Fair value measurement (Changes in Contingent Consideration Obligation) (Details) - USD ($) $ in Millions		12 Months Ended
	Oct. 14, 2015	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Input Reconciliation [Roll Forward]
Payment to former BioVex Group, Inc. shareholders		$ 0	$ 253	$ 92
Dezima [Member]
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Input Reconciliation [Roll Forward]
Additions from Dezima acquisition	$ 110
Biovex [Member]
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Input Reconciliation [Roll Forward]
Payment to former BioVex Group, Inc. shareholders			125
Recurring [Member]
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Input Reconciliation [Roll Forward]
Beginning balance		188
Ending balance		179	188
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		188	215
Net changes in valuation		(9)	(12)
Ending balance		179	188	$ 215
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]
Additions from Dezima acquisition		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]
Payment to former BioVex Group, Inc. shareholders		$ 0	$ 125