Stock-based compensation - Stock Options (Details) - USD ($)	1 Months Ended		6 Months Ended		12 Months Ended
	Oct. 31, 2016	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016	Dec. 31, 2016
Shares
Outstanding at the beginning of the period (in shares)			5,848,470
Granted (in shares)			1,905,000
Forfeited/cancelled (in shares)			(180,315)
Outstanding at the end of the period (in shares)			7,573,155		5,848,470
Vested at the end of the period (in shares)			3,921,304
Vested and expected to vest at end of period (in shares)			7,323,155
Weighted-average exercise price per share
Outstanding at the beginning of the period (in dollars per share)			$ 6.35
Granted (in dollars per share)			1.32
Forfeited/cancelled (in dollars per share)			2.31
Outstanding at the end of the period (in dollars per share)			5.19		$ 6.35
Vested at the end of the period (in dollars per share)			7.74
Vested and expected to vest at end of the period (in dollars per share)			$ 5.32
Weighted-average remaining contractual term
Outstanding at the end of the period					8 years
Vested at the end of the period			7 years 1 month 6 days
Vested and expected to vest at the end of the period			8 years
Exercisable at the end of the period			8 years
Aggregate intrinsic value
Outstanding at the end of the period (in dollars)			$ 2,939,000		$ 62,000
Vested at the end of the period (in dollars)			694,000
Vested and expected to vest at end of the period (in dollars)			$ 2,737,000
Assumptions used to estimate fair value of each stock-based award on the grant date
Risk-free interest rate (as a percent)			2.05%	1.47%
Volatility (as a percent)			79.00%	75.00%
Expected term			6 years 4 months 24 days	5 years 10 months 24 days
Performance shares
Shares
Granted (in shares)		500,000
Vested at the end of the period (in shares)	250,000
Additional disclosures
Grant date fair value of options		$ 445,000
Percentage of performance conditions achieved	50.00%
Stock-based compensation expense					$ 222,000
Percentage of performance conditions not considered probable of achievement			50.00%