Income taxes (Details 6) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Reconciliation of the beginning and ending amount of gross unrecognized tax benefits
Unrecognized tax benefits, beginning	$ 1,032	$ 968
Additions for tax positions related to current year	270	73
Additions for tax positions related to prior years	20	55
Reductions for tax positions related to prior years	(27)	(36)
Reductions for settlements	(9)	(24)
Reductions for expiration of statute of limitations	0	(4)
Unrecognized tax benefits, ending	1,286	1,032	$ 968
Unrecognized tax benefits that, if recognized, would impact the effective tax rate	1,209	963
Net provision for interest and penalties	38	34	$ 20
Interest and penalties, accrued	157	$ 120
Income tax examination, proposed liability increase/(decrease)	$ 2,300