Vessels and vessel improvements - Narrative (Details)			1 Months Ended	3 Months Ended			12 Months Ended
	Jan. 04, 2019 USD ($)	Dec. 13, 2018 USD ($)	Jan. 31, 2019 vessel	Mar. 31, 2019 USD ($)	Dec. 31, 2016 USD ($)	Mar. 31, 2016 USD ($)	Dec. 31, 2018 USD ($) vessel option	Dec. 31, 2017 USD ($) vessel	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($) vessel	Jan. 23, 2019 option	Dec. 21, 2018 USD ($)	Nov. 20, 2018 vessel	Sep. 04, 2018 USD ($) vessel	Aug. 14, 2018 USD ($) vessel	Sep. 30, 2016 USD ($)
Property, Plant and Equipment [Line Items]
Number of vessels | vessel							47
Number of vessels expected to be sold | vessel							2			6
Impairment of assets to be sold					$ 122,900,000	$ 6,200,000				$ 50,900,000
Carrying value of assets to be sold										$ 76,300,000						$ 234,900,000
Cost savings period									2 years
Number of vessels sold | vessel							4	4
Number of vessels evaluated as impaired | vessel							16	16
Proceeds from sale of vessels							$ 20,545,202	$ 26,042,000	$ 13,001,000
Gain on sale of vessel							335,160	2,134,767	(101,860)
Advance for vessel purchase							$ 2,040,000	$ 2,201,773	$ 1,926,886
Ultramax Vessels
Property, Plant and Equipment [Line Items]
Number of vessels delivered | vessel							2
Vessel agreement, purchase price per vessel							$ 21,300,000
Avocet and Thrush Vessels
Property, Plant and Equipment [Line Items]
Number of vessels sold | vessel							2
Proceeds from sale of vessels							$ 20,500,000
Gain on sale of vessel							300,000
Vessel Condor
Property, Plant and Equipment [Line Items]
Proceeds from sale of vessels		$ 6,500,000
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
Ballast Water Treatment System
Property, Plant and Equipment [Line Items]
Number of vessels | vessel															47
Projected project costs															$ 500,000
Advance for vessel purchase							1,000,000
Scrubber Systems
Property, Plant and Equipment [Line Items]
Projected project costs														$ 2,200,000
Advance for vessel purchase							$ 16,900,000
Number of scrubbers | vessel													15	19
Number of additional units to be purchased | vessel														18
Number of purchase options exercised | option							3
Maximum | Scrubber Systems
Property, Plant and Equipment [Line Items]
Number of scrubbers | vessel														37
Subsequent Event
Property, Plant and Equipment [Line Items]
Number of vessels delivered | vessel			2
Subsequent Event | Vessel Merlin
Property, Plant and Equipment [Line Items]
Proceeds from sale of vessels	$ 6,100,000
Subsequent Event | Scrubber Systems
Property, Plant and Equipment [Line Items]
Number of purchase options exercised | option											3
Scenario, Forecast | Vessel Condor
Property, Plant and Equipment [Line Items]
Gain on sale of vessel				$ 2,200,000
Scenario, Forecast | Vessel Merlin
Property, Plant and Equipment [Line Items]
Gain on sale of vessel				$ 1,900,000