Fair value measurement (Changes in Contingent Consideration Obligation) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Dezima [Member]
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Input Reconciliation [Roll Forward]
Addition from Dezima acquisition	$ 0		$ 110
Net changes in valuation	116
Biovex [Member]
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Input Reconciliation [Roll Forward]
Settlement of contingent consideration obligations			(125)
Recurring [Member]
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Input Reconciliation [Roll Forward]
Beginning balance	179
Ending balance	69	$ 179
Recurring [Member] | Significant unobservable inputs (Level 3) [Member]
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Input Reconciliation [Roll Forward]
Beginning balance	179	188	215
Net changes in valuation	(110)	(9)	(12)
Ending balance	69	179	188
Recurring [Member] | Significant unobservable inputs (Level 3) [Member] | Dezima [Member]
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Input Reconciliation [Roll Forward]
Addition from Dezima acquisition	0	0	110
Recurring [Member] | Significant unobservable inputs (Level 3) [Member] | Biovex [Member]
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Input Reconciliation [Roll Forward]
Settlement of contingent consideration obligations	$ 0	$ 0	$ (125)