AVAILABLE-FOR-SALE SECURITIES - Narrative (Details)	3 Months Ended
	Mar. 31, 2018 USD ($) security	Mar. 31, 2017 USD ($)	Dec. 31, 2017 USD ($) security	Dec. 31, 2016 USD ($)
Schedule of Available-for-sale Securities [Line Items]
Number of AFS securities | security	51		59
Unpaid principal balance	$ 1,095,189,264		$ 1,179,035,377
Credit-related OTTI losses	$ 3,074,728	$ 3,074,728	$ 3,074,728	$ 3,074,728
Number of AFS securities, unrealized loss position for less than twelve months | security	46		49
Number of AFS securities, unrealized loss position for more than twelve months | security	5		5
Unrealized gains (losses) on AFS securities	$ (10,865,347)	3,550,902
AFS Securities
Schedule of Available-for-sale Securities [Line Items]
Credit-related OTTI losses	0	0
Unrealized gains (losses) on AFS securities	$ (10,865,347)	$ 3,550,902
Minimum
Schedule of Available-for-sale Securities [Line Items]
Maturity of Available-for-Sale Securities	10 years