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, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014
Business Combination, Contingent Consideration [Roll Forward]
Beginning balance			$ 215
Ending balance	$ 197		197
Significant unobservable inputs (Level 3) [Member]
Business Combination, Contingent Consideration [Roll Forward]
Beginning balance	215	$ 610	215	$ 595
Net changes in valuation	(18)	(62)	(18)	(47)
Ending balance	$ 197	$ 548	$ 197	$ 548