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Investments (Details 5) - USD ($)
$ in Millions
Mar. 31, 2017
Dec. 31, 2016
Investments [Abstracts]    
Available-for-sale Securities, Debt Maturities, Next Twelve Months, Amortized Cost Basis $ 1,996  
Available-for-sale Securities, Debt Maturities, Next Twelve Months, Fair Value 1,996  
Available-for-sale Securities, Debt Maturities, after One Through Five Years, Amortized Cost Basis 2,318  
Available-for-sale Securities, Debt Maturities, after One Through Five Years, Fair Value 2,353  
Available-for-sale Securities, Debt Maturities, after Five Through Ten Years, Amortized Cost Basis 176  
Available-for-sale Securities, Debt Maturities, after Five Through Ten Years, Fair Value 193  
Available-for-sale Securities, Debt Maturities, after Ten Years, Amortized Cost Basis 276  
Available-for-sale Securities, Debt Maturities, after Ten Years, Fair Value 288  
Available-for-sale Securities, Amortized Cost Basis 6,053 $ 5,952
Available-for-sale Securities Fair Value 6,764 6,413
Cost Method Investments [Abstract]    
Cost-method Investments, Aggregate Carrying Amount 142 140
Equity Securities [Member]    
Investments [Abstracts]    
Available-for-sale Securities, Amortized Cost Basis 1,287 1,252
Available-for-sale Securities Fair Value $ 1,934 $ 1,655