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Stockholders' equity (Details Textual) - USD ($)
$ / shares in Units, shares in Millions
1 Months Ended 3 Months Ended 6 Months Ended
Mar. 08, 2018
Jun. 30, 2018
Mar. 31, 2018
Dec. 31, 2017
Jun. 30, 2018
Mar. 31, 2018
Jun. 30, 2017
Mar. 31, 2017
Jun. 30, 2018
Jun. 30, 2017
Apr. 30, 2018
Jan. 31, 2018
Equity [Abstract]                        
Stock repurchase program, additional authorized amount                     $ 5,000,000,000.0 $ 10,000,000,000.0
Number of shares repurchased 52.1       18.2 56.4 6.2 3.4 74.6 9.6    
Stock repurchase program, tender offer amount           $ 10,000,000,000            
Amount available for stock repurchases under a board approved stock repurchase plan   $ 5,400,000,000     $ 5,400,000,000       $ 5,400,000,000      
Dividends declared per share (in usd per share)     $ 1.32 $ 1.32                
Dividends paid per share (in usd per share)   $ 1.32 $ 1.32   $ 1.32   $ 1.15   $ 2.64 $ 2.3