6)	(0.5)	(2.9)
Foreign Currency Transaction Gain (Loss), Unrealized	(0.1)	0	0	0
Net Unrealized Gain (Loss) and Foreign Currency Gain (Loss) on Trading Securities	2.3	(0.6)	(0.5)	(2.9)
Other long-term investments
Investment securities
Trading Securities, Change in Unrealized Holding Gain (Loss)	3.1	1.6	7.6	18.2
Foreign Currency Transaction Gain (Loss), Unrealized	(1.4)	(3.7)	3.2	(2.7)
Net Unrealized Gain (Loss) and Foreign Currency Gain (Loss) on Trading Securities	1.7	(2.1)	10.8	15.5
Forward Contracts
Investment securities
Trading Securities, Change in Unrealized Holding Gain (Loss)	0	0	0	0
Foreign Currency Transaction Gain (Loss), Unrealized	0	0	0	0
Net Unrealized Gain (Loss) and Foreign Currency Gain (Loss) on Trading Securities	$ 0	$ 0	$ 0	$ 0