Realized Investment Gains and Losses Reported in Consolidated Statements of Income (Detail) (USD $) In Millions	3 Months Ended	6 Months Ended
Jun. 30, 2011	Jun. 30, 2010	Jun. 30, 2011	Jun. 30, 2010
Available-for-sale Securities [Abstract]
Gross Gains on Sales	$ 8.7	$ 11.4	$ 18.2	$ 29.4
Gross Losses on Sales	(9.8)	(7.2)	(16.7)	(21.7)
Other-Than-Temporary Impairment Loss	(0.7)	(10.2)	(3.0)	(10.4)
Other Investments [Abstract]
Gross Gains on Sales	4.3	1.6	5.5	6.6
Gross Losses on Sales	(1.3)	(0.3)	(1.7)	(0.8)
Impairment Loss	0	(1.2)	0	(1.7)
Embedded Derivative in Modified Coinsurance Arrangement	(4.8)	(23.6)	9.3	(5.3)
Net Realized Investment Gain (Loss)	$ (3.6)	$ (29.5)	$ 11.6	$ (3.9)

This item represents the gross profit realized on the sale of investments not otherwise specified in the taxonomy

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This item represents the gross loss realized on the sale of investments not otherwise specified in the taxonomy

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This item represents the gross profit realized on the sale of debt or equity securities categorized neither as held-to-maturity nor trading securities.

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This item represents the gross loss realized on the sale of debt or equity securities categorized neither as held-to-maturity nor trading securities.

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Net Increase or Decrease in the fair value of the embedded derivative or group of embedded derivatives included in earnings in the period.

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This item represents the net total realized and unrealized gain (loss) included in earnings for the period as a result of selling or holding marketable securities categorized as trading, available-for-sale, or held-to-maturity, including the unrealized holding gain (loss) of held-to-maturity securities transferred to the trading security category and the cumulative unrealized gain (loss) which was included in other comprehensive income (a separate component of shareholders' equity) for available-for-sale securities transferred to trading securities during the period. Additionally, this item would include any gains (losses) realized during the period from the sale of investments accounted for under the cost method of accounting and losses recognized for other than temporary impairments (OTTI) of the subject investments.

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The amount by which the fair value of an investment is less than the amortized cost basis or carrying amount of that investment at the balance sheet date and the decline in fair value is deemed to be other than temporary, before considering whether or not such amount is recognized in earnings or other comprehensive income.

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This element represents the amount of loss recognized for other than temporary impairments (OTTI) of investments in debt and equity securities.

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