Basis of presentation, and significant concentrations and risks (Sales and Purchase Concentration) (Details) - USD ($)	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Concentration Risk [Line Items]
Revenues	$ 301,466,007	$ 310,453,033
Customer Concentration Risk [Member] | Revenues [Member] | Distributor A [Member]
Concentration Risk [Line Items]
Revenues	$ 56,499,732	$ 47,731,909
Concentration risk, percentage	18.70%	15.40%
Customer Concentration Risk [Member] | Revenues [Member] | Distributor B, located in PRC [Member]
Concentration Risk [Line Items]
Revenues	$ 33,931,975	$ 29,867,832
Concentration risk, percentage	11.30%	9.60%
Customer Concentration Risk [Member] | Revenues [Member] | Distributor C, located in PRC [Member]
Concentration Risk [Line Items]
Revenues	$ 32,491,923	$ 35,567,286
Concentration risk, percentage	10.80%	11.50%
Customer Concentration Risk [Member] | Revenues [Member] | Distributor D, located in PRC [Member]
Concentration Risk [Line Items]
Revenues	$ 21,968,499	$ 32,353,013
Concentration risk, percentage	7.30%	10.40%
Customer Concentration Risk [Member] | Revenues [Member] | Major Distributors Aggregate [Member]
Concentration Risk [Line Items]
Revenues	$ 144,892,129	$ 145,520,040
Concentration risk, percentage	48.10%	46.90%
Raw Materials Supplier Concentration Risk [Member] | Purchase of Raw Materials [Member] | Major Raw Materials Distributors [Member]
Concentration Risk [Line Items]
Concentration risk, percentage	37.80%	30.30%