Accounts receivable (Schedule of Aging of Accounts Receivable) (Details) - USD ($)	Mar. 31, 2019	Dec. 31, 2018
Aging:
current	$ 95,781,698	$ 218,458,862
Total accounts receivable	154,090,385	294,726,804
1-3 months past due [Member]
Aging:
Past due	15,305,123	31,386,341
4-6 months past due [Member]
Aging:
Past due	89,204	109,412
7-12 months past due [Member]
Aging:
Past due	10,746,881	42,532,170
Greater than one year past due [Member]
Aging:
Past due	$ 32,167,479	$ 2,240,019