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Stockholders' equity (Share Repurchase Program) (Details) - USD ($)
shares in Millions, $ in Millions
3 Months Ended 6 Months Ended
Jun. 30, 2019
Mar. 31, 2019
Jun. 30, 2018
Mar. 31, 2018
Jun. 30, 2019
Jun. 30, 2018
Equity [Abstract]            
Stock repurchased, Shares 13.1 15.9 18.2 56.4 28.9 74.6
Stock repurchased $ 2,349 $ 3,031 $ 3,190 $ 10,787 $ 5,380 $ 13,977