Fair value of financial instruments and marketable securities - Narrative (Details) - USD ($)	9 Months Ended	12 Months Ended
	Sep. 30, 2017	Dec. 31, 2016
Financial assets and liabilities measured at fair value on recurring basis
Transfers of assets between Level 1 and Level 2 of the fair value measurement hierarchy	$ 0	$ 0
Realized gains	$ 30,000	$ 0
Warrants
Financial assets and liabilities measured at fair value on recurring basis
Volatility	70.00%
Risk-free interest rate (as a percent)	1.47%
Minimum | Warrants
Financial assets and liabilities measured at fair value on recurring basis
Volatility		62.00%
Risk-free interest rate (as a percent)		0.62%
Strike price (in dollars per share)	$ 128.00	$ 128.00
Expected life	1 year 10 months 7 days	4 months 24 days
Maximum | Warrants
Financial assets and liabilities measured at fair value on recurring basis
Volatility		67.00%
Risk-free interest rate (as a percent)		1.34%
Strike price (in dollars per share)	$ 2,520.00	$ 2,520.00
Expected life	1 year 11 months 23 days	2 years 8 months 12 days
Common stock | Warrants
Financial assets and liabilities measured at fair value on recurring basis
Fair value of shares (in dollars per share)	$ 20.01	$ 10.91
SARs | Minimum
Financial assets and liabilities measured at fair value on recurring basis
Volatility	63.00%	48.00%
Risk-free interest rate (as a percent)	1.06%	0.44%
Strike price (in dollars per share)	$ 6.76	$ 6.76
Expected life	3 months 7 days	4 days
SARs | Maximum
Financial assets and liabilities measured at fair value on recurring basis
Volatility	71.00%	71.00%
Risk-free interest rate (as a percent)	1.55%	1.47%
Strike price (in dollars per share)	$ 30.86	$ 30.86
Expected life	2 years 3 months 7 days	3 years
SARs | Common stock
Financial assets and liabilities measured at fair value on recurring basis
Fair value of shares (in dollars per share)	$ 20.01	$ 10.91