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AVAILABLE-FOR-SALE SECURITIES - Narrative (Details)
3 Months Ended 9 Months Ended
Sep. 30, 2018
USD ($)
Sep. 30, 2017
USD ($)
Sep. 30, 2018
USD ($)
Sep. 30, 2017
USD ($)
Dec. 31, 2017
USD ($)
security
Debt Securities, Available-for-sale [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 $ 0 $ 2,187,048 $ 12,617,794 $ 3,625,531  
AFS Securities          
Debt Securities, Available-for-sale [Line Items]          
Credit-related OTTI losses $ 0 $ 0 $ 0 0  
Unrealized gains (losses) on AFS securities       $ (2,954,193)  
Minimum          
Debt Securities, Available-for-sale [Line Items]          
Maturity of Available-for-Sale Securities     10 years