Other accrued liabilities (Details) - USD ($)	Dec. 31, 2018	Dec. 31, 2017
Other Accrued Liabilities [Line Items]
Other accrued liabilities	$ 10,064,017	$ 11,810,366
Vessel and voyage expenses
Other Accrued Liabilities [Line Items]
Other accrued liabilities	4,981,596	5,373,389
General and administrative expenses
Other Accrued Liabilities [Line Items]
Other accrued liabilities	4,768,244	6,050,078
Other expenses
Other Accrued Liabilities [Line Items]
Other accrued liabilities	$ 314,177	$ 386,899