Income taxes - Uncertain tax positions (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Reconciliation of Unrecognized Tax Benefits, Excluding Amounts Pertaining to Examined Tax Returns [Roll Forward]
Balance, January 1	$ 243	$ 84	$ 108
Additions based on tax positions related to the current year	17	4	11
Additions for tax positions of prior years	42	189	3
Reductions for tax positions of prior years	(1)	(2)	(21)
Settlements with tax authorities	(1)	(32)	(17)
Balance, December 31	300	243	84
Interest income (expense) recognized in the year ended December 31	(19)	4	8
Interest receivable (payable) as of December 31	$ (38)	$ 13	$ 9