AVAILABLE-FOR-SALE SECURITIES - Amortized Cost and Fair Value of AFS Investments by Collateral Type (Details) - USD ($)	Jun. 30, 2017	Dec. 31, 2016	Dec. 31, 2015
Schedule of Available-for-sale Securities [Line Items]
Face Value	$ 1,240,398,562	$ 884,520,701
Unamortized premium	26,191,776	17,748,138
Unamortized discount
Designated credit reserve and OTTI	0	(1,929,833)	$ (8,891,565)
Net, unamortized	(8,820,927)	(27,841,262)	$ (57,280,275)
Amortized Cost	1,257,769,411	872,497,744
Gross unrealized gain	2,833,444	3,408,141
Gross unrealized (loss)	(8,226,905)	(10,240,388)
Fair Value	1,252,375,950	865,665,497
Agency
Schedule of Available-for-sale Securities [Line Items]
Face Value	1,201,732,176	779,219,115
Unamortized premium	26,191,776	17,748,138
Unamortized discount
Designated credit reserve and OTTI		0
Net, unamortized	(718,551)	(1,311,292)
Amortized Cost	1,227,205,401	795,655,961
Gross unrealized gain	2,756,978	2,663,975
Gross unrealized (loss)	(8,093,665)	(8,129,703)
Fair Value	1,221,868,714	790,190,233
Non-Agency
Schedule of Available-for-sale Securities [Line Items]
Face Value	0	4,393,771
Unamortized premium	0	0
Unamortized discount
Designated credit reserve and OTTI		(1,929,833)
Net, unamortized	0	(369,887)
Amortized Cost	0	2,094,051
Gross unrealized gain	0	234,647
Gross unrealized (loss)	0
Fair Value	0	2,328,698
Multi-Family
Schedule of Available-for-sale Securities [Line Items]
Face Value	38,666,386	100,907,815
Unamortized premium	0	0
Unamortized discount
Designated credit reserve and OTTI		0
Net, unamortized	(8,102,376)	(26,160,083)
Amortized Cost	30,564,010	74,747,732
Gross unrealized gain	76,466	509,519
Gross unrealized (loss)	(133,240)	(2,110,685)
Fair Value	$ 30,507,236	$ 73,146,566