Income taxes (Details 6) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Reconciliation of the beginning and ending amount of gross unrecognized tax benefits
Unrecognized tax benefits, beginning	$ 968	$ 846
Additions for tax positions related to current year	73	73
Unrecognized tax benefits, increase resulting from prior period tax positions	55	118
Reductions for tax positions related to prior years	(36)	(30)
Reductions for settlements	(24)	(30)
Reductions for expiration of statute of limitations	(4)	(9)
Unrecognized tax benefits, ending	1,032	968	$ 846
Unrecognized tax benefits that, if recognized, would impact the effective tax rate	963	934
Net provision for interest and penalties	34	20	$ 3
Interest and penalties, accrued	120	79
Income tax examination, proposed liability increase/(decrease)	2,000
Income Tax Contingency [Line Items]
Unrecognized tax benefits, increase resulting from prior period tax positions	$ 55	118
Tax Year Prior Years
Reconciliation of the beginning and ending amount of gross unrecognized tax benefits
Unrecognized tax benefits, increase resulting from prior period tax positions		68
Income Tax Contingency [Line Items]
Unrecognized tax benefits, increase resulting from prior period tax positions		$ 68