Segment information (Details 3) - USD ($)	Dec. 31, 2018	Dec. 31, 2017
Revenues from External Customers and Long-Lived Assets [Line Items]
Accounts receivable	$ 77,279,817	$ 58,252,999
China Mainland [Member]
Revenues from External Customers and Long-Lived Assets [Line Items]
Accounts receivable	38,048,651	37,636,478
Asia [Member]
Revenues from External Customers and Long-Lived Assets [Line Items]
Accounts receivable	33,237,051	15,294,527
Europe [Member]
Revenues from External Customers and Long-Lived Assets [Line Items]
Accounts receivable	5,413,343	5,189,859
North America [Member]
Revenues from External Customers and Long-Lived Assets [Line Items]
Accounts receivable	566,769	94,585
Others [Member]
Revenues from External Customers and Long-Lived Assets [Line Items]
Accounts receivable	$ 14,003	$ 37,550