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Investments (Details 7) (USD $)
In Millions, unless otherwise specified
Dec. 31, 2012
Dec. 31, 2011
Agricultural Mortgage Loans - by Credit Quality Indicator:    
Recorded investment in Mortgage Loan $ 12,843 $ 13,129
% of Total 100.00% 100.00%
Residential Mortgage Loans - by Credit Quality Indicator:    
Recorded investment in Mortgage Loan 958 689
% of Total 100.00% 100.00%
Less than 65% [Member]
   
Agricultural Mortgage Loans - by Credit Quality Indicator:    
Recorded investment in Mortgage Loan 11,908 11,802
% of Total 92.70% 89.90%
65% to 75% [Member]
   
Agricultural Mortgage Loans - by Credit Quality Indicator:    
Recorded investment in Mortgage Loan 590 874
% of Total 4.60% 6.70%
76% to 80% [Member]
   
Agricultural Mortgage Loans - by Credit Quality Indicator:    
Recorded investment in Mortgage Loan 92 76
% of Total 0.70% 0.60%
Greater than 80% [Member]
   
Agricultural Mortgage Loans - by Credit Quality Indicator:    
Recorded investment in Mortgage Loan 253 377
% of Total 2.00% 2.80%
Performing [Member]
   
Residential Mortgage Loans - by Credit Quality Indicator:    
Recorded investment in Mortgage Loan 929 671
% of Total 97.00% 97.40%
Nonperforming [Member]
   
Residential Mortgage Loans - by Credit Quality Indicator:    
Recorded investment in Mortgage Loan $ 29 $ 18
% of Total 3.00% 2.60%