Income taxes (Reconciliation of Total Gross Amounts of UTBs) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
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	$ 3,061	$ 2,953	$ 2,543
Additions based on tax positions related to the current year	215	173	447
Additions based on tax positions related to prior years	22	13	1
Reductions for tax positions of prior years	(11)	(17)	(5)
Reductions for expiration of statute of limitations	0	0	(5)
Settlements	0	(61)	(28)
Ending balance	$ 3,287	$ 3,061	$ 2,953