AVAILABLE-FOR-SALE SECURITIES - Amortized Cost and Fair Value of AFS Investments by Collateral Type (Details) - USD ($)	Mar. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Schedule of Available-for-sale Securities [Line Items]
Face Value	$ 1,098,998,936	$ 1,281,829,317
Unamortized premium	20,077,893	23,818,687
Unamortized discount	(404,423)	(2,204,562)	$ (27,841,262)
Amortized Cost	1,118,672,406	1,303,443,442
Gross unrealized gain	0	751,458
Gross unrealized (loss)	(23,483,142)	(13,369,252)
Fair Value	1,095,189,264	1,290,825,648
Agency
Schedule of Available-for-sale Securities [Line Items]
Face Value	1,098,998,936	1,274,329,317
Unamortized premium	20,077,893	23,818,687
Unamortized discount	(404,423)	(491,020)
Amortized Cost	1,118,672,406	1,297,656,984
Gross unrealized gain	0	751,458
Gross unrealized (loss)	(23,483,142)	(13,324,794)
Fair Value	1,095,189,264	1,285,083,648
Multi-Family
Schedule of Available-for-sale Securities [Line Items]
Face Value	0	7,500,000
Unamortized premium	0	0
Unamortized discount	0	(1,713,542)
Amortized Cost	0	5,786,458
Gross unrealized gain	0	0
Gross unrealized (loss)	0	(44,458)
Fair Value	$ 0	$ 5,742,000