Vessels and vessel improvements - Schedule of Vessels and vessel improvements (Details) - USD ($)	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Movement in Property, Plant and Equipment [Roll Forward]
Beginning balance, vessels and vessel improvements, at cost, net of accumulated depreciation	$ 682,944,936
Advance Payment For Vessel Purchase	0	$ 2,040,000	$ 2,201,773
Ending balance, vessels and vessel improvements, at cost, net of accumulated depreciation	835,959,084	682,944,936
Vessels and Vessel Improvements
Movement in Property, Plant and Equipment [Roll Forward]
Beginning balance, vessels and vessel improvements, at cost, net of accumulated depreciation	682,944,936	690,236,419
Advance Payment For Vessel Purchase	2,040,000	2,201,773
Purchase of vessels and vessel improvements	143,477,720	41,487,795
Sale of vessels	(14,757,027)	(10,354,855)
Reclassification to vessels held for sale	0	(8,458,444)
Scrubbers and BWTS	56,267,925	0
Depreciation expense	(34,014,470)	(32,167,752)
Ending balance, vessels and vessel improvements, at cost, net of accumulated depreciation	$ 835,959,084	$ 682,944,936	$ 690,236,419