Stock-based compensation - Stock options (Details) - USD ($)	1 Months Ended	3 Months Ended		9 Months Ended		12 Months Ended
	Jun. 30, 2017	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016	Dec. 31, 2016	Jun. 21, 2017
Shares
Granted (in shares)		192,500	737,500	4,207,856	2,224,600
Weighted average exercise price per share
Granted (in dollars per share)		$ 1.40	$ 7.15	$ 1.51	$ 7.37
Additional disclosures
Weighted average grant date fair value (in dollars per share)		$ 1.05	$ 5.19	$ 1.14	$ 5.24
Non-Employee Stock
Shares
Granted (in shares)				150,000	10,000
Weighted average exercise price per share
Granted (in dollars per share)				$ 1.60	$ 9.69
Stock options
Shares
Outstanding at the beginning of the period (in shares)				4,611,392
Granted (in shares)	143,000			4,357,856
Forfeited/Canceled (in shares)				(2,500,789)
Outstanding at the end of the period (in shares)		6,468,459		6,468,459		4,611,392
Exercisable at the end of the period (in shares)		2,562,854		2,562,854
Vested and expected to vest at the end of the period (in shares)		6,468,459		6,468,459
Weighted average exercise price per share
Outstanding at the beginning of the period (in dollars per share)				$ 14.42
Granted (in dollars per share)				1.51
Forfeited/Canceled (in dollars per share)				8.95
Outstanding at the end of period (in dollars per share)		$ 7.84		7.84		$ 14.42
Exercisable at the end of the period (in dollars per share)		14.38		14.38
Vested and expected to vest at the end of the period (in dollars per share)		$ 7.84		$ 7.84
Weighted average remaining contractual term (years)
Outstanding				8 years 5 months 20 days		8 years 2 months 23 days
Exercisable at the end of the period				7 years 1 month 9 days
Vested and expected to vest at the end of the period				8 years 5 months 20 days
Aggregate intrinsic value
Outstanding		$ 48,000		$ 48,000		$ 45,000
Exercisable at the end of period		43,000		43,000
Vested and expected to vest at the end of the period		48,000		48,000
Additional disclosures
Total intrinsic value of options exercised		0	$ 200,000	0	$ 300,000
Total unrecognized compensation cost, net of estimated forfeitures, related to unvested stock options and restricted stock units		$ 8,800,000		$ 8,800,000
Weighted average period for recognition of compensation cost related to unvested stock awards				2 years 1 month 6 days
Assumptions used to estimate fair value of each stock-based option award on the grant date using the Black-Scholes option pricing model
Risk-free interest rate			1.60%
Dividend yield		0.00%	0.00%	0.00%	0.00%
Volatility			86.00%
Expected term (years)		6 years 1 month 6 days	6 years 1 month 6 days
Stock options | Minimum
Assumptions used to estimate fair value of each stock-based option award on the grant date using the Black-Scholes option pricing model
Risk-free interest rate		1.90%		1.30%	1.40%
Volatility		92.00%		89.00%	78.00%
Expected term (years)				2 years	5 years 3 months 19 days
Stock options | Maximum
Assumptions used to estimate fair value of each stock-based option award on the grant date using the Black-Scholes option pricing model
Risk-free interest rate				2.20%	2.00%
Volatility				109.00%	89.00%
Expected term (years)				6 years 10 months 24 days	9 years 10 months 24 days
Stock Compensation Plan | President
Additional disclosures
Total unrecognized compensation cost, net of estimated forfeitures, related to unvested stock options and restricted stock units							$ 2,700,000
Weighted average period for recognition of compensation cost related to unvested stock awards				2 years
Modified share based compensation expense				$ 100,000
Stock Compensation Plan | President | Selling, General and Administrative Expenses
Additional disclosures
Share-based compensation expense				$ 2,800,000