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SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Concentration risks (Details)
3 Months Ended 9 Months Ended
Sep. 30, 2018
USD ($)
customer
Sep. 30, 2017
USD ($)
customer
Sep. 30, 2018
USD ($)
customer
Sep. 30, 2017
USD ($)
customer
Concentration risks        
Number of significant customers | customer 3 3 2 2
Aggregate dollar amount of net sales to significant customers $ 9,863,272 $ 10,412,036 $ 25,011,066 $ 24,981,833
Sales | Customer Concentration Risk        
Concentration risks        
Aggregate dollar amount of net sales to significant customers $ 5,400,000 $ 5,100,000 $ 10,000,000 $ 9,400,000
Percentage of net sales to significant customers (as a percent) 55.20% 49.10% 39.90% 37.50%
Cost of Goods, Product Line | Supplier Concentration Risk        
Concentration risks        
Percentage of net sales to significant customers (as a percent) 79.90% 80.10% 85.10% 81.70%