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Stockholders' equity (Share Repurchase Program) (Details) - USD ($)
shares in Millions, $ in Millions
3 Months Ended 9 Months Ended
Sep. 30, 2017
Jun. 30, 2017
Mar. 31, 2017
Sep. 30, 2016
Jun. 30, 2016
Mar. 31, 2016
Sep. 30, 2017
Sep. 30, 2016
Equity [Abstract]                
Stock repurchased, Shares 4.4 6.2 3.4 4.4 3.9 4.7 14.0 12.9
Stock repurchased $ 769 $ 1,006 $ 555 $ 747 $ 591 $ 690 $ 2,330 $ 2,028