Investment Securities (Net Unrealized Investment Gains (Losses)) (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2012	Sep. 30, 2011	Sep. 30, 2012	Sep. 30, 2011
Investment securities
Net unrealized gains (losses), pre-tax	$ 93.3	$ (110.0)	$ 122.3	$ (110.3)
Trading Securities, Unrealized Gain (Loss) Tax	16.7	(26.4)	25.9	(28.5)
Trading Securities, Unrealized Gain (Loss), Net of Tax	76.6	(83.6)	96.4	(81.8)
Net unrealized investment gains (losses), net foreign exchange gains (losses), pre-tax	(44.3)	74.6	(39.6)	79.0
Trading Securities, Unrealized Foreign Currency Transaction Gain (Loss) Tax	(11.8)	19.5	(10.4)	20.5
Trading Securities, Unrealized Foreign Currency Gain (Loss) Net of Tax	(32.5)	55.1	(29.2)	58.5
Net Unrealized Gain (Loss) and Foreign Currency Gain (Loss) on Trading Securities	49.0	(35.4)	82.7	(31.3)
Net Unrealized Gain (Loss) and Foreign Currency Gain (Loss) on Trading Securities Tax	4.9	(6.9)	15.5	(8.0)
Net Unrealized Gain (Loss) and Foreign Currency Gain (Loss) on Trading Securities Net of Tax	44.1	(28.5)	67.2	(23.3)
Fixed maturity investments.
Investment securities
Net unrealized gains (losses), pre-tax	27.2	3.5	35.5	12.6
Net unrealized investment gains (losses), net foreign exchange gains (losses), pre-tax	(40.6)	67.5	(36.9)	77.7
Net Unrealized Gain (Loss) and Foreign Currency Gain (Loss) on Trading Securities	(13.4)	71.0	(1.4)	90.3
Short-term investments
Investment securities
Net unrealized gains (losses), pre-tax	0	(0.1)	0	(0.1)
Net unrealized investment gains (losses), net foreign exchange gains (losses), pre-tax	0.1	(0.4)	0.1	(1.4)
Net Unrealized Gain (Loss) and Foreign Currency Gain (Loss) on Trading Securities	0.1	(0.5)	0.1	(1.5)
Common equity securities
Investment securities
Net unrealized gains (losses), pre-tax	65.1	(101.0)	71.5	(95.1)
Net unrealized investment gains (losses), net foreign exchange gains (losses), pre-tax	(0.1)	(0.9)	(0.1)	(1.8)
Net Unrealized Gain (Loss) and Foreign Currency Gain (Loss) on Trading Securities	65.0	(101.9)	71.4	(96.9)
Convertible Debt Securities [Member]
Investment securities
Net unrealized gains (losses), pre-tax	(0.6)	(11.2)	(2.9)	(19.3)
Net unrealized investment gains (losses), net foreign exchange gains (losses), pre-tax	0	0	0	0
Net Unrealized Gain (Loss) and Foreign Currency Gain (Loss) on Trading Securities	(0.6)	(11.2)	(2.9)	(19.3)
Other long-term investments
Investment securities
Net unrealized gains (losses), pre-tax	1.6	(1.2)	18.2	(8.4)
Net unrealized investment gains (losses), net foreign exchange gains (losses), pre-tax	(3.7)	8.4	(2.7)	4.5
Net Unrealized Gain (Loss) and Foreign Currency Gain (Loss) on Trading Securities	$ (2.1)	$ 7.2	$ 15.5	$ (3.9)