Fair value of financial instruments and marketable securities - Hierarchy (Details) - USD ($) $ in Thousands	Sep. 30, 2017	Dec. 31, 2016
Financial assets and liabilities measured at fair value on recurring basis
Marketable securities	$ 27,472	$ 173,345
Recurring basis | Total
Financial assets and liabilities measured at fair value on recurring basis
Marketable securities	27,472	173,345
Warrant Liability	4	1
Stock appreciation rights liability	1,723	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	27,472	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	4	1
Stock appreciation rights liability	$ 1,723	$ 865