AVAILABLE-FOR-SALE SECURITIES - Narrative (Details)	3 Months Ended		6 Months Ended
	Jun. 30, 2018 USD ($)	Jun. 30, 2017 USD ($)	Jun. 30, 2018 USD ($)	Jun. 30, 2017 USD ($)	Dec. 31, 2017 USD ($) security
Schedule of Available-for-sale Securities [Line Items]
Number of AFS securities | security					59
Unpaid principal balance					$ 1,179,035,377
Number of AFS securities, unrealized loss position for less than twelve months | security					49
Number of AFS securities, unrealized loss position for more than twelve months | security					5
Unrealized gains (losses) on AFS securities	$ 23,483,141	$ (2,112,419)	$ 12,617,794	$ 1,438,483
AFS Securities
Schedule of Available-for-sale Securities [Line Items]
Credit-related OTTI losses	$ 0	$ 0	$ 0	0
Unrealized gains (losses) on AFS securities				$ 1,220,918
Minimum
Schedule of Available-for-sale Securities [Line Items]
Maturity of Available-for-Sale Securities			10 years