AVAILABLE-FOR-SALE SECURITIES - Changes of Unamortized Net Discount and Designated Credit Reserves (Details) - USD ($)	3 Months Ended	12 Months Ended
	Mar. 31, 2018	Dec. 31, 2017
Designated credit reserve
Beginning Balance	$ 0	$ (1,929,833)
Dispositions	0	1,929,833
Accretion of net discount	0	0
Ending Balance	0	0
Unamortized net discount
Beginning Balance	(2,204,562)	(27,841,262)
Dispositions	1,736,855	22,685,756
Accretion of net discount	63,284	2,950,944
Ending Balance	(404,423)	(2,204,562)
Total
Beginning Balance	(2,204,562)	(29,771,095)
Dispositions	1,736,855	24,615,589
Accretion of net discount	63,284	2,950,944
Ending Balance	$ (404,423)	$ (2,204,562)