Cash, cash equivalents, and Restricted cash (Details) - USD ($)	Jun. 30, 2020	Dec. 31, 2019	Jun. 30, 2019	Dec. 31, 2018
Cash and Cash Equivalents [Abstract]
Cash and cash equivalents	$ 98,532,141	$ 53,583,898	$ 38,598,904	$ 67,209,753
Restricted cash - current	0	5,471,470	0	0
Restricted cash - noncurrent	74,917	74,917	26,863,979	10,953,885
Total cash, cash equivalents and restricted cash shown in the condensed consolidated statements of cash flows	$ 98,607,058	$ 59,130,285	$ 65,462,883	$ 78,163,638