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Before-Tax Credit Related Portion of Other-Than-Temporary Impairments on Fixed Maturity Securities Still Held and Recognized in Other Comprehensive Income (Detail) (USD $)
In Millions, unless otherwise specified
3 Months Ended 12 Months Ended
Sep. 30, 2010
Dec. 31, 2011
Dec. 31, 2010
Dec. 31, 2009
Other than Temporary Impairment, Credit Losses Recognized in Earnings [Line Items]        
Balance at Beginning of Year $ 0 $ 8.5 $ 18.3 $ 0
Credit Losses Remaining in Retained Earnings Related to the Adoption of Accounting Standard   0 0 30.8
Impairment Recognized on Securities not Previously Impaired   0 0 38.4
Additional Impairment Recognized on Securities Previously Impaired   0 0 4.4
Sales or Maturities of Securities in the Period 0 8.5 9.8 38.3
Reduction for Credit Loss Impairments Previously Recognized due to Change in Intent to Sell   0 0 (17.0)
Balance at End of Year   $ 0 $ 8.5 $ 18.3