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Summary of significant accounting policies - Software development costs and amortization (Details) - USD ($)
12 Months Ended
Dec. 31, 2016
Dec. 31, 2015
Dec. 31, 2014
Software development costs and amortization      
Capitalized software development costs $ 4,197,142 $ 3,964,454  
Amortization expense 1,200,000 1,900,000 $ 1,700,000
Goodwill and intangible assets      
Impairment charges of Capitalized software   $ 300,000  
Impairment of goodwill $ 0    
Minimum      
Software development costs and amortization      
Estimated useful live 3 years    
Goodwill and intangible assets      
Excessive Fair Value Over Book Value (in percent) 61.00%    
Maximum      
Software development costs and amortization      
Estimated useful live 5 years    
Goodwill and intangible assets      
Excessive Fair Value Over Book Value (in percent) 67.00%