AVAILABLE-FOR-SALE SECURITIES - Amortized Cost and Fair Value of AFS Investments by Collateral Type (Details) - USD ($)	Mar. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Schedule of Available-for-sale Securities [Line Items]
Face Value	$ 1,030,557,309	$ 884,520,701
Unamortized premium	21,113,937	17,748,138
Unamortized discount
Designated credit reserve and OTTI	0	(1,929,833)	$ (8,891,565)
Net, unamortized	(12,669,025)	(27,841,262)	$ (57,280,275)
Amortized Cost	1,039,002,221	872,497,744
Gross unrealized gain	3,968,851	3,408,141
Gross unrealized (loss)	(7,250,202)	(10,240,388)
Fair Value	1,035,720,870	865,665,497
Agency
Schedule of Available-for-sale Securities [Line Items]
Face Value	981,890,924	779,219,115
Unamortized premium	21,113,937	17,748,138
Unamortized discount
Designated credit reserve and OTTI	0	0
Net, unamortized	(887,983)	(1,311,292)
Amortized Cost	1,002,116,878	795,655,961
Gross unrealized gain	3,968,851	2,663,975
Gross unrealized (loss)	(6,175,059)	(8,129,703)
Fair Value	999,910,670	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	0	(1,929,833)
Net, unamortized	0	(369,887)
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	48,666,385	100,907,815
Unamortized premium	0	0
Unamortized discount
Designated credit reserve and OTTI	0	0
Net, unamortized	(11,781,042)	(26,160,083)
Amortized Cost	36,885,343	74,747,732
Gross unrealized gain	0	509,519
Gross unrealized (loss)	(1,075,143)	(2,110,685)
Fair Value	$ 35,810,200	$ 73,146,566