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Investments - Securities with a Decline in Fair Value (Details) - Debt Securities [Member]
$ in Millions
Mar. 31, 2019
USD ($)
Dec. 31, 2018
USD ($)
Investment Grade [Member]    
Available-for-sale Securities, Continuous Unrealized Loss Position [Abstract]    
Fair value, one year or less $ 715 $ 7,127
Amortized cost, one year or less 734 7,367
Unrealized depreciation, one year or less (19) (240)
Fair value, more than one year 4,431 3,023
Amortized cost, more than one year 4,532 3,181
Unrealized depreciation, more than one year $ (101) $ (158)
Investment Grade [Member] | One Year Or Less [Member]    
Available-for-sale Securities, Continuous Unrealized Loss Position [Abstract]    
Number of issues, total 190 1,324
Investment Grade [Member] | More Than One Year [Member]    
Available-for-sale Securities, Continuous Unrealized Loss Position [Abstract]    
Number of issues, total 919 784
Below Investment Grade [Member]    
Available-for-sale Securities, Continuous Unrealized Loss Position [Abstract]    
Fair value, one year or less $ 324 $ 1,185
Amortized cost, one year or less 328 1,240
Unrealized depreciation, one year or less (4) (55)
Fair value, more than one year 455 249
Amortized cost, more than one year 476 270
Unrealized depreciation, more than one year $ (21) $ (21)
Below Investment Grade [Member] | One Year Or Less [Member]    
Available-for-sale Securities, Continuous Unrealized Loss Position [Abstract]    
Number of issues, total 393 1,190
Below Investment Grade [Member] | More Than One Year [Member]    
Available-for-sale Securities, Continuous Unrealized Loss Position [Abstract]    
Number of issues, total 355 245