AVAILABLE-FOR-SALE SECURITIES - Realized Gain (Loss) from Sale of AFS Securities (Details) - AFS Securities - USD ($)	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Schedule of Available-for-sale Securities [Line Items]
AFS securities sold, at cost	$ 1,113,596,615	$ 6,832,799	$ 1,260,655,159	$ 62,435,106
Proceeds from AFS securities sold	1,083,104,038	6,681,250	1,227,314,575	52,966,554
Net realized gain (loss) on sale of AFS securities	$ (30,492,577)	$ (151,549)	$ (33,340,584)	$ (9,468,552)