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Fair value of financial instruments and investments - Recurring Basis (Details) - USD ($)
$ in Thousands
Dec. 31, 2017
Dec. 31, 2016
Financial assets and liabilities measured at fair value on recurring basis    
Marketable securities $ 79,454 $ 173,345
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 79,454 173,345
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 1,665 865
Recurring basis | Total    
Financial assets and liabilities measured at fair value on recurring basis    
Marketable securities 79,454 173,345
Warrant liability 1 1
Stock appreciation rights liability $ 1,665 $ 865