Income taxes - Uncertain tax positions (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Reconciliation of Unrecognized Tax Benefits, Excluding Amounts Pertaining to Examined Tax Returns [Roll Forward]
Balance, January 1	$ 300	$ 243	$ 84
Additions based on tax positions related to the current year	3	17	4
Additions for tax positions of prior years	1	42	189
Reductions for tax positions of prior years	0	(1)	(2)
Settlements with tax authorities	(18)	(1)	(32)
Balance, December 31	286	300	243
Interest income (expense) recognized in the year ended December 31	(15)	(19)	4
Interest receivable (payable) as of December 31	$ (49)	$ (38)	$ 13