Investment Securities (Details 2) (USD $) In Millions, unless otherwise specified	12 Months Ended
Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009
Net Realized and Unrealized Investment Gains and Losses
Net realized investment gains (losses), pre-tax	$ 117.0	$ 136.4	$ (69.9)
Net unrealized gains (losses), pre-tax	(63.6)	17.2	498.5
Net realized and unrealized investment gains (losses), pre-tax	122.8	82.6	361.7
Income taxes attributable to realized and unrealized investment gains (losses)	(30.9)	(22.8)	(124.4)
Net realized and unrealized investment gains (losses), after-tax	91.9	59.8	237.3
Net realized investment gains (losses)
Net realized investment gains (losses), pre-tax	117.0	136.4	(69.9)
Income taxes attributable to realized investment gains (losses)	(37.0)	(42.1)	14.9
Net realized investment gains (losses), after-tax	80.0	94.3	(55.0)
Gross realized investment gains recognized during the period	191.6	205.0	157.3
Gross realized investment losses recognized during the period	74.6	68.6	227.2
Accounts payable on unsettled investment purchases	34.6	22.3
Accounts receivable on unsettled investment sales	4.7	39.5
Net unrealized investment gains (losses)
Net unrealized gains (losses), pre-tax	(63.6)	17.2	498.5
Income taxes attributable to unrealized investment gains (losses)	24.0	0.5	(154.3)
Net unrealized investment gains (losses), after-tax	(39.6)	17.7	344.2
Net unrealized investment gains (losses), net foreign exchange gains (losses), pre-tax	69.4	(71.0)	(66.9)
Income taxes attributable to unrealized investment gains, net foreign exchange gains (losses)	(17.9)	18.8	15.0
Net unrealized investment gains (losses), net foreign currency gains (losses), after-tax	51.5	(52.2)	(51.9)
Net unrealized investment gains (losses) pre-tax, total changes in fair value reflected in earnings	5.8	(53.8)	431.6
Income taxes attributable to unrealized investment gains (losses), total changes in fair value reflected in earnings	6.1	19.3	(139.3)
Net unrealized investment gains (losses) after-tax, total changes in fair value reflected in earnings	11.9	(34.5)	292.3
Fixed maturity investments.
Net Realized and Unrealized Investment Gains and Losses
Net realized investment gains (losses), pre-tax	49.4	96.9	(7.4)
Net unrealized gains (losses), pre-tax	7.6	(67.4)	333.9
Net realized investment gains (losses)
Net realized investment gains (losses), pre-tax	49.4	96.9	(7.4)
Net unrealized investment gains (losses)
Net unrealized gains (losses), pre-tax	7.6	(67.4)	333.9
Net unrealized investment gains (losses), net foreign exchange gains (losses), pre-tax	68.9	(53.8)	(53.6)
Net unrealized investment gains (losses) pre-tax, total changes in fair value reflected in earnings	76.5	(121.2)	280.3
Short-term investments
Net Realized and Unrealized Investment Gains and Losses
Net realized investment gains (losses), pre-tax			0.2
Net unrealized gains (losses), pre-tax			(0.3)
Net realized investment gains (losses)
Net realized investment gains (losses), pre-tax			0.2
Net unrealized investment gains (losses)
Net unrealized gains (losses), pre-tax			(0.3)
Net unrealized investment gains (losses), net foreign exchange gains (losses), pre-tax	(1.1)	0.2	1.6
Net unrealized investment gains (losses) pre-tax, total changes in fair value reflected in earnings	(1.1)	0.2	1.3
Common equity securities
Net Realized and Unrealized Investment Gains and Losses
Net realized investment gains (losses), pre-tax	34.6	6.9	(46.4)
Net unrealized gains (losses), pre-tax	(40.6)	72.4	57.9
Net realized investment gains (losses)
Net realized investment gains (losses), pre-tax	34.6	6.9	(46.4)
Net unrealized investment gains (losses)
Net unrealized gains (losses), pre-tax	(40.6)	72.4	57.9
Net unrealized investment gains (losses), net foreign exchange gains (losses), pre-tax	(1.8)	(9.6)	(9.9)
Net unrealized investment gains (losses) pre-tax, total changes in fair value reflected in earnings	(42.4)	62.8	48.0
Convertible fixed maturities
Net Realized and Unrealized Investment Gains and Losses
Net realized investment gains (losses), pre-tax	(2.2)	16.7	15.6
Net unrealized gains (losses), pre-tax	(11.5)	(5.4)	39.8
Net realized investment gains (losses)
Net realized investment gains (losses), pre-tax	(2.2)	16.7	15.6
Net unrealized investment gains (losses)
Net unrealized gains (losses), pre-tax	(11.5)	(5.4)	39.8
Net unrealized investment gains (losses) pre-tax, total changes in fair value reflected in earnings	(11.5)	(5.4)	39.8
Other long-term investments
Net Realized and Unrealized Investment Gains and Losses
Net realized investment gains (losses), pre-tax	35.2	15.9	(31.9)
Net unrealized gains (losses), pre-tax	(19.1)	17.6	67.2
Net realized investment gains (losses)
Net realized investment gains (losses), pre-tax	35.2	15.9	(31.9)
Net unrealized investment gains (losses)
Net unrealized gains (losses), pre-tax	(19.1)	17.6	67.2
Net unrealized investment gains (losses), net foreign exchange gains (losses), pre-tax	3.4	(7.8)	(5.0)
Net unrealized investment gains (losses) pre-tax, total changes in fair value reflected in earnings	$ (15.7)	$ 9.8	$ 62.2

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

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Aggregate amounts payable to broker-dealers and clearing organizations include amounts payable for securities failed to receive, certain deposits received for securities loaned, amounts payable to clearing organizations on open transactions, and floor-brokerage payables. In addition, the net receivable or payable arising from unsettled trades would be reflected in those captions. A broker-dealer that purchases securities, either for its own account or for a customer's account, but does not receive the securities on the settlement date records the amount of money representing the purchase price of the securities, including any accrued interest, as a liability to the selling broker-dealer. This liability is paid when the securities are received by the purchasing broker-dealer.

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Amount receivable from securities failed to deliver, certain deposits for securities borrowed, open transactions, good faith and margin deposits, commissions and floor brokerage items. Also cash and securities [guarantee] on deposit with [commodities] clearing organizations and other entities which provide services necessary to operations of the Company. This item would include, for instance, cash and securities advanced as [refundable] collateral for securities borrowed.

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The net change in the difference between the fair value and the carrying value, or in the comparative fair values, of trading securities held at each balance sheet date, that was included in earnings for the period. Trading securities are bought and held principally for the purpose of selling them in the near term (thus held for only a short period of time) and the change in net unrealized holding gain (loss) is included in earnings for the period.

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The aggregate of the change in unrealized holding gain (loss) and foreign currency gain (loss) included in earnings during the reporting period.

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The after-tax aggregate of the change in unrealized holding gain (loss) and foreign currency gain (loss) included in earnings during the reporting period.

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No definition available.

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The tax expense (benefit) attributable to the aggregate of the change in unrealized holding gain (loss) and foreign currency gain (loss) included in earnings during the reporting period.

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No definition available.

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Represents the net realized and unrealized investment gains (losses) after income taxes.

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No definition available.

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This item represents the after-tax gain (loss) realized during the period from the sale of trading securities.

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The tax expense (benefit) attributable to the gain (loss) realized during the period from the sale of trading securities.

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Represents the income taxes attributable to realized and unrealized gains (losses) on trading securities.

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The after-tax gain (loss) attributable to the foreign currency transaction gain (loss) on trading securities held at each balance sheet date, that was included in earnings in the period.

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The unrealized foreign currency transaction gain (loss) as of the balance sheet date which is included in net income before tax during the reporting period.

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The tax expense (benefit) attributable to the foreign currency transaction gain (loss) on trading securities held at each balance sheet date, that was included in earnings in the period.

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The after-tax gain (loss) attributable to the net change in the difference between the fair value and the carrying value, or in the comparative fair values, of trading securities, that was included in earnings in the period.

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The tax expense (benefit) attributable to the net change in the difference between the fair value and the carrying value, or in the comparative fair values, of trading securities, that was included in earnings in the period.

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