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Investment Securities (Details 2) (USD $) In Millions, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2012	Jun. 30, 2011	Jun. 30, 2012	Jun. 30, 2011
Net Realized and Unrealized Investment Gains and Losses
Net realized investment gains (losses), pre-tax	$ (22.4)	$ 30.6	$ 16.8	$ 29.4
Net unrealized investment gains (losses) pre-tax, total changes in fair value reflected in earnings	14.3	12.2	33.7	4.2
Net realized and unrealized investment gains (losses), pre-tax	(8.1)	42.8	50.5	33.6
Income taxes attributable to realized and unrealized investment gains (losses)	(2.0)	(12.2)	(16.0)	(11.3)
Net realized and unrealized investment gains (losses), after-tax	(10.1)	30.6	34.5	22.3
Net realized investment gains (losses)
Net realized investment gains (losses), pre-tax	(22.4)	30.6	16.8	29.4
Income taxes attributable to realized investment gains (losses)	6.3	(7.4)	(5.4)	(12.4)
Net realized investment gains (losses), after-tax	(16.1)	23.2	11.4	17.0
Net unrealized investment gains (losses)
Net unrealized gains (losses), pre-tax	(13.0)	(14.0)	29.0	(0.2)
Income taxes attributable to unrealized investment gains (losses)	(1.0)	1.9	(9.2)	2.1
Net unrealized investment gains (losses), after-tax	(14.0)	(12.1)	19.8	1.9
Net unrealized investment gains (losses), net foreign exchange gains (losses), pre-tax	27.3	26.2	4.7	4.4
Income taxes attributable to unrealized investment gains, net foreign exchange gains (losses)	(7.3)	(6.7)	(1.4)	(1.0)
Net unrealized investment gains (losses), net foreign currency gains (losses), after-tax	20.0	19.5	3.3	3.4
Net unrealized investment gains (losses) pre-tax, total changes in fair value reflected in earnings	14.3	12.2	33.7	4.2
Income taxes attributable to unrealized investment gains (losses), total changes in fair value reflected in earnings	(8.3)	(4.8)	(10.6)	1.1
Net unrealized investment gains (losses) after-tax, total changes in fair value reflected in earnings	6.0	7.4	23.1	5.3
Fixed maturity investments.
Net Realized and Unrealized Investment Gains and Losses
Net realized investment gains (losses), pre-tax	9.0	2.8	40.5	(10.8)
Net unrealized investment gains (losses) pre-tax, total changes in fair value reflected in earnings	34.2	42.7	12.0	19.4
Net realized investment gains (losses)
Net realized investment gains (losses), pre-tax	9.0	2.8	40.5	(10.8)
Net unrealized investment gains (losses)
Net unrealized gains (losses), pre-tax	11.2	17.2	8.3	9.2
Net unrealized investment gains (losses), net foreign exchange gains (losses), pre-tax	23.0	25.5	3.7	10.2
Net unrealized investment gains (losses) pre-tax, total changes in fair value reflected in earnings	34.2	42.7	12.0	19.4
Short-term investments
Net Realized and Unrealized Investment Gains and Losses
Net realized investment gains (losses), pre-tax	(0.2)	0.9	(0.5)	(11.3)
Net unrealized investment gains (losses) pre-tax, total changes in fair value reflected in earnings	0	(0.4)	0	(1.0)
Net realized investment gains (losses)
Net realized investment gains (losses), pre-tax	(0.2)	0.9	(0.5)	(11.3)
Net unrealized investment gains (losses)
Net unrealized gains (losses), pre-tax	0	0	0	0
Net unrealized investment gains (losses), net foreign exchange gains (losses), pre-tax	0	(0.4)	0	(1.0)
Net unrealized investment gains (losses) pre-tax, total changes in fair value reflected in earnings	0	(0.4)	0	(1.0)
Common equity securities
Net Realized and Unrealized Investment Gains and Losses
Net realized investment gains (losses), pre-tax	(10.3)	15.6	(6.1)	17.9
Net unrealized investment gains (losses) pre-tax, total changes in fair value reflected in earnings	(26.0)	(18.9)	6.4	5.0
Net realized investment gains (losses)
Net realized investment gains (losses), pre-tax	(10.3)	15.6	(6.1)	17.9
Net unrealized investment gains (losses)
Net unrealized gains (losses), pre-tax	(26.0)	(18.8)	6.4	5.9
Net unrealized investment gains (losses), net foreign exchange gains (losses), pre-tax	0	(0.1)	0	(0.9)
Net unrealized investment gains (losses) pre-tax, total changes in fair value reflected in earnings	(26.0)	(18.9)	6.4	5.0
Convertible fixed maturity investments
Net Realized and Unrealized Investment Gains and Losses
Net realized investment gains (losses), pre-tax	0.6	2.9	2.1	7.0
Net unrealized investment gains (losses) pre-tax, total changes in fair value reflected in earnings	(7.0)	(6.0)	(2.3)	(8.1)
Net realized investment gains (losses)
Net realized investment gains (losses), pre-tax	0.6	2.9	2.1	7.0
Net unrealized investment gains (losses)
Net unrealized gains (losses), pre-tax	(7.0)	(6.0)	(2.3)	(8.1)
Net unrealized investment gains (losses), net foreign exchange gains (losses), pre-tax	0	0	0	0
Net unrealized investment gains (losses) pre-tax, total changes in fair value reflected in earnings	(7.0)	(6.0)	(2.3)	(8.1)
Other long-term investments
Net Realized and Unrealized Investment Gains and Losses
Net realized investment gains (losses), pre-tax	(21.5)	8.4	(19.2)	26.6
Net unrealized investment gains (losses) pre-tax, total changes in fair value reflected in earnings	13.1	(5.2)	17.6	(11.1)
Net realized investment gains (losses)
Net realized investment gains (losses), pre-tax	(21.5)	8.4	(19.2)	26.6
Net unrealized investment gains (losses)
Net unrealized gains (losses), pre-tax	8.8	(6.4)	16.6	(7.2)
Net unrealized investment gains (losses), net foreign exchange gains (losses), pre-tax	4.3	1.2	1.0	(3.9)
Net unrealized investment gains (losses) pre-tax, total changes in fair value reflected in earnings	$ 13.1	$ (5.2)	$ 17.6	$ (11.1)

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