AVAILABLE-FOR-SALE SECURITIES - Realized Gain (Loss) from Sale of AFS Securities (Details) - AFS Securities - USD ($)	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Schedule of Available-for-sale Securities [Line Items]
AFS securities sold, at cost	$ 421,186,153	$ 41,467,395	$ 483,621,259	$ 233,054,347
AFS principal payments, at cost	0	32,736,219	0	66,872,340
Proceeds from AFS securities sold	416,037,708	41,283,816	469,004,262	230,557,238
Proceeds from AFS principal payments	0	32,170,194	0	66,007,840
Net realized gain (loss) on sale of AFS securities	$ (5,148,445)	$ (749,604)	$ (14,616,997)	$ (3,361,609)