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Goodwill and intangible assets - Intangible amortization expense (Details) - USD ($)
Dec. 31, 2016
Dec. 31, 2015
Intangible amortization expense    
2017 $ 261,530  
2018 78,392  
2019 75,406  
2020 66,291  
2021 57,181  
Thereafter 288,674  
Net book value $ 827,474 $ 1,378,264