SECURITIES - Portfolio (Details) - USD ($) $ in Thousands	Mar. 31, 2021	Dec. 31, 2020
Mortgage-Backed Securities Portfolio [Line Items]
Principal / Notional	$ 69,291,915	$ 72,253,465
Remaining Premium	4,115,966	3,971,492
Remaining Discount	(88,496)	(95,734)
Total	70,015,346	72,487,080
Unrealized Gains	2,414,654	3,332,263
Unrealized Losses	(580,563)	(166,947)
Estimated Fair Value	71,849,437	75,652,396
Agency
Mortgage-Backed Securities Portfolio [Line Items]
Principal / Notional	66,677,820	70,144,080
Remaining Premium	4,094,580	3,948,694
Remaining Discount	(27,535)	(26,569)
Total	67,825,943	70,921,106
Unrealized Gains	2,377,641	3,294,122
Unrealized Losses	(566,355)	(148,169)
Estimated Fair Value	69,637,229	74,067,059
Agency | Fixed-rate pass-through
Mortgage-Backed Securities Portfolio [Line Items]
Principal / Notional	62,221,818	64,800,235
Remaining Premium	3,509,679	3,325,020
Remaining Discount	(22,954)	(22,143)
Total	65,708,543	68,103,112
Unrealized Gains	2,323,973	3,200,542
Unrealized Losses	(418,896)	(1,076)
Estimated Fair Value	67,613,620	71,302,578
Agency | Adjustable-rate pass-through
Mortgage-Backed Securities Portfolio [Line Items]
Principal / Notional	416,963	455,675
Remaining Premium	2,425	2,869
Remaining Discount	(3,550)	(3,369)
Total	415,838	455,175
Unrealized Gains	23,035	22,341
Unrealized Losses	(1)	0
Estimated Fair Value	438,872	477,516
Agency | CMO
Mortgage-Backed Securities Portfolio [Line Items]
Principal / Notional	133,977	139,664
Remaining Premium	2,140	2,177
Remaining Discount	0	0
Total	136,117	141,841
Unrealized Gains	6,755	7,926
Unrealized Losses	0	0
Estimated Fair Value	142,872	149,767
Agency | Interest-only
Mortgage-Backed Securities Portfolio [Line Items]
Principal / Notional	2,564,531	2,790,537
Remaining Premium	534,913	564,297
Remaining Discount	0	0
Total	534,913	564,297
Unrealized Gains	2,058	3,513
Unrealized Losses	(126,807)	(145,901)
Estimated Fair Value	410,164	421,909
Agency | Multifamily
Mortgage-Backed Securities Portfolio [Line Items]
Principal / Notional	1,294,527	1,910,384
Remaining Premium	41,353	50,148
Remaining Discount	(1,031)	(1,057)
Total	980,458	1,604,913
Unrealized Gains	21,820	59,548
Unrealized Losses	(20,163)	(954)
Estimated Fair Value	982,115	1,663,507
Agency | Reverse mortgages
Mortgage-Backed Securities Portfolio [Line Items]
Principal / Notional	46,004	47,585
Remaining Premium	4,070	4,183
Remaining Discount	0	0
Total	50,074	51,768
Unrealized Gains	0	252
Unrealized Losses	(488)	(238)
Estimated Fair Value	49,586	51,782
Residential credit
Mortgage-Backed Securities Portfolio [Line Items]
Principal / Notional	2,610,095	2,019,527
Remaining Premium	21,386	22,798
Remaining Discount	(60,850)	(61,694)
Total	2,185,514	1,483,587
Unrealized Gains	36,781	38,087
Unrealized Losses	(14,208)	(17,079)
Estimated Fair Value	2,208,087	1,504,595
Residential credit | Multifamily Interest-Only Security
Mortgage-Backed Securities Portfolio [Line Items]
Principal / Notional	354,400	354,600
Residential credit | CRT
Mortgage-Backed Securities Portfolio [Line Items]
Principal / Notional	940,130	544,780
Remaining Premium	7,087	7,324
Remaining Discount	(2,299)	(2,430)
Total	934,425	538,941
Unrealized Gains	3,455	3,062
Unrealized Losses	(6,897)	(9,600)
Estimated Fair Value	930,983	532,403
Residential credit | CRT interest-only security
Mortgage-Backed Securities Portfolio [Line Items]
Principal / Notional	10,500	14,900
Residential credit | Alt-A
Mortgage-Backed Securities Portfolio [Line Items]
Principal / Notional	72,234	93,001
Remaining Premium	50	51
Remaining Discount	(17,179)	(17,368)
Total	55,105	75,684
Unrealized Gains	3,398	4,644
Unrealized Losses	(5)	0
Estimated Fair Value	58,498	80,328
Residential credit | Prime
Mortgage-Backed Securities Portfolio [Line Items]
Principal / Notional	174,335	177,852
Remaining Premium	4,932	5,126
Remaining Discount	(15,778)	(15,999)
Total	163,489	166,979
Unrealized Gains	12,026	14,607
Unrealized Losses	(285)	(77)
Estimated Fair Value	175,230	181,509
Residential credit | Prime interest-only
Mortgage-Backed Securities Portfolio [Line Items]
Principal / Notional	146,141	194,687
Remaining Premium	1,513	1,882
Remaining Discount	0	0
Total	1,513	1,882
Unrealized Gains	0	0
Unrealized Losses	(554)	(642)
Estimated Fair Value	959	1,240
Residential credit | Subprime
Mortgage-Backed Securities Portfolio [Line Items]
Principal / Notional	199,639	197,779
Remaining Premium	531	584
Remaining Discount	(17,986)	(18,181)
Total	182,184	180,182
Unrealized Gains	7,479	8,312
Unrealized Losses	(1,165)	(61)
Estimated Fair Value	188,498	188,433
Residential credit | NPL/RPL
Mortgage-Backed Securities Portfolio [Line Items]
Principal / Notional	804,824	475,108
Remaining Premium	1,307	821
Remaining Discount	(2,374)	(2,416)
Total	803,757	473,513
Unrealized Gains	6,297	3,782
Unrealized Losses	(650)	(1,448)
Estimated Fair Value	809,404	475,847
Residential credit | Prime jumbo (>=2010 vintage)
Mortgage-Backed Securities Portfolio [Line Items]
Principal / Notional	44,309	44,696
Remaining Premium	196	207
Remaining Discount	(5,234)	(5,300)
Total	39,271	39,603
Unrealized Gains	4,126	3,680
Unrealized Losses	(48)	0
Estimated Fair Value	43,349	43,283
Residential credit | Prime jumbo (>=2010 vintage) Interest-only
Mortgage-Backed Securities Portfolio [Line Items]
Principal / Notional	228,483	291,624
Remaining Premium	5,770	6,803
Remaining Discount	0	0
Total	5,770	6,803
Unrealized Gains	0	0
Unrealized Losses	(4,604)	(5,251)
Estimated Fair Value	1,166	1,552
Total Residential Securities
Mortgage-Backed Securities Portfolio [Line Items]
Principal / Notional	69,287,915	72,163,607
Remaining Premium	4,115,966	3,971,492
Remaining Discount	(88,385)	(88,263)
Total	70,011,457	72,404,693
Unrealized Gains	2,414,422	3,332,209
Unrealized Losses	(580,563)	(165,248)
Estimated Fair Value	71,845,316	75,571,654
Commercial Securities
Mortgage-Backed Securities Portfolio [Line Items]
Principal / Notional	4,000	89,858
Remaining Premium	0	0
Remaining Discount	(111)	(7,471)
Total	3,889	82,387
Unrealized Gains	232	54
Unrealized Losses	0	(1,699)
Estimated Fair Value	$ 4,121	$ 80,742