Accounts receivable (Schedule of Accounts Receivable) (Details) - USD ($)	Mar. 31, 2019	Dec. 31, 2018
Accounts Receivable, after Allowance for Credit Loss [Abstract]
Accounts receivable	$ 154,090,385	$ 294,726,804
Allowance for doubtful accounts	(39,258)	(38,516)
Accounts receivable, net	154,051,127	294,688,288
Notes receivable	148,511	27,392
Accounts receivable pledged as collateral for short-term bank loans	$ 96,402,983	$ 94,581,170