SECURITIES - Portfolio (Details) - USD ($) $ in Thousands	Mar. 31, 2020	Dec. 31, 2019
Mortgage-Backed Securities Portfolio [Line Items]
Principal / notional	$ 77,593,064	$ 113,123,301
Remaining premium	3,913,867	5,353,597
Remaining discount	(106,340)	(166,320)
Amortized cost	76,947,610	112,863,385
Unrealized gains	2,896,205	2,261,636
Unrealized losses	(486,219)	(291,441)
Estimated fair value	79,357,596	114,833,580
Agency
Mortgage-Backed Securities Portfolio [Line Items]
Principal / notional	75,465,615	110,241,618
Remaining premium	3,885,902	5,307,771
Remaining discount	(30,782)	(50,294)
Amortized cost	75,727,702	111,012,250
Unrealized gains	2,890,817	2,167,941
Unrealized losses	(161,673)	(286,824)
Estimated fair value	78,456,846	112,893,367
Agency | Fixed-rate pass-through
Mortgage-Backed Securities Portfolio [Line Items]
Principal / notional	69,788,774	102,448,565
Remaining premium	3,167,897	4,345,053
Remaining discount	(27,245)	(46,614)
Amortized cost	72,929,426	106,747,004
Unrealized gains	2,798,718	2,071,583
Unrealized losses	(234)	(95,173)
Estimated fair value	75,727,910	108,723,414
Agency | Adjustable-rate pass-through
Mortgage-Backed Securities Portfolio [Line Items]
Principal / notional	609,737	1,474,818
Remaining premium	9,043	72,245
Remaining discount	(2,404)	(1,400)
Amortized cost	616,376	1,545,663
Unrealized gains	16,817	10,184
Unrealized losses	(1,757)	(31,516)
Estimated fair value	631,436	1,524,331
Agency | CMO
Mortgage-Backed Securities Portfolio [Line Items]
Principal / notional	153,405	156,937
Remaining premium	2,436	2,534
Remaining discount	0	0
Amortized cost	155,841	159,471
Unrealized gains	4,749	545
Unrealized losses	0	0
Estimated fair value	160,590	160,016
Agency | Interest-only
Mortgage-Backed Securities Portfolio [Line Items]
Principal / notional	3,593,033	4,486,845
Remaining premium	684,208	862,905
Remaining discount	0	0
Amortized cost	684,208	862,905
Unrealized gains	3,170	2,787
Unrealized losses	(157,742)	(157,130)
Estimated fair value	529,636	708,562
Agency | Multifamily
Mortgage-Backed Securities Portfolio [Line Items]
Principal / notional	1,266,927	1,619,900
Remaining premium	17,490	19,981
Remaining discount	(1,133)	(2,280)
Amortized cost	1,283,284	1,637,601
Unrealized gains	67,345	82,292
Unrealized losses	(1,408)	(2,696)
Estimated fair value	1,349,221	1,717,197
Agency | Reverse mortgages
Mortgage-Backed Securities Portfolio [Line Items]
Principal / notional	53,739	54,553
Remaining premium	4,828	5,053
Remaining discount	0	0
Amortized cost	58,567	59,606
Unrealized gains	18	550
Unrealized losses	(532)	(309)
Estimated fair value	58,053	59,847
Residential credit
Mortgage-Backed Securities Portfolio [Line Items]
Principal / notional	2,012,897	2,617,718
Remaining premium	27,685	34,953
Remaining discount	(67,656)	(106,633)
Amortized cost	1,112,978	1,585,690
Unrealized gains	5,388	85,985
Unrealized losses	(309,541)	(4,485)
Estimated fair value	808,825	1,667,190
Residential credit | CRT
Mortgage-Backed Securities Portfolio [Line Items]
Principal / notional	470,229	517,110
Remaining premium	10,811	15,850
Remaining discount	(1,325)	(2,085)
Amortized cost	464,790	515,950
Unrealized gains	0	16,605
Unrealized losses	(241,919)	(1,233)
Estimated fair value	222,871	531,322
Residential credit | CRT interest-only security
Mortgage-Backed Securities Portfolio [Line Items]
Principal / notional	14,900	14,900
Residential credit | Alt-A
Mortgage-Backed Securities Portfolio [Line Items]
Principal / notional	108,312	160,957
Remaining premium	52	250
Remaining discount	(20,861)	(22,306)
Amortized cost	87,503	138,901
Unrealized gains	723	12,482
Unrealized losses	(11,966)	0
Estimated fair value	76,260	151,383
Residential credit | Prime
Mortgage-Backed Securities Portfolio [Line Items]
Principal / notional	230,056	277,076
Remaining premium	4,451	3,362
Remaining discount	(15,780)	(17,794)
Amortized cost	218,727	262,644
Unrealized gains	3,008	14,142
Unrealized losses	(13,099)	(529)
Estimated fair value	208,636	276,257
Residential credit | Prime interest-only
Mortgage-Backed Securities Portfolio [Line Items]
Principal / notional	332,185	391,234
Remaining premium	3,387	3,757
Remaining discount	0	0
Amortized cost	3,387	3,757
Unrealized gains	0	0
Unrealized losses	(1,114)	(590)
Estimated fair value	2,273	3,167
Residential credit | Subprime
Mortgage-Backed Securities Portfolio [Line Items]
Principal / notional	160,887	370,263
Remaining premium	100	1,356
Remaining discount	(25,138)	(59,727)
Amortized cost	135,849	311,892
Unrealized gains	1,125	37,205
Unrealized losses	(7,931)	(118)
Estimated fair value	129,043	348,979
Residential credit | NPL/RPL
Mortgage-Backed Securities Portfolio [Line Items]
Principal / notional	132,603	164,180
Remaining premium	321	351
Remaining discount	(352)	(440)
Amortized cost	132,572	164,091
Unrealized gains	0	191
Unrealized losses	(26,489)	(14)
Estimated fair value	106,083	164,268
Residential credit | Prime jumbo (2010 vintage)
Mortgage-Backed Securities Portfolio [Line Items]
Principal / notional	65,787	182,709
Remaining premium	0	1,026
Remaining discount	(4,200)	(4,281)
Amortized cost	61,587	179,454
Unrealized gains	532	5,360
Unrealized losses	(3,454)	(150)
Estimated fair value	58,665	184,664
Residential credit | Prime jumbo (2010 vintage) Interest-only
Mortgage-Backed Securities Portfolio [Line Items]
Principal / notional	512,838	554,189
Remaining premium	8,563	9,001
Remaining discount	0	0
Amortized cost	8,563	9,001
Unrealized gains	0	0
Unrealized losses	(3,569)	(1,851)
Estimated fair value	4,994	7,150
Total Residential Securities
Mortgage-Backed Securities Portfolio [Line Items]
Principal / notional	77,478,512	112,859,336
Remaining premium	3,913,587	5,342,724
Remaining discount	(98,438)	(156,927)
Amortized cost	76,840,680	112,597,940
Unrealized gains	2,896,205	2,253,926
Unrealized losses	(471,214)	(291,309)
Estimated fair value	79,265,671	114,560,557
Commercial Securities
Mortgage-Backed Securities Portfolio [Line Items]
Principal / notional	114,552	263,965
Remaining premium	280	10,873
Remaining discount	(7,902)	(9,393)
Amortized cost	106,930	265,445
Unrealized gains	0	7,710
Unrealized losses	(15,005)	(132)
Estimated fair value	$ 91,925	$ 273,023