AVAILABLE-FOR-SALE SECURITIES - Realized Gain (Loss) from Sale of AFS Securities (Details) - AFS Securities - USD ($)	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Schedule of Available-for-sale Securities [Line Items]
AFS securities sold, at cost	$ 509,084,520	$ 268,849,640	$ 267,741,325
AFS principal payments, at cost	0	98,166,335	70,836,624
Proceeds from AFS securities sold	495,030,356	263,143,871	267,567,905
Proceeds from AFS principal payments	0	96,655,967	70,476,212
Net realized gain (loss) on sale of AFS securities	$ (14,054,164)	$ (7,216,137)	$ (533,832)