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Common stock, treasury stock and warrants - Additional Information (Detail) - USD ($)
$ / shares in Units, $ in Thousands
 6 Months Ended
Apr. 09, 2018
Jan. 10, 2018
Jun. 30, 2018
Dec. 31, 2017
Equity [Line Items]        
Common stock, shares issued     76,509,709 61,631,573
Treasury stock, shares     1,225,085 352,523
Proceeds from issuance of shares, net of issuance costs     $ 13,392  
Treasury stock, value     $ 3,253 $ 1,274
Warrants outstanding     2,623,776 1,273,776
Minimum [Member]        
Equity [Line Items]        
Exercise price of warrants     $ 3.75  
Maximum [Member]        
Equity [Line Items]        
Exercise price of warrants     $ 8.00  
Accelerated Vesting of RSUs [Member]        
Equity [Line Items]        
Treasury stock, repurchased 729,167      
Treasury stock, repurchased value $ 1,581      
Registered Direct Offering [Member] | Common Stock [Member]        
Equity [Line Items]        
Issuance of shares, net of offering costs, shares   2,700,000    
Common stock purchase price per share   $ 5.00    
Proceeds from issuance of shares, net of issuance costs   $ 13,392    
Issuance costs   $ 108    
Registered Direct Offering [Member] | Warrant [Member]        
Equity [Line Items]        
Issuance of shares, net of offering costs, shares   1,350,000    
Exercise price of warrants   $ 6.00    
Warrants exercisable period   2 years    
Restricted Stock Units [Member]        
Equity [Line Items]        
Shares issued from vesting     12,178,136  
Common shares withheld to pay withholding taxes     872,562