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Mortgage Loans, Sorted by Applicable Credit Quality Indicators, Loan to Value Ratio (Detail) (USD $)
In Millions, unless otherwise specified
Dec. 31, 2012
Dec. 31, 2011
Mortgage Loans on Real Estate [Line Items]    
Mortgage Loans $ 1,712.7 [1] $ 1,612.3
Loan to Value Ratio Below or Equal to 65 Percent
   
Mortgage Loans on Real Estate [Line Items]    
Mortgage Loans 624.7 578.4
Loan To Value Ratio Above 65 To 75 Percent
   
Mortgage Loans on Real Estate [Line Items]    
Mortgage Loans 858.8 802.3
Loan To Value Ratio Above 75 To 85 Percent
   
Mortgage Loans on Real Estate [Line Items]    
Mortgage Loans 142.5 165.1
Loan To Value Ratio Above 85 Percent
   
Mortgage Loans on Real Estate [Line Items]    
Mortgage Loans $ 86.7 $ 66.5
[1] Amortized cost for fixed maturity securities and mortgage loans represents original cost reduced by repayments, write-downs from other-than-temporary declines in fair value, amortization of premiums, and accretion of discounts.