Annual report pursuant to Section 13 and 15(d)

Stockholders' equity - Common Stock and Receivable from Shareholder (Details)

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Stockholders' equity - Common Stock and Receivable from Shareholder (Details) - USD ($)
 12 Months Ended 24 Months Ended
Dec. 31, 2015
Dec. 31, 2014
Dec. 31, 2015
Feb. 13, 2013
Class of Stock [Line Items]        
Common stock, shares outstanding 18,478,990 15,905,881 18,478,990  
Preferred stock, shares authorized       10,000,000
Common stock        
Class of Stock [Line Items]        
Shares of common stock issued in private placements 647,706      
Private placements, price in usd per share $ 4.75      
Stock issuance costs $ 742,709.51      
Net cash proceeds from private placements $ 2,333,894      
Private Placement | Common stock        
Class of Stock [Line Items]        
Shares of common stock issued in private placements 2,350,734 1,400    
Net cash proceeds from private placements $ 8,859,767 $ 6,300    
Private Placement | Common stock | Minimum        
Class of Stock [Line Items]        
Private placements, price in usd per share     $ 3.37  
Private Placement | Common stock | Maximum        
Class of Stock [Line Items]        
Private placements, price in usd per share     $ 4.75