AVAILABLE-FOR-SALE SECURITIES - Fair Value of AFS Investment Securities by Maturity Date (Details) - USD ($)	Mar. 31, 2018	Dec. 31, 2017
Investments, Debt and Equity Securities [Abstract]
Greater than or equal to one year and less than five years	$ 992,367,597	$ 1,187,909,353
Greater than or equal to five years	102,821,667	102,916,295
Total	$ 1,095,189,264	$ 1,290,825,648