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Mortgage Loans, Sorted by Applicable Credit Quality Indicators, Internal rating (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
Aa
   
Mortgage Loans on Real Estate [Line Items]    
Mortgage Loans 11.5 10.9
A
   
Mortgage Loans on Real Estate [Line Items]    
Mortgage Loans 659.4 712.6
Baa
   
Mortgage Loans on Real Estate [Line Items]    
Mortgage Loans 994.5 855.0
Ba
   
Mortgage Loans on Real Estate [Line Items]    
Mortgage Loans 34.2 20.7
B
   
Mortgage Loans on Real Estate [Line Items]    
Mortgage Loans $ 13.1 $ 13.1
[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.