Vessels and vessel improvements - Schedule of Vessels and vessel improvements (Details) - USD ($)	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Movement in Property, Plant and Equipment [Roll Forward]
Vessel and vessel improvements at the beginning of the year	$ 690,236,419
Vessels and vessel improvements at the end of the year	682,944,936	$ 690,236,419
Vessels and Vessel Improvements
Movement in Property, Plant and Equipment [Roll Forward]
Vessel and vessel improvements at the beginning of the year	690,236,419	567,592,950
Advance paid for vessel purchase	2,201,773	1,926,886
Purchase of Vessels and vessel improvements	41,487,795	174,400,746
Disposal of Vessels	(10,354,855)	(15,218,633)
Reclassification to vessels held for sale	(8,458,444)	(9,316,095)
Depreciation Expense	(32,167,752)	(29,149,435)
Vessels and vessel improvements at the end of the year	$ 682,944,936	$ 690,236,419