SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Concentration risks (Details)	3 Months Ended		6 Months Ended
	Jun. 30, 2018 USD ($) customer	Jun. 30, 2017 USD ($) customer	Jun. 30, 2018 USD ($) customer	Jun. 30, 2017 USD ($) customer
Concentration risks
Number of significant customers | customer	2	2	2	1
Aggregate dollar amount of net sales to significant customers	$ 7,474,993	$ 7,646,117	$ 15,147,794	$ 14,569,797
Sales | Customer Concentration Risk
Concentration risks
Aggregate dollar amount of net sales to significant customers	$ 3,100,000	$ 3,100,000	$ 6,300,000	$ 4,100,000
Percentage of net sales to significant customers (as a percent)	41.60%	40.20%	41.80%	28.00%
Cost of Goods, Product Line | Supplier Concentration Risk
Concentration risks
Percentage of net sales to significant customers (as a percent)	90.80%	90.10%	87.80%	89.10%