Fair value measurement (Changes in Contingent Consideration Obligation) (Details) - Fair Value, Measurements, Recurring [Member] - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Business Combination, Contingent Consideration [Roll Forward]
Beginning balance			$ 179
Ending balance	$ 68		68
Significant unobservable inputs (Level 3) [Member]
Business Combination, Contingent Consideration [Roll Forward]
Beginning balance	182	$ 171	179	$ 188
Net changes in valuation	(114)	5	(111)	(12)
Ending balance	$ 68	$ 176	$ 68	$ 176