Realized Investment Gains and Losses Reported in Consolidated Statements of Income (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended
Mar. 31, 2013	Mar. 31, 2012	Dec. 31, 2012
Gain (Loss) on Investments [Line Items]
Available for Sale Securities Other Than Temporary Impairments in Accumulated Other Comprehensive Income Loss	$ 0		$ 0
Fixed Maturity Securities
Gross Gains on Sales	6.6	5.9
Gross Losses on Sales	(5.5)	(5.0)
Other-Than-Temporary Impairment Loss	(0.8)	0
Mortgage Loans and Other Invested Assets
Gross Gains on Sales	7.6	0
Impairment Loss	(1.0)	(0.5)
Embedded Derivative, Gain (Loss) on Embedded Derivative, Net	6.4	12.3
Derivative, Gain (Loss) on Derivative, Net	(0.3)	0
Foreign Currency Transactions	(2.7)	(0.3)
Net Realized Investment Gain (Loss)	$ 10.3	$ 12.4

Available-for-Sale Securities Other-Than-Temporary Impairments in Accumulated Other Comprehensive Income (Loss)

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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 profit 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 derivative or group of derivatives included in earnings.

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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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The aggregate foreign currency transaction gain (loss) (both realized and unrealized) included in determining net income for the reporting period. Excludes foreign currency transactions designated as hedges of net investment in a foreign entity and intercompany foreign currency transactions that are of a long-term nature, when the entities to the transaction are consolidated, combined, or accounted for by the equity method in the reporting enterprise's financial statements. For certain enterprises, primarily banks, that are dealers in foreign exchange, foreign currency transaction gains (losses) may be disclosed as dealer gains (losses).

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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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The amount of other than temporary impairment (OTTI) losses on equity securities, OTTI related to credit losses on debt securities, and OTTI losses on debt securities when the entity intends to sell the securities or it is more likely than not that the entity will be required to sell the securities before recovery of its amortized cost basis. Additionally, this item includes OTTI losses recognized during the period on investments accounted for under the cost method of accounting.

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