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Investments - Securities with a Decline in Fair Value (Details)
$ in Millions
Dec. 31, 2018
USD ($)
Dec. 31, 2017
USD ($)
Investment Grade [Member]    
Available-for-sale Securities, Continuous Unrealized Loss Position [Abstract]    
Fair value, one year or less   $ 3,272
Amortized cost, one year or less   3,309
Unrealized depreciation, one year or less   (37)
Fair value, more than one year   1,503
Amortized cost, more than one year   1,549
Unrealized depreciation, more than one year   $ (46)
Investment Grade [Member] | One Year Or Less [Member]    
Available-for-sale Securities, Continuous Unrealized Loss Position [Abstract]    
Number of issues, total   797
Investment Grade [Member] | More Than One Year [Member]    
Available-for-sale Securities, Continuous Unrealized Loss Position [Abstract]    
Number of issues, total   373
Below Investment Grade [Member]    
Available-for-sale Securities, Continuous Unrealized Loss Position [Abstract]    
Fair value, one year or less   $ 543
Amortized cost, one year or less   553
Unrealized depreciation, one year or less   (10)
Fair value, more than one year   155
Amortized cost, more than one year   162
Unrealized depreciation, more than one year   $ (7)
Below Investment Grade [Member] | One Year Or Less [Member]    
Available-for-sale Securities, Continuous Unrealized Loss Position [Abstract]    
Number of issues, total   643
Below Investment Grade [Member] | More Than One Year [Member]    
Available-for-sale Securities, Continuous Unrealized Loss Position [Abstract]    
Number of issues, total   42
Fixed maturities [Member] | Investment Grade [Member]    
Available-for-sale Securities, Continuous Unrealized Loss Position [Abstract]    
Fair value, one year or less $ 7,127  
Amortized cost, one year or less 7,367  
Unrealized depreciation, one year or less (240)  
Fair value, more than one year 3,023  
Amortized cost, more than one year 3,181  
Unrealized depreciation, more than one year $ (158)  
Fixed maturities [Member] | Investment Grade [Member] | One Year Or Less [Member]    
Available-for-sale Securities, Continuous Unrealized Loss Position [Abstract]    
Number of issues, total 1,324  
Fixed maturities [Member] | Investment Grade [Member] | More Than One Year [Member]    
Available-for-sale Securities, Continuous Unrealized Loss Position [Abstract]    
Number of issues, total 784  
Fixed maturities [Member] | Below Investment Grade [Member]    
Available-for-sale Securities, Continuous Unrealized Loss Position [Abstract]    
Fair value, one year or less $ 1,185  
Amortized cost, one year or less 1,240  
Unrealized depreciation, one year or less (55)  
Fair value, more than one year 249  
Amortized cost, more than one year 270  
Unrealized depreciation, more than one year $ (21)  
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 1,190  
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 245