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, 2016	Sep. 30, 2015	Sep. 30, 2016	Sep. 30, 2015
Business Combination, Contingent Consideration [Roll Forward]
Beginning balance			$ 188
Ending balance	$ 176		176
Significant unobservable inputs (Level 3) [Member]
Business Combination, Contingent Consideration [Roll Forward]
Beginning balance	171	$ 215	188	$ 215
Net changes in valuation	5	(18)	(12)	(18)
Ending balance	$ 176	$ 197	$ 176	$ 197