Investment Securities (Net Unrealized Investment Gains (Losses) (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended
	Mar. 31, 2013	Mar. 31, 2012
Investment securities
Net realized and unrealized gain (loss) on trading securities	$ 85.0	$ 83.7
Trading securities, tax on realized and unrealized holding gain (loss) on investments	(15.9)	(20.6)
Trading securities realized and unrealized holding gain (loss) on investments after tax	69.1	63.1
Net unrealized investment gains (losses), net foreign exchange gains (losses), pre-tax	(9.8)	(25.1)
Trading securities, unrealized foreign currency transaction gain (loss) tax	2.2	6.6
Trading securities, unrealized foreign currency gain (loss) net of tax	(7.6)	(18.5)
Net unrealized gain (loss) and foreign currency gain (loss) on trading securities	75.2	58.6
Net realized and unrealized gain (loss) and foreign currency gain (loss) on trading securities tax	(13.7)	(14.0)
Net realized and unrealized gain (loss) and foreign currency gain (loss) on trading securities net of tax	61.5	44.6
Fixed maturity investments.
Investment securities
Net realized and unrealized gain (loss) on trading securities	(8.0)	30.7
Net unrealized investment gains (losses), net foreign exchange gains (losses), pre-tax	(10.0)	(21.4)
Net unrealized gain (loss) and foreign currency gain (loss) on trading securities	(18.0)	9.3
Short-term investments
Investment securities
Net realized and unrealized gain (loss) on trading securities	0	0
Net unrealized investment gains (losses), net foreign exchange gains (losses), pre-tax	0.1	(0.3)
Net unrealized gain (loss) and foreign currency gain (loss) on trading securities	0.1	(0.3)
Common equity securities
Investment securities
Net realized and unrealized gain (loss) on trading securities	84.5	36.6
Net unrealized investment gains (losses), net foreign exchange gains (losses), pre-tax	(0.2)	0
Net unrealized gain (loss) and foreign currency gain (loss) on trading securities	84.3	36.6
Convertible Debt Securities
Investment securities
Net realized and unrealized gain (loss) on trading securities	1.6	6.2
Net unrealized investment gains (losses), net foreign exchange gains (losses), pre-tax	(0.2)	0
Net unrealized gain (loss) and foreign currency gain (loss) on trading securities	1.4	6.2
Other long-term investments
Investment securities
Net realized and unrealized gain (loss) on trading securities	6.5	10.2
Net unrealized investment gains (losses), net foreign exchange gains (losses), pre-tax	0.5	(3.4)
Net unrealized gain (loss) and foreign currency gain (loss) on trading securities	7.0	6.8
Forward Contracts
Investment securities
Net realized and unrealized gain (loss) on trading securities	0.4	0
Net unrealized investment gains (losses), net foreign exchange gains (losses), pre-tax	0	0
Net unrealized gain (loss) and foreign currency gain (loss) on trading securities	$ 0.4	$ 0