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Stockholders' equity (Share Repurchase Program) (Details) - USD ($)
shares in Millions, $ in Millions
3 Months Ended
Mar. 08, 2018
Mar. 31, 2018
Mar. 31, 2017
Equity [Abstract]      
Stock repurchased, Shares 52.1 56.4 3.4
Stock repurchased   $ 10,787 $ 555