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Stockholders' Equity - Impact to Equity as a Result of Stock Repurchases (Details) - USD ($)
$ in Millions
 12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Equity, Repurchased Stock [Line Items]      
Stock Repurchased and Retired During Period, Value $ 954 $ 11,001 $ 10,002
Common Stock Including Additional Paid in Capital [Member]      
Equity, Repurchased Stock [Line Items]      
Stock Repurchased and Retired During Period, Value 34 302 223
Retained Earnings [Member]      
Equity, Repurchased Stock [Line Items]      
Stock Repurchased and Retired During Period, Value $ 1,028 $ 10,883 $ 10,115