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Summary of significant accounting policies - Narrative (Details)
3 Months Ended 12 Months Ended
Dec. 31, 2018
USD ($)
Sep. 30, 2018
USD ($)
Jun. 30, 2018
USD ($)
Mar. 31, 2018
USD ($)
Dec. 31, 2017
USD ($)
Sep. 30, 2017
USD ($)
Jun. 30, 2017
USD ($)
Mar. 31, 2017
USD ($)
Dec. 31, 2018
USD ($)
segment
Dec. 31, 2017
USD ($)
Dec. 31, 2016
USD ($)
Entity Location [Line Items]                      
Number of operating segments | segment                 2    
Advertising expenses                 $ 822,000 $ 3,566,000 $ 2,936,000
Cost of product revenue $ 74,934 $ 91,937 $ 179,945 $ 83,934 $ 133,343 $ 148,756 $ 145,325 $ 79,106 430,750 506,530 662,747
Shipping and handling                      
Entity Location [Line Items]                      
Cost of product revenue                 $ 98,000 $ 261,000 $ 170,000
UNITED STATES                      
Entity Location [Line Items]                      
Number of geographic segments | segment                 1