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Investment Securities (Details 2) (USD $) In Millions, unless otherwise specified	3 Months Ended
	Mar. 31, 2012	Mar. 31, 2011
Net Realized and Unrealized Investment Gains and Losses
Net realized investment gains (losses), pre-tax	$ 39.2	$ (1.2)
Net unrealized investment gains (losses) pre-tax, total changes in fair value reflected in earnings	19.4	(8.0)
Net realized and unrealized investment gains (losses), pre-tax	58.6	(9.2)
Income taxes attributable to realized and unrealized investment gains (losses)	(14.0)	0.9
Net realized and unrealized investment gains (losses), after-tax	44.6	(8.3)
Net realized investment gains (losses)
Net realized investment gains (losses), pre-tax	39.2	(1.2)
Income taxes attributable to realized investment gains (losses)	(11.7)	(5.0)
Net realized investment gains (losses), after-tax	27.5	(6.2)
Net unrealized investment gains (losses)
Net unrealized gains (losses), pre-tax	42.0	13.8
Income taxes attributable to unrealized investment gains (losses)	(8.2)	0.2
Net unrealized investment gains (losses), after-tax	33.8	14.0
Net unrealized investment gains (losses), net foreign exchange gains (losses), pre-tax	(22.6)	(21.8)
Income taxes attributable to unrealized investment gains, net foreign exchange gains (losses)	5.9	5.7
Net unrealized investment gains (losses), net foreign currency gains (losses), after-tax	(16.7)	(16.1)
Net unrealized investment gains (losses) pre-tax, total changes in fair value reflected in earnings	19.4	(8.0)
Income taxes attributable to unrealized investment gains (losses), total changes in fair value reflected in earnings	(2.3)	5.9
Net unrealized investment gains (losses) after-tax, total changes in fair value reflected in earnings	17.1	(2.1)
Fixed maturity investments.
Net Realized and Unrealized Investment Gains and Losses
Net realized investment gains (losses), pre-tax	31.5	(13.6)
Net unrealized investment gains (losses) pre-tax, total changes in fair value reflected in earnings	(22.2)	(23.3)
Net realized investment gains (losses)
Net realized investment gains (losses), pre-tax	31.5	(13.6)
Net unrealized investment gains (losses)
Net unrealized gains (losses), pre-tax	(2.9)	(8.0)
Net unrealized investment gains (losses), net foreign exchange gains (losses), pre-tax	(19.3)	(15.3)
Net unrealized investment gains (losses) pre-tax, total changes in fair value reflected in earnings	(22.2)	(23.3)
Short-term investments
Net Realized and Unrealized Investment Gains and Losses
Net realized investment gains (losses), pre-tax	(0.3)	(12.2)
Net unrealized investment gains (losses) pre-tax, total changes in fair value reflected in earnings		(0.6)
Net realized investment gains (losses)
Net realized investment gains (losses), pre-tax	(0.3)	(12.2)
Net unrealized investment gains (losses)
Net unrealized investment gains (losses), net foreign exchange gains (losses), pre-tax		(0.6)
Net unrealized investment gains (losses) pre-tax, total changes in fair value reflected in earnings		(0.6)
Common equity securities
Net Realized and Unrealized Investment Gains and Losses
Net realized investment gains (losses), pre-tax	4.2	2.3
Net unrealized investment gains (losses) pre-tax, total changes in fair value reflected in earnings	32.4	23.9
Net realized investment gains (losses)
Net realized investment gains (losses), pre-tax	4.2	2.3
Net unrealized investment gains (losses)
Net unrealized gains (losses), pre-tax	32.4	24.7
Net unrealized investment gains (losses), net foreign exchange gains (losses), pre-tax		(0.8)
Net unrealized investment gains (losses) pre-tax, total changes in fair value reflected in earnings	32.4	23.9
Convertible fixed maturity investments
Net Realized and Unrealized Investment Gains and Losses
Net realized investment gains (losses), pre-tax	1.5	4.1
Net unrealized investment gains (losses) pre-tax, total changes in fair value reflected in earnings	4.7	(2.1)
Net realized investment gains (losses)
Net realized investment gains (losses), pre-tax	1.5	4.1
Net unrealized investment gains (losses)
Net unrealized gains (losses), pre-tax	4.7	(2.1)
Net unrealized investment gains (losses) pre-tax, total changes in fair value reflected in earnings	4.7	(2.1)
Other long-term investments
Net Realized and Unrealized Investment Gains and Losses
Net realized investment gains (losses), pre-tax	2.3	18.2
Net unrealized investment gains (losses) pre-tax, total changes in fair value reflected in earnings	4.5	(5.9)
Net realized investment gains (losses)
Net realized investment gains (losses), pre-tax	2.3	18.2
Net unrealized investment gains (losses)
Net unrealized gains (losses), pre-tax	7.8	(0.8)
Net unrealized investment gains (losses), net foreign exchange gains (losses), pre-tax	(3.3)	(5.1)
Net unrealized investment gains (losses) pre-tax, total changes in fair value reflected in earnings	$ 4.5	$ (5.9)

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The sum of the realized and unrealized net gain (loss) on trading securities during the period.

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