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Property and equipment (Details) - USD ($)
12 Months Ended
Dec. 31, 2015
Dec. 31, 2014
Dec. 31, 2013
Property and equipment      
Total property and equipment, gross $ 5,163,309 $ 4,852,419  
Less: Accumulated depreciation and amortization (3,351,690) (2,712,672)  
Total property and equipment, net 1,811,619 2,139,747  
Depreciation and amortization expense 664,303 621,695 $ 554,765
Equipment      
Property and equipment      
Total property and equipment, gross $ 958,175 945,116  
Equipment | Minimum      
Property and equipment      
Estimated useful life 2 years    
Equipment | Maximum      
Property and equipment      
Estimated useful life 7 years    
Computer hardware      
Property and equipment      
Total property and equipment, gross $ 3,266,233 2,963,629  
Computer hardware | Minimum      
Property and equipment      
Estimated useful life 3 years    
Computer hardware | Maximum      
Property and equipment      
Estimated useful life 5 years    
Furniture and fixtures      
Property and equipment      
Total property and equipment, gross $ 192,565 194,399  
Furniture and fixtures | Minimum      
Property and equipment      
Estimated useful life 5 years    
Furniture and fixtures | Maximum      
Property and equipment      
Estimated useful life 7 years    
Leasehold improvements      
Property and equipment      
Total property and equipment, gross $ 746,336 $ 749,275  
Leasehold improvements | Minimum      
Property and equipment      
Estimated useful life 3 years    
Leasehold improvements | Maximum      
Property and equipment      
Estimated useful life 10 years