AVAILABLE-FOR-SALE SECURITIES - Changes of Unamortized Net Discount and Designated Credit Reserves (Details) - USD ($)	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Designated credit reserve
Beginning Balance as of January 1, 2017	$ (1,929,833)	$ (8,891,565)
Acquisitions	0	0
Dispositions	1,929,833	4,893,913
Accretion of net discount	0	0
Realized gain on paydowns	0	0
Realized credit losses	0	3,023,911
Addition to credit reserves	0	(1,021,433)
Release of credit reserves	0	65,341
December 31, 2017	0	(1,929,833)
Unamortized net discount
Beginning Balance as of January 1, 2017	(27,841,262)	(57,280,275)
Acquisitions	0	0
Dispositions	22,685,756	21,637,637
Accretion of net discount	2,950,944	6,703,365
Realized gain on paydowns	0	325,709
Realized credit losses	0	(183,790)
Addition to credit reserves	0	1,021,433
Release of credit reserves	0	(65,341)
December 31, 2017	(2,204,562)	(27,841,262)
Total
Beginning Balance as of January 1, 2017		(29,771,095)	$ (66,171,840)
Acquisitions	0	0
Dispositions	24,615,589	26,531,550
Accretion of net discount	2,950,944	6,703,365
Realized gain on paydowns	0	325,709
Realized credit losses	0	2,840,121
Addition to credit reserves	0	0
Release of credit reserves	0	0
December 31, 2017	$ (2,204,562)	$ (29,771,095)