AVAILABLE-FOR-SALE SECURITIES - Carrying Value of AFS Securities Not Deemed to be Other Than Temporarily Impaired (Details) - USD ($)	Mar. 31, 2018	Dec. 31, 2017
Investments, Debt and Equity Securities [Abstract]
Less than 12 months, Estimated Fair Value	$ 1,003,823,817	$ 1,084,010,586
Less than 12 months, Gross Unrealized Losses	(20,481,607)	(11,135,736)
Greater than 12 months, Estimated Fair Value	91,365,447	95,024,791
Greater than 12 months, Gross Unrealized Losses	(3,001,535)	(2,233,516)
Total, Estimated Fair Value	1,095,189,264	1,179,035,377
Total, Gross Unrealized Losses	$ (23,483,142)	$ (13,369,252)