SECURITIES - Portfolio (Details) - USD ($) $ in Thousands	Sep. 30, 2021	Dec. 31, 2020
Mortgage-Backed Securities Portfolio [Line Items]
Principal / Notional	$ 65,710,241	$ 72,253,465
Remaining Premium	3,864,993	3,971,492
Remaining Discount	(82,277)	(95,734)
Amortized Cost	64,164,022	72,487,080
Unrealized Gains	1,915,388	3,332,263
Unrealized Losses	(457,058)	(166,947)
Estimated Fair Value	65,622,352	75,652,396
Total Residential Securities
Mortgage-Backed Securities Portfolio [Line Items]
Principal / Notional	65,441,241	72,163,607
Remaining Premium	3,864,987	3,971,492
Remaining Discount	(82,174)	(88,263)
Amortized Cost	63,895,119	72,404,693
Unrealized Gains	1,915,179	3,332,209
Unrealized Losses	(457,052)	(165,248)
Estimated Fair Value	65,353,246	75,571,654
Agency
Mortgage-Backed Securities Portfolio [Line Items]
Principal / Notional	62,684,604	70,144,080
Remaining Premium	3,840,505	3,948,694
Remaining Discount	(23,522)	(26,569)
Amortized Cost	61,400,340	70,921,106
Unrealized Gains	1,866,103	3,294,122
Unrealized Losses	(448,364)	(148,169)
Estimated Fair Value	62,818,079	74,067,059
Agency | Fixed-rate pass-through
Mortgage-Backed Securities Portfolio [Line Items]
Principal / Notional	55,667,603	64,800,235
Remaining Premium	3,107,102	3,325,020
Remaining Discount	(19,552)	(22,143)
Amortized Cost	58,755,153	68,103,112
Unrealized Gains	1,815,792	3,200,542
Unrealized Losses	(272,823)	(1,076)
Estimated Fair Value	60,298,122	71,302,578
Agency | Adjustable-rate pass-through
Mortgage-Backed Securities Portfolio [Line Items]
Principal / Notional	331,164	455,675
Remaining Premium	1,644	2,869
Remaining Discount	(2,991)	(3,369)
Amortized Cost	329,817	455,175
Unrealized Gains	19,204	22,341
Unrealized Losses	0	0
Estimated Fair Value	349,021	477,516
Agency | CMO
Mortgage-Backed Securities Portfolio [Line Items]
Principal / Notional	120,598	139,664
Remaining Premium	1,906	2,177
Remaining Discount	0	0
Amortized Cost	122,504	141,841
Unrealized Gains	6,740	7,926
Unrealized Losses	0	0
Estimated Fair Value	129,244	149,767
Agency | Interest-only
Mortgage-Backed Securities Portfolio [Line Items]
Principal / Notional	2,091,601	2,790,537
Remaining Premium	482,544	564,297
Remaining Discount	0	0
Amortized Cost	482,544	564,297
Unrealized Gains	919	3,513
Unrealized Losses	(153,522)	(145,901)
Estimated Fair Value	329,941	421,909
Agency | Multifamily
Mortgage-Backed Securities Portfolio [Line Items]
Principal / Notional	4,435,186	1,910,384
Remaining Premium	243,588	50,148
Remaining Discount	(979)	(1,057)
Amortized Cost	1,668,149	1,604,913
Unrealized Gains	23,448	59,548
Unrealized Losses	(21,125)	(954)
Estimated Fair Value	1,670,472	1,663,507
Agency | Reverse mortgages
Mortgage-Backed Securities Portfolio [Line Items]
Principal / Notional	38,452	47,585
Remaining Premium	3,721	4,183
Remaining Discount	0	0
Amortized Cost	42,173	51,768
Unrealized Gains	0	252
Unrealized Losses	(894)	(238)
Estimated Fair Value	41,279	51,782
Agency | Multifamily Interest-Only Security
Mortgage-Backed Securities Portfolio [Line Items]
Principal / Notional	3,000,000	354,600
Residential credit
Mortgage-Backed Securities Portfolio [Line Items]
Principal / Notional	2,756,637	2,019,527
Remaining Premium	24,482	22,798
Remaining Discount	(58,652)	(61,694)
Amortized Cost	2,494,779	1,483,587
Unrealized Gains	49,076	38,087
Unrealized Losses	(8,688)	(17,079)
Estimated Fair Value	2,535,167	1,504,595
Residential credit | Credit risk transfer
Mortgage-Backed Securities Portfolio [Line Items]
Principal / Notional	780,039	544,780
Remaining Premium	6,639	7,324
Remaining Discount	(1,593)	(2,430)
Amortized Cost	776,784	538,941
Unrealized Gains	11,598	3,062
Unrealized Losses	(1,147)	(9,600)
Estimated Fair Value	787,235	532,403
Residential credit | Alt-A
Mortgage-Backed Securities Portfolio [Line Items]
Principal / Notional	66,805	93,001
Remaining Premium	31	51
Remaining Discount	(16,879)	(17,368)
Amortized Cost	49,957	75,684
Unrealized Gains	3,584	4,644
Unrealized Losses	0	0
Estimated Fair Value	53,541	80,328
Residential credit | Prime
Mortgage-Backed Securities Portfolio [Line Items]
Principal / Notional	343,013	372,539
Remaining Premium	10,197	7,008
Remaining Discount	(15,226)	(15,999)
Amortized Cost	269,586	168,861
Unrealized Gains	12,191	14,607
Unrealized Losses	(2,040)	(719)
Estimated Fair Value	279,737	182,749
Residential credit | Subprime
Mortgage-Backed Securities Portfolio [Line Items]
Principal / Notional	183,962	197,779
Remaining Premium	403	584
Remaining Discount	(16,984)	(18,181)
Amortized Cost	167,381	180,182
Unrealized Gains	9,340	8,312
Unrealized Losses	(285)	(61)
Estimated Fair Value	176,436	188,433
Residential credit | NPL/RPL
Mortgage-Backed Securities Portfolio [Line Items]
Principal / Notional	1,130,807	475,108
Remaining Premium	1,107	821
Remaining Discount	(2,126)	(2,416)
Amortized Cost	1,129,788	473,513
Unrealized Gains	7,700	3,782
Unrealized Losses	(357)	(1,448)
Estimated Fair Value	1,137,131	475,847
Residential credit | Prime jumbo (>=2010 vintage)
Mortgage-Backed Securities Portfolio [Line Items]
Principal / Notional	252,011	336,320
Remaining Premium	6,105	7,010
Remaining Discount	(5,844)	(5,300)
Amortized Cost	101,283	46,406
Unrealized Gains	4,663	3,680
Unrealized Losses	(4,859)	(5,251)
Estimated Fair Value	101,087	44,835
Residential credit | CRT interest-only security
Mortgage-Backed Securities Portfolio [Line Items]
Principal / Notional	8,300	10,700
Residential credit | Prime interest-only
Mortgage-Backed Securities Portfolio [Line Items]
Principal / Notional	68,400	194,700
Residential credit | Prime Jumbo interest-only
Mortgage-Backed Securities Portfolio [Line Items]
Principal / Notional	151,000	291,600
Commercial Securities
Mortgage-Backed Securities Portfolio [Line Items]
Principal / Notional	269,000	89,858
Remaining Premium	6	0
Remaining Discount	(103)	(7,471)
Amortized Cost	268,903	82,387
Unrealized Gains	209	54
Unrealized Losses	(6)	(1,699)
Estimated Fair Value	$ 269,106	$ 80,742