Cash, cash equivalents, and Restricted cash (Details) - USD ($)	Mar. 31, 2020	Dec. 31, 2019	Mar. 31, 2019	Dec. 31, 2018
Cash and Cash Equivalents [Abstract]
Cash and cash equivalents	$ 69,229,926	$ 53,583,898	$ 60,715,204	$ 67,209,753
Restricted cash - current	2,917,885	5,471,470	0	0
Restricted cash - noncurrent	74,917	74,917	19,283,145	10,953,885
Total cash, cash equivalents and restricted cash shown in the condensed consolidated statements of cash flows	$ 72,222,728	$ 59,130,285	$ 79,998,349	$ 78,163,638