Investment Securities (Details 2) (USD $) In Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2011	Sep. 30, 2010	Sep. 30, 2011	Sep. 30, 2010
Net Realized and Unrealized Investment Gains and Losses
Net realized investment gains (losses), pre-tax	$ 37.1	$ 31.4	$ 109.6	$ 93.7
Net unrealized investment gains (losses), pre-tax	(35.4)	(19.8)	(31.3)	11.0
Net realized and unrealized investment gains (losses), pre-tax	1.7	11.6	78.3	104.7
Income taxes attributable to realized and unrealized investment gains (losses)	(1.1)	(22.7)	(23.7)	(24.5)
Net realized and unrealized investment gains (losses), after-tax	0.6	(11.1)	54.6	80.2
Net realized investment gains (losses)
Net realized investment gains (losses), pre-tax	37.1	31.4	109.6	93.7
Income taxes attributable to realized investment gains (losses)	(8.0)	(4.9)	(31.7)	(20.5)
Net realized investment gains (losses), after-tax	29.1	26.5	77.9	73.2
Net unrealized investment gains (losses)
Net unrealized gains (losses), pre-tax	(110.0)	79.4	(110.3)	80.4
Income taxes attributable to unrealized investment gains (losses)	26.4	(28.8)	28.5	(22.4)
Net unrealized investment gains (losses), after-tax	(83.6)	50.6	(81.8)	58.0
Net unrealized investment gains (losses), net foreign exchange gains (losses), pre-tax	74.6	(99.2)	79.0	(69.4)
Income taxes attributable to unrealized investment gains, net foreign exchange gains (losses)	(19.5)	11.0	(20.5)	18.4
Net unrealized investment gains (losses), net foreign exchange gains (losses), after tax	55.1	(88.2)	58.5	(51.0)
Net unrealized investment gains (losses) pre-tax, total changes in fair value reflected in earnings	(35.4)	(19.8)	(31.3)	11.0
Income taxes attributable to unrealized investment gains (losses), total changes in fair value reflected in earnings	6.9	(17.8)	8.0	(4.0)
Net unrealized investment gains (losses) after-tax, total changes in fair value reflected in earnings	(28.5)	(37.6)	(23.3)	7.0
Fixed maturity investments
Net Realized and Unrealized Investment Gains and Losses
Net realized investment gains (losses), pre-tax	14.0	34.7	33.6	68.1
Net unrealized investment gains (losses), pre-tax	71.0	(77.9)	90.3	(17.2)
Net realized investment gains (losses)
Net realized investment gains (losses), pre-tax	14.0	34.7	33.6	68.1
Net unrealized investment gains (losses)
Net unrealized gains (losses), pre-tax	3.5	3.3	12.6	36.4
Net unrealized investment gains (losses), net foreign exchange gains (losses), pre-tax	67.5	(81.2)	77.7	(53.6)
Net unrealized investment gains (losses) pre-tax, total changes in fair value reflected in earnings	71.0	(77.9)	90.3	(17.2)
Short-term investments
Net Realized and Unrealized Investment Gains and Losses
Net unrealized investment gains (losses), pre-tax	(0.5)	1.4	(1.5)	0.6
Net unrealized investment gains (losses)
Net unrealized gains (losses), pre-tax	(0.1)		(0.1)
Net unrealized investment gains (losses), net foreign exchange gains (losses), pre-tax	(0.4)	1.4	(1.4)	0.6
Net unrealized investment gains (losses) pre-tax, total changes in fair value reflected in earnings	(0.5)	1.4	(1.5)	0.6
Common Equity Securities
Net Realized and Unrealized Investment Gains and Losses
Net realized investment gains (losses), pre-tax	23.1	(13.7)	41.0	6.7
Net unrealized investment gains (losses), pre-tax	(101.9)	53.5	(96.9)	22.1
Net realized investment gains (losses)
Net realized investment gains (losses), pre-tax	23.1	(13.7)	41.0	6.7
Net unrealized investment gains (losses)
Net unrealized gains (losses), pre-tax	(101.0)	64.3	(95.1)	31.8
Net unrealized investment gains (losses), net foreign exchange gains (losses), pre-tax	(0.9)	(10.8)	(1.8)	(9.7)
Net unrealized investment gains (losses) pre-tax, total changes in fair value reflected in earnings	(101.9)	53.5	(96.9)	22.1
Convertible fixed maturity investments
Net Realized and Unrealized Investment Gains and Losses
Net realized investment gains (losses), pre-tax	(0.7)	6.2	6.3	19.9
Net unrealized investment gains (losses), pre-tax	(11.2)	2.8	(19.3)	(8.3)
Net realized investment gains (losses)
Net realized investment gains (losses), pre-tax	(0.7)	6.2	6.3	19.9
Net unrealized investment gains (losses)
Net unrealized gains (losses), pre-tax	(11.2)	2.8	(19.3)	(8.3)
Net unrealized investment gains (losses) pre-tax, total changes in fair value reflected in earnings	(11.2)	2.8	(19.3)	(8.3)
Other long-term investments.
Net Realized and Unrealized Investment Gains and Losses
Net realized investment gains (losses), pre-tax	0.7	4.2	28.7	(1.0)
Net unrealized investment gains (losses), pre-tax	7.2	0.4	(3.9)	13.8
Net realized investment gains (losses)
Net realized investment gains (losses), pre-tax	0.7	4.2	28.7	(1.0)
Net unrealized investment gains (losses)
Net unrealized gains (losses), pre-tax	(1.2)	9.0	(8.4)	20.5
Net unrealized investment gains (losses), net foreign exchange gains (losses), pre-tax	8.4	(8.6)	4.5	(6.7)
Net unrealized investment gains (losses) pre-tax, total changes in fair value reflected in earnings	$ 7.2	$ 0.4	$ (3.9)	$ 13.8