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Employee Benefit Plans - Stock unit activity (Details) - USD ($)
$ / shares in Units, $ in Millions
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Summary of stock unit activity      
Share-based Compensation Arrangement by Share-based Payment Award, Options, Vested and Expected to Vest, Exercisable, Number 18,074,088    
Restricted Stock Units (RSUs) [Member]      
Summary of stock unit activity      
Stock Units, Outstanding Beginning of year 8,507,504    
Stock Units, Granted 3,011,903    
Stock Units, Forfeited (372,871)    
Stock Units, Issued/Settled 3,319,684    
Stock Units, Outstanding End of year 7,826,852 8,507,504  
Not Vested at End of Year 5,396,027    
Weighted-Average Grant-Date Fair Value, beginning of year $ 48.65    
Weighted-Average Grant-Date Fair Value, Granted 48.77    
Weighted-Average Grant-Date Fair Value, Forfeited 45.99    
Weighted-Average Grant-Date Fair Value, ending of year 45.75 $ 48.65  
Weighted-Average Grant-Date Fair Value Not Vested, ending of year $ 45.58    
Total Fair Value, Issued $ 159 $ 191 $ 316