AVAILABLE-FOR-SALE SECURITIES - Amortized Cost and Fair Value of AFS Investments by Collateral Type (Details) - USD ($)	Jun. 30, 2018	Dec. 31, 2017	Dec. 31, 2016
Schedule of Available-for-sale Securities [Line Items]
Face Value		$ 1,281,829,317
Unamortized premium		23,818,687
Unamortized discount	$ 0	(2,204,562)	$ (27,841,262)
Amortized Cost		1,303,443,442
Gross unrealized gain		751,458
Gross unrealized (loss)		(13,369,252)
Fair Value		1,290,825,648
Agency
Schedule of Available-for-sale Securities [Line Items]
Face Value		1,274,329,317
Unamortized premium		23,818,687
Unamortized discount		(491,020)
Amortized Cost		1,297,656,984
Gross unrealized gain		751,458
Gross unrealized (loss)		(13,324,794)
Fair Value		1,285,083,648
Multi-Family
Schedule of Available-for-sale Securities [Line Items]
Face Value		7,500,000
Unamortized premium		0
Unamortized discount		(1,713,542)
Amortized Cost		5,786,458
Gross unrealized gain		0
Gross unrealized (loss)		(44,458)
Fair Value		$ 5,742,000