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Retirement plan (Details) - USD ($)
12 Months Ended
Dec. 31, 2018
Dec. 31, 2017
Dec. 31, 2016
United States      
Defined Contribution Plan Disclosure [Line Items]      
Employer contributions to retirement plan $ 43,866 $ 31,308 $ 33,422
Canada      
Defined Contribution Plan Disclosure [Line Items]      
Employer contributions to retirement plan $ 25,900 $ 26,578 $ 21,777