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Investments (Credit Quality of Commercial Mortgage Loans) (Details) - USD ($)
$ in Millions
Mar. 31, 2018
Dec. 31, 2017
Mortgage Loans on Real Estate [Line Items]    
Commercial mortgage loans $ 7,601 $ 7,233
% of Total 100.00% 100.00%
Estimated Fair Value $ 7,660 $ 7,392
% of Total 100.00% 100.00%
Less than 65%    
Mortgage Loans on Real Estate [Line Items]    
Commercial mortgage loans $ 6,902 $ 6,493
% of Total 90.80% 89.70%
Estimated Fair Value $ 6,968 $ 6,654
% of Total 91.00% 90.00%
65% to 75%    
Mortgage Loans on Real Estate [Line Items]    
Commercial mortgage loans $ 616 $ 656
% of Total 8.10% 9.10%
Estimated Fair Value $ 611 $ 658
% of Total 8.00% 8.90%
76% to 80%    
Mortgage Loans on Real Estate [Line Items]    
Commercial mortgage loans $ 83 $ 51
% of Total 1.10% 0.70%
Estimated Fair Value $ 81 $ 50
% of Total 1.00% 0.70%
Greater than 80%    
Mortgage Loans on Real Estate [Line Items]    
Commercial mortgage loans   $ 33
% of Total   0.50%
Estimated Fair Value   $ 30
% of Total   0.40%
Greater than 1.20x    
Mortgage Loans on Real Estate [Line Items]    
Commercial mortgage loans $ 7,228 $ 6,851
Greater than 1.20x | Less than 65%    
Mortgage Loans on Real Estate [Line Items]    
Commercial mortgage loans 6,631 6,167
Greater than 1.20x | 65% to 75%    
Mortgage Loans on Real Estate [Line Items]    
Commercial mortgage loans 578 642
Greater than 1.20x | 76% to 80%    
Mortgage Loans on Real Estate [Line Items]    
Commercial mortgage loans 19 42
Greater than 1.20x | Greater than 80%    
Mortgage Loans on Real Estate [Line Items]    
Commercial mortgage loans   0
1.00x - 1.20x    
Mortgage Loans on Real Estate [Line Items]    
Commercial mortgage loans 270 302
1.00x - 1.20x | Less than 65%    
Mortgage Loans on Real Estate [Line Items]    
Commercial mortgage loans 238 293
1.00x - 1.20x | 65% to 75%    
Mortgage Loans on Real Estate [Line Items]    
Commercial mortgage loans 0 0
1.00x - 1.20x | 76% to 80%    
Mortgage Loans on Real Estate [Line Items]    
Commercial mortgage loans 32 0
1.00x - 1.20x | Greater than 80%    
Mortgage Loans on Real Estate [Line Items]    
Commercial mortgage loans   9
Less than 1.00x    
Mortgage Loans on Real Estate [Line Items]    
Commercial mortgage loans 103 80
Less than 1.00x | Less than 65%    
Mortgage Loans on Real Estate [Line Items]    
Commercial mortgage loans 33 33
Less than 1.00x | 65% to 75%    
Mortgage Loans on Real Estate [Line Items]    
Commercial mortgage loans 38 14
Less than 1.00x | 76% to 80%    
Mortgage Loans on Real Estate [Line Items]    
Commercial mortgage loans $ 32 9
Less than 1.00x | Greater than 80%    
Mortgage Loans on Real Estate [Line Items]    
Commercial mortgage loans   $ 24