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Fair value of financial instruments and marketable securities - Hierachy (Details) - USD ($)
$ in Thousands
Jun. 30, 2016
Dec. 31, 2015
Financial assets and liabilities measured at fair value on recurring basis    
Marketable securities $ 237,235 $ 280,903
Recurring basis | Total    
Financial assets and liabilities measured at fair value on recurring basis    
Marketable securities 237,235 280,903
Warrant liability 1 48
Stock appreciation rights liability 140  
Recurring basis | Significant other observable inputs (level 2)    
Financial assets and liabilities measured at fair value on recurring basis    
Marketable securities 237,235 280,903
Recurring basis | Significant unobservable inputs (level 3)    
Financial assets and liabilities measured at fair value on recurring basis    
Warrant liability 1 $ 48
Stock appreciation rights liability $ 140