Vessels and vessel improvements - Narrative (Details)	3 Months Ended		12 Months Ended
	Sep. 30, 2019 USD ($) vessel	Dec. 31, 2018 USD ($)	Dec. 31, 2019 USD ($) vessel	Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)	Dec. 21, 2018 USD ($)	Sep. 04, 2018 USD ($) vessel	Aug. 14, 2018 USD ($) vessel
Property, Plant and Equipment [Line Items]
Number of vessels | vessel			50
Number of sellers | vessel	2
Number of vessels sold | vessel			4
Proceeds from sale of vessels			$ 29,560,746	$ 20,545,202	$ 26,042,000
Gain on sale of vessel			$ 5,978,566	335,160	2,134,767
Number of vessels installed | vessel			9
Advance Payment For Vessel Purchase			$ 0	2,040,000	$ 2,201,773
Vessels and vessel improvements, at cost, net of accumulated depreciation		$ 682,944,936	$ 835,959,084	$ 682,944,936
Ultramax Vessels
Property, Plant and Equipment [Line Items]
Number of vessels purchased | vessel	6
Vessel purchase price	$ 124,300,000
Vessels Kestrel. Thrasher, Condor and Merlin
Property, Plant and Equipment [Line Items]
Number of vessels sold | vessel			4
Proceeds from sale of vessels			$ 29,600,000
Gain on sale of vessel			6,000,000.0
2015 Built Ultramax Vessel
Property, Plant and Equipment [Line Items]
Vessel purchase price						$ 20,400,000
Deposit paid on purchase of vessel		$ 2,000,000.0
Ballast Water Treatment System
Property, Plant and Equipment [Line Items]
Number of vessels | vessel								40
Projected project costs								$ 500,000
Scrubbers and BWTS			3,800,000
Advance Payment For Vessel Purchase			2,900,000
Scrubber Systems
Property, Plant and Equipment [Line Items]
Projected project costs							$ 2,200,000
Advance Payment For Vessel Purchase			23,600,000
Vessels and vessel improvements, at cost, net of accumulated depreciation			$ 52,400,000
Commissioned scrubbers | vessel			24
Maximum | Scrubber Systems
Property, Plant and Equipment [Line Items]
Number of scrubbers | vessel							37