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Fair value measurements (Details) - USD ($)
$ in Thousands
1 Months Ended 3 Months Ended 6 Months Ended
Jan. 29, 2014
Jun. 30, 2017
Jun. 30, 2016
Jun. 30, 2017
Jun. 30, 2016
Unobservable Input Reconciliation [Roll Forward]          
Balance, beginning of period       $ 13,185  
Expense included in earnings       433  
Settlements       (2,405)  
Purchases, sales and issuances       0  
Transfers in/(out) of Level 3       0  
Balance, end of period   $ 11,213   11,213  
Convertible Senior Notes Due 2021 [Member]          
Unobservable Input Reconciliation [Roll Forward]          
Face amount of debt instrument $ 250,000        
Interest rate, stated percentage 2.875%        
Maturity date Jan. 15, 2021        
Fair value of the notes   323,200   323,200  
Evolva Holding SA 035 & Broad Spectrum Antiviral Platform [Member]          
Unobservable Input Reconciliation [Roll Forward]          
Change in fair value of contingent obligations     $ (400) (100) $ (400)
RSDL [Member]          
Unobservable Input Reconciliation [Roll Forward]          
Change in fair value of contingent obligations   $ 200 $ 500 $ 500 $ 1,300