Stock-based compensation - Summary of status of company's stock option activity (Details) - Employee Stock Option - USD ($)					12 Months Ended
	Sep. 19, 2016	Sep. 06, 2016	Apr. 08, 2016	Apr. 01, 2016	Dec. 31, 2016	Dec. 31, 2015
Shares
Beginning balance outstanding (in shares)					1,672,578
Granted (in shares)	30,000	30,000	1,000,000	250,000	560,000	0
Exercised (in shares)					0
Cancelled, expired, or forfeited (in shares)					(375,566)
Ending balance outstanding (in shares)					1,857,012	1,672,578
Exercisable (in shares)					832,442
Weighted Average Exercise Price
Beginning balance outstanding (in dollars per share)					$ 5.05
Granted (in dollars per share)	$ 2.39	$ 2.61	$ 2.73	$ 2.83	2.72
Exercised (in dollars per share)					0.00
Cancelled, expired, or forfeited (in dollars per share)					5.01
Ending balance outstanding (in dollars per share)					4.28	$ 5.05
Exercisable (in dollars per share)					$ 4.71
Weighted Average Remaining Contractual Life (Years)
Outstanding					7 years 9 months 18 days
Exercisable					7 years 4 months 6 days
Intrinsic Value
Outstanding					$ 0
Exercisable					$ 0