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Common stock, treasury stock and warrants - Additional Information (Detail) - USD ($)
$ / shares in Units, $ in Thousands
 3 Months Ended
Jan. 10, 2018
Mar. 31, 2018
Dec. 31, 2017
Equity [Line Items]      
Common stock, shares issued   76,437,209 61,631,573
Treasury stock, shares   495,918 352,523
Proceeds from issuance of shares, net of issuance costs   $ 13,392  
Treasury stock, value   $ 1,672 $ 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  
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    
Restricted Stock Units [Member]      
Equity [Line Items]      
Shares issued from vesting   12,105,636  
Common shares withheld to pay withholding taxes   143,395