Fair value measurement (Changes in Contingent Consideration Obligation) (Details) - Fair Value, Measurements, Recurring [Member] - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016
Business Combination, Contingent Consideration [Roll Forward]
Beginning balance			$ 179
Ending balance	$ 182		182
Significant unobservable inputs (Level 3) [Member]
Business Combination, Contingent Consideration [Roll Forward]
Beginning balance	184	$ 194	179	$ 188
Net changes in valuation	(2)	(23)	3	(17)
Ending balance	$ 182	$ 171	$ 182	$ 171