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Income taxes (Reconciliation of Total Gross Amounts of UTBs) (Details) - USD ($)
$ in Millions
12 Months Ended
Dec. 31, 2020
Dec. 31, 2019
Dec. 31, 2018
Reconciliation of total gross amounts of unrecognized tax benefits (excluding interest, penalties, foreign tax credits and the federal tax benefit of state taxes related to unrecognized tax benefits)      
Beginning balance $ 3,287 $ 3,061 $ 2,953
Additions based on tax positions related to the current year 165 215 173
Additions based on tax positions related to prior years 3 22 13
Reductions for tax positions of prior years (35) (11) (17)
Settlements (68) 0 (61)
Ending balance $ 3,352 $ 3,287 $ 3,061