Income taxes - Uncertain tax positions (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Reconciliation of Unrecognized Tax Benefits, Excluding Amounts Pertaining to Examined Tax Returns [Roll Forward]
Balance, January 1	$ 303	$ 286	$ 300
Additions based on tax positions related to the current year	3	3	3
Additions for tax positions of prior years	35	63	1
Reductions for tax positions of prior years	(249)	(41)	0
Settlements with tax authorities	0	(8)	(18)
Expiration of the statute of limitations for assessing taxes	(3)	0	0
Balance, December 31	89	303	286
Interest income (expense) recognized in the year ended December 31	39	9	(15)
Interest payable as of December 31	$ 8	$ 44	$ 49