Investment Securities (Details 3) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009
Components of the entity's net realized and unrealized investment gains,(losses), after-tax
Net change in pre-tax unrealized gains (losses) on investments in unconsolidated affiliates	$ (63.6)	$ 72.7	$ 192.7
Income taxes	5.1	0.8	(0.3)
Net change in unrealized gains (losses) from investments in unconsolidated affiliates	(58.5)	73.5	192.4
Net unrealized foreign currency gains (losses) on investments	(90.1)	102.9	185.1
Total investment gains (losses) recognized through other comprehensive income	(148.6)	176.4	377.5
Change in net unrealized investment gains (losses), after-tax	11.9	(34.5)	292.3
Net realized investment gains (losses), after-tax	80.0	94.3	(55.0)
Total investment gains (losses) recorded during the period, after-tax	(56.7)	236.2	614.8
Level 3 Inputs
Investment securities
Total pre-tax gains (losses) included in earnings attributable to unrealized investment gains (losses) for Level 3 investments	(45.6)	30.0	127.1
Fixed maturity investments. | Level 3 Inputs
Investment securities
Total pre-tax gains (losses) included in earnings attributable to unrealized investment gains (losses) for Level 3 investments	(12.2)	10.2	47.3
Common equity securities | Level 3 Inputs
Investment securities
Total pre-tax gains (losses) included in earnings attributable to unrealized investment gains (losses) for Level 3 investments	(16.6)	(19.2)	14.7
Convertible fixed maturities | Level 3 Inputs
Investment securities
Total pre-tax gains (losses) included in earnings attributable to unrealized investment gains (losses) for Level 3 investments			0.1
Other long-term investments | Level 3 Inputs
Investment securities
Total pre-tax gains (losses) included in earnings attributable to unrealized investment gains (losses) for Level 3 investments	$ (16.8)	$ 39.0	$ 65.0