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Marketable securities - Additional Information (Details) - USD ($)
$ in Millions
Dec. 31, 2020
Dec. 31, 2019
Dec. 31, 2018
Jul. 02, 2018
Jul. 01, 2018
Dec. 31, 2017
Gain (Loss) on Securities [Line Items]            
Adoption of accounting standard update $ (3,140) $ 1,793 $ 3,035 $ 3,655 $ 154 $ 6,959
Retained earnings            
Gain (Loss) on Securities [Line Items]            
Adoption of accounting standard update $ (6,628) $ (1,851) $ (611) $ 0 $ 0 $ 225