Investment Securities (Net Unrealized Investment Gains (Losses) (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2013	Jun. 30, 2012	Jun. 30, 2013	Jun. 30, 2012
Investment securities
Net realized and unrealized gain (loss) on trading securities	$ (72.9)	$ (36.2)	$ 12.1	$ 47.5
Trading securities, tax on realized and unrealized holding gain (loss) on investments	20.3	5.6	4.4	(15.0)
Trading securities realized and unrealized holding gain (loss) on investments after tax	(52.6)	(30.6)	16.5	32.5
Net unrealized and realized investment gains (losses), net foreign exchange gains (losses), pre-tax	35.6	28.1	25.8	3.0
Trading securities, unrealized foreign currency transaction gain (loss) tax	(7.8)	(7.6)	(5.6)	(1.0)
Trading securities, unrealized foreign currency gain (loss) net of tax	27.8	20.5	20.2	2.0
Net unrealized gain (loss) and foreign currency gain (loss) on trading securities	(37.3)	(8.1)	37.9	50.5
Net realized and unrealized gain (loss) and foreign currency gain (loss) on trading securities tax	12.5	(2.0)	(1.2)	(16.0)
Net realized and unrealized gain (loss) and foreign currency gain (loss) on trading securities net of tax	(24.8)	(10.1)	36.7	34.5
Fixed maturity investments.
Investment securities
Net realized and unrealized gain (loss) on trading securities	(74.0)	19.0	(82.0)	49.7
Net unrealized and realized investment gains (losses), net foreign exchange gains (losses), pre-tax	32.5	24.2	22.5	2.8
Net unrealized gain (loss) and foreign currency gain (loss) on trading securities	(41.5)	43.2	(59.5)	52.5
Short-term investments
Investment securities
Net realized and unrealized gain (loss) on trading securities	0.2	(0.1)	0.2	(0.1)
Net unrealized and realized investment gains (losses), net foreign exchange gains (losses), pre-tax	0.5	(0.1)	0.6	(0.4)
Net unrealized gain (loss) and foreign currency gain (loss) on trading securities	0.7	(0.2)	0.8	(0.5)
Common equity securities
Investment securities
Net realized and unrealized gain (loss) on trading securities	2.3	(36.3)	86.8	0.3
Net unrealized and realized investment gains (losses), net foreign exchange gains (losses), pre-tax	1.2	0	1.0	0
Net unrealized gain (loss) and foreign currency gain (loss) on trading securities	3.5	(36.3)	87.8	0.3
Convertible Debt Securities
Investment securities
Net realized and unrealized gain (loss) on trading securities	(5.1)	(6.4)	(3.5)	(0.2)
Net unrealized and realized investment gains (losses), net foreign exchange gains (losses), pre-tax	0.3	0	0.1	0
Net unrealized gain (loss) and foreign currency gain (loss) on trading securities	(4.8)	(6.4)	(3.4)	(0.2)
Other long-term investments
Investment securities
Net realized and unrealized gain (loss) on trading securities	3.9	(12.4)	10.4	(2.2)
Net unrealized and realized investment gains (losses), net foreign exchange gains (losses), pre-tax	1.1	4.0	1.6	0.6
Net unrealized gain (loss) and foreign currency gain (loss) on trading securities	5.0	(8.4)	12.0	(1.6)
Forward Contracts
Investment securities
Net realized and unrealized gain (loss) on trading securities	(0.2)	0	0.2	0
Net unrealized and realized 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	$ (0.2)	$ 0	$ 0.2	$ 0