Income taxes - Reconciliation of Total Gross Amounts of UTBs (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
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	$ 3,770	$ 3,546	$ 3,352
Additions based on tax positions related to the current year	196	151	171
Additions based on tax positions related to prior years	56	90	35
Reductions for tax positions of prior years	0	(14)	(4)
Reductions for expiration of statute of limitations	(4)	(3)	0
Settlements	(6)	0	(8)
Ending balance	$ 4,012	$ 3,770	$ 3,546