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Derivative Financial Instruments and Off-balance sheet Financial Instruments - CDS notional amounts by credit rating (Details)
$ in Millions
12 Months Ended
Dec. 31, 2017
USD ($)
entity
Dec. 31, 2016
USD ($)
Derivative Financial Instruments and Off-balance sheet Financial Instruments    
Term of credit default swaps (in years) 5 years  
Credit Derivatives    
Notional amount $ 2,626 $ 2,919
The number of reference entities generally included in a CDX index | entity 125  
Credit default swaps    
Credit Derivatives    
Notional amount $ 81 180
Fair value 1 (2)
Credit default swaps | Single name | Corporate debt    
Credit Derivatives    
Notional amount 1  
Fair value 0  
Credit default swaps | First-to-default Basket | Municipal    
Credit Derivatives    
Notional amount   100
Fair value   (3)
Credit default swaps | Index | Corporate debt    
Credit Derivatives    
Notional amount 80 80
Fair value 1 1
AA | Credit default swaps    
Credit Derivatives    
Notional amount 1 1
AA | Credit default swaps | Single name | Corporate debt    
Credit Derivatives    
Notional amount 0  
AA | Credit default swaps | First-to-default Basket | Municipal    
Credit Derivatives    
Notional amount   0
AA | Credit default swaps | Index | Corporate debt    
Credit Derivatives    
Notional amount 1 1
A | Credit default swaps    
Credit Derivatives    
Notional amount 19 19
A | Credit default swaps | Single name | Corporate debt    
Credit Derivatives    
Notional amount 0  
A | Credit default swaps | First-to-default Basket | Municipal    
Credit Derivatives    
Notional amount   0
A | Credit default swaps | Index | Corporate debt    
Credit Derivatives    
Notional amount 19 19
BBB | Credit default swaps    
Credit Derivatives    
Notional amount 45 150
BBB | Credit default swaps | Single name | Corporate debt    
Credit Derivatives    
Notional amount 0  
BBB | Credit default swaps | First-to-default Basket | Municipal    
Credit Derivatives    
Notional amount   100
BBB | Credit default swaps | Index | Corporate debt    
Credit Derivatives    
Notional amount 45 50
BB and lower | Credit default swaps    
Credit Derivatives    
Notional amount 16 10
BB and lower | Credit default swaps | Single name | Corporate debt    
Credit Derivatives    
Notional amount 1  
BB and lower | Credit default swaps | First-to-default Basket | Municipal    
Credit Derivatives    
Notional amount   0
BB and lower | Credit default swaps | Index | Corporate debt    
Credit Derivatives    
Notional amount $ 15 $ 10