Revenues (Details) - USD ($)	3 Months Ended								12 Months Ended
	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2018	Mar. 31, 2017	Mar. 31, 2016
Revenues.
Net pool revenues—related party									$ 106,958,576	$ 115,753,153	$ 202,918,232
Time charter revenues									50,176,166	49,474,510	38,737,172
Voyage charter revenues									2,068,491	1,296,952	46,194,134
Other revenues, net									131,527	922,556	1,358,291
Total revenues	$ 39,034,678	$ 44,545,589	$ 34,729,021	$ 41,025,472	$ 47,585,174	$ 35,734,988	$ 33,611,233	$ 50,515,776	$ 159,334,760	$ 167,447,171	$ 289,207,829