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Investments - Securities with a Decline in Fair Value (Details)
$ in Millions
Dec. 31, 2017
USD ($)
Dec. 31, 2016
USD ($)
Investment Grade [Member]    
Available-for-sale Securities, Continuous Unrealized Loss Position [Abstract]    
Fair value, one year or less   $ 4,346
Fair value, more than one year   308
Amortized cost, one year or less   4,475
Amortized cost, more than one year   327
Unrealized depreciation, one year or less   (129)
Unrealized depreciation, more than one year   $ (19)
Investment Grade [Member] | One Year Or Less [Member]    
Available-for-sale Securities, Continuous Unrealized Loss Position [Abstract]    
Number of issues, total   992
Investment Grade [Member] | More Than One Year [Member]    
Available-for-sale Securities, Continuous Unrealized Loss Position [Abstract]    
Number of issues, total   53
Below Investment Grade [Member]    
Available-for-sale Securities, Continuous Unrealized Loss Position [Abstract]    
Fair value, one year or less   $ 724
Fair value, more than one year   186
Amortized cost, one year or less   736
Amortized cost, more than one year   203
Unrealized depreciation, one year or less   (12)
Unrealized depreciation, more than one year   $ (17)
Below Investment Grade [Member] | One Year Or Less [Member]    
Available-for-sale Securities, Continuous Unrealized Loss Position [Abstract]    
Number of issues, total   591
Below Investment Grade [Member] | More Than One Year [Member]    
Available-for-sale Securities, Continuous Unrealized Loss Position [Abstract]    
Number of issues, total   28
Fixed Maturities [Member] | Investment Grade [Member]    
Available-for-sale Securities, Continuous Unrealized Loss Position [Abstract]    
Fair value, one year or less $ 3,272  
Fair value, more than one year 1,503  
Amortized cost, one year or less 3,309  
Amortized cost, more than one year 1,549  
Unrealized depreciation, one year or less (37)  
Unrealized depreciation, more than one year $ (46)  
Fixed Maturities [Member] | Investment Grade [Member] | One Year Or Less [Member]    
Available-for-sale Securities, Continuous Unrealized Loss Position [Abstract]    
Number of issues, total 797  
Fixed Maturities [Member] | Investment Grade [Member] | More Than One Year [Member]    
Available-for-sale Securities, Continuous Unrealized Loss Position [Abstract]    
Number of issues, total 373  
Fixed Maturities [Member] | Below Investment Grade [Member]    
Available-for-sale Securities, Continuous Unrealized Loss Position [Abstract]    
Fair value, one year or less $ 543  
Fair value, more than one year 155  
Amortized cost, one year or less 553  
Amortized cost, more than one year 162  
Unrealized depreciation, one year or less (10)  
Unrealized depreciation, more than one year $ (7)  
Fixed Maturities [Member] | Below Investment Grade [Member] | One Year Or Less [Member]    
Available-for-sale Securities, Continuous Unrealized Loss Position [Abstract]    
Number of issues, total 643  
Fixed Maturities [Member] | Below Investment Grade [Member] | More Than One Year [Member]    
Available-for-sale Securities, Continuous Unrealized Loss Position [Abstract]    
Number of issues, total 42