AVAILABLE-FOR-SALE SECURITIES - Amortized Cost and Fair Value of AFS Investments by Collateral Type (Details) - USD ($)	Sep. 30, 2017	Dec. 31, 2016	Dec. 31, 2015
Schedule of Available-for-sale Securities [Line Items]
Face Value	$ 1,292,557,791	$ 884,520,701
Unamortized premium	23,409,353	17,748,138
Unamortized discount
Net, unamortized	(8,274,696)	(27,841,262)	$ (57,280,275)
Designated credit reserve and OTTI	0	(1,929,833)	$ (8,891,565)
Amortized Cost	1,307,692,448	872,497,744
Gross unrealized gain	1,987,281	3,408,141
Gross unrealized (loss)	(5,193,689)	(10,240,388)
Fair Value	1,304,486,040	865,665,497
Agency
Schedule of Available-for-sale Securities [Line Items]
Face Value	1,253,891,405	779,219,115
Unamortized premium	23,409,353	17,748,138
Unamortized discount
Net, unamortized	(643,743)	(1,311,292)
Designated credit reserve and OTTI		0
Amortized Cost	1,276,657,015	795,655,961
Gross unrealized gain	1,970,238	2,663,975
Gross unrealized (loss)	(4,891,632)	(8,129,703)
Fair Value	1,273,735,621	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
Net, unamortized	0	(369,887)
Designated credit reserve and OTTI		(1,929,833)
Amortized Cost	0	2,094,051
Gross unrealized gain	0	234,647
Gross unrealized (loss)	0	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
Net, unamortized	(7,630,953)	(26,160,083)
Designated credit reserve and OTTI		0
Amortized Cost	31,035,433	74,747,732
Gross unrealized gain	17,043	509,519
Gross unrealized (loss)	(302,057)	(2,110,685)
Fair Value	$ 30,750,419	$ 73,146,566