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Fair value of financial instruments and marketable securities - Hierarchy (Details) - USD ($)
$ in Thousands
Jun. 30, 2018
Dec. 31, 2017
Financial assets and liabilities measured at fair value on recurring basis    
Marketable securities $ 72,318 $ 79,454
Recurring basis | Total    
Financial assets and liabilities measured at fair value on recurring basis    
Marketable securities 72,318 79,454
Warrant Liability 1 1
Stock appreciation rights liability 2,274 1,665
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 72,318 79,454
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 $ 2,274 $ 1,665