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Fair value of financial instruments and marketable securities - Hierarchy (Details) - USD ($)
$ in Thousands
Mar. 31, 2017
Dec. 31, 2016
Financial assets and liabilities measured at fair value on recurring basis    
Marketable securities $ 148,471 $ 173,345
Recurring basis | Total    
Financial assets and liabilities measured at fair value on recurring basis    
Marketable securities 148,471 173,345
Warrant Liability 1 1
Stock appreciation rights liability 265 865
Recurring basis | Quoted prices in active markets for identical assets (level 1)    
Financial assets and liabilities measured at fair value on recurring basis    
Marketable securities 0 0
Warrant Liability 0 0
Stock appreciation rights liability 0 0
Recurring basis | Significant other observable inputs (level 2)    
Financial assets and liabilities measured at fair value on recurring basis    
Marketable securities 148,471 173,345
Warrant Liability 0 0
Stock appreciation rights liability 0 0
Recurring basis | Significant unobservable inputs (level 3)    
Financial assets and liabilities measured at fair value on recurring basis    
Marketable securities 0 0
Warrant Liability 1 1
Stock appreciation rights liability $ 265 $ 865