Income taxes - Uncertain tax positions (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Reconciliation of Unrecognized Tax Benefits, Excluding Amounts Pertaining to Examined Tax Returns [Roll Forward]
Balance, January 1	$ 108	$ 91	$ 184
Additions based on tax positions related to the current year	11	10	7
Additions for tax positions of prior years	3	52	19
Reductions for tax positions of prior years	(21)	(9)	(10)
Settlements with tax authorities	(17)	(36)	(96)
Expiration of the statute of limitations for assessing taxes	0	0	(13)
Balance, December 31	84	108	91
Interest income (expense) recognized in the year ended December 31	8	6	(10)
Interest receivable (payable) as of December 31	$ 9	$ 0	$ (5)