Summary of Significant Accounting Policies - Cash, Cash Equivalents, Restricted Cash (Details) - USD ($)	Mar. 31, 2018	Dec. 31, 2017	Mar. 31, 2017	Dec. 31, 2016
Accounting Policies [Abstract]
Cash and cash equivalents	$ 13,934,807	$ 19,857,833	$ 30,103,574	$ 28,701,824
Restricted cash-short term	10,850,136	10,701,305	10,138,890	10,138,890
Restricted cash-long term	3,754,940	6,542,448	14,805,554	17,333,332
Cash, cash equivalents and restricted cash	$ 28,539,883	$ 37,101,586	$ 55,048,018	$ 56,174,046