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Income taxes (Details 3) - USD ($)
12 Months Ended
Dec. 31, 2018
Dec. 31, 2017
Income Taxes Valuation And Qualifying Accounts Details    
Balance at Beginning of Period $ 2,557,309 $ 4,063,436
Charged to Cost and Expenses 171,868 (1,506,127)
Write-offs
Other Charges
Balance at End of Period $ 2,729,177 $ 2,557,309