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Goodwill and intangible assets - Intangible amortization expense (Details) - USD ($)
Dec. 31, 2015
Dec. 31, 2014
Intangible amortization expense    
2016 $ 563,353  
2017 265,849  
2018 76,848  
2019 75,994  
2020 73,366  
Thereafter 322,854  
Net book value $ 1,378,264 $ 2,046,700