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Stock-based compensation - Stock options (Details) - USD ($)
 1 Months Ended 3 Months Ended 6 Months Ended 12 Months Ended
Jun. 30, 2017
Jun. 30, 2018
Jun. 30, 2017
Jun. 30, 2018
Jun. 30, 2017
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Shares                
Outstanding at the beginning of the period (in shares)       5,745,815 4,611,392 4,611,392    
Granted (in shares)   1,179,877 2,183,106   4,015,356 4,386,856    
Forfeited/Canceled (in shares)           (3,252,433)    
Outstanding at the end of the period (in shares)           5,745,815 4,611,392  
Exercisable at the end of the period (in shares)           2,370,335    
Weighted average exercise price per share                
Outstanding at the beginning of the period (in USD per share)       $ 7.28 $ 14.42 $ 14.42    
Granted (in dollars per share)   $ 0.96 $ 1.46   1.51 1.51    
Forfeited/Canceled (in dollars per share)           9.63    
Outstanding at the end of period (in USD per share)           7.28 $ 14.42  
Exercisable at the end of the period (in USD per share)           $ 13.37    
Weighted average remaining contractual term (years)                
Outstanding           8 years 3 months 26 days 8 years 2 months 23 days  
Exercisable at the end of the period           7 years 29 days    
Aggregate intrinsic value                
Outstanding           $ 43,000 $ 45,000  
Exercisable at the end of period           $ 42,000    
Assumptions used to estimate fair value of each stock-based option award on the grant date using the Black-Scholes option pricing model                
Dividend yield           0.00% 0.00% 0.00%
Expected term (years)              
Additional disclosures                
Total unrecognized compensation cost           $ 4,200,000    
Weighted average period for recognition of compensation cost           2 years 7 months 17 days    
Weighted average grant date fair value (in dollars per share)   $ 0.70 $ 1.11   $ 1.15      
Non-Employee Stock                
Shares                
Granted (in shares)     150,000   150,000      
Weighted average exercise price per share                
Granted (in dollars per share)     $ 1.60   $ 1.60      
Minimum [Member]                
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           1.30% 1.30% 1.60%
Volatility           87.00% 78.00% 72.00%
Expected term (years)           2 years 5 years 3 months 18 days 5 years 3 months 18 days
Maximum [Member]                
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% 2.30%
Volatility           109.00% 89.00% 78.00%
Expected term (years)           6 years 10 months 25 days 9 years 10 months 24 days 9 years 10 months 24 days
Stock options                
Shares                
Outstanding at the beginning of the period (in shares)       5,745,815        
Granted (in shares) 143,000     1,179,877        
Forfeited/Canceled (in shares)       (1,529,717)        
Outstanding at the end of the period (in shares)   5,395,975   5,395,975   5,745,815    
Exercisable at the end of the period (in shares)   2,576,138   2,576,138        
Weighted average exercise price per share                
Outstanding at the beginning of the period (in USD per share)       $ 7.28        
Granted (in dollars per share)       0.96        
Forfeited/Canceled (in dollars per share)       5.90        
Outstanding at the end of period (in USD per share)   $ 6.28   6.28   $ 7.28    
Exercisable at the end of the period (in USD per share)   $ 10.88   $ 10.88        
Weighted average remaining contractual term (years)                
Outstanding       8 years 18 days   8 years 3 months 25 days    
Exercisable at the end of the period       6 years 11 months 15 days        
Aggregate intrinsic value                
Outstanding   $ 1,000   $ 1,000   $ 43,000    
Exercisable at the end of period   $ 1,000   $ 1,000        
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   0.00%   2.70%        
Dividend yield   0.00% 0.00% 0.00% 0.00%      
Volatility   0.00%            
Expected term (years)       6 years 1 month 9 days        
Additional disclosures                
Stock options exercised   $ 0 $ 0          
Total unrecognized compensation cost   $ 2,800,000   $ 2,800,000        
Weighted average period for recognition of compensation cost       2 years 10 months 24 days        
Stock options | Minimum [Member]                
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     1.30%   1.30%      
Volatility     89.00% 83.00% 89.00%      
Expected term (years)     1 year 9 months 18 days   1 year 9 months 18 days      
Stock options | Maximum [Member]                
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     2.00%   2.20%      
Volatility     109.00% 85.00% 109.00%      
Expected term (years)     6 years 10 months 24 days   6 years 10 months 24 days