Income taxes - Uncertain tax positions (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Reconciliation of Unrecognized Tax Benefits, Excluding Amounts Pertaining to Examined Tax Returns [Roll Forward]
Balance, January 1	$ 286	$ 300	$ 243
Additions based on tax positions related to the current year	3	3	17
Additions for tax positions of prior years	63	1	42
Reductions for tax positions of prior years	(41)	0	(1)
Settlements with tax authorities	(8)	(18)	(1)
Balance, December 31	303	286	300
Interest income (expense) recognized in the year ended December 31	9	(15)	(19)
Interest payable as of December 31	$ 44	$ 49	$ 38