SECURITIES - Portfolio (Details) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Mortgage-Backed Securities Portfolio [Line Items]
Principal / Notional	$ 96,782,059	$ 95,582,210
Remaining Premium	5,937,065	6,173,318
Remaining Discount	(177,203)	(158,022)
Amortized Cost	95,408,711	93,803,739
Unrealized Gains	136,990	288,667
Unrealized Losses	(4,207,090)	(1,528,834)
Estimated Fair Value	91,338,611	92,563,572
Agency Securities
Mortgage-Backed Securities Portfolio [Line Items]
Principal / Notional	93,832,975	92,600,794
Remaining Premium	5,884,735	6,128,353
Remaining Discount	(17,453)	(4,043)
Amortized Cost	93,451,372	91,920,389
Unrealized Gains	40,060	157,875
Unrealized Losses	(4,201,304)	(1,526,501)
Estimated Fair Value	89,290,128	90,551,763
Agency Securities | Fixed-rate pass-through
Mortgage-Backed Securities Portfolio [Line Items]
Principal / Notional	80,917,596	78,509,335
Remaining Premium	4,376,927	4,514,815
Remaining Discount	(7,643)	(1,750)
Amortized Cost	85,286,880	83,022,400
Unrealized Gains	28,156	140,115
Unrealized Losses	(3,664,420)	(1,178,673)
Estimated Fair Value	81,650,616	81,983,842
Agency Securities | Adjustable-rate pass-through
Mortgage-Backed Securities Portfolio [Line Items]
Principal / Notional	5,415,559	6,760,991
Remaining Premium	268,852	277,212
Remaining Discount	(1,568)	(1,952)
Amortized Cost	5,682,843	7,036,251
Unrealized Gains	9,098	15,776
Unrealized Losses	(164,520)	(103,121)
Estimated Fair Value	5,527,421	6,948,906
Agency Securities | CMO
Mortgage-Backed Securities Portfolio [Line Items]
Principal / Notional	11,610
Remaining Premium	55
Remaining Discount	0
Amortized Cost	11,665
Unrealized Gains	0
Unrealized Losses	(124)
Estimated Fair Value	11,541
Agency Securities | Interest-only
Mortgage-Backed Securities Portfolio [Line Items]
Principal / Notional	6,248,885	6,804,715
Remaining Premium	1,226,033	1,326,761
Remaining Discount	0	0
Amortized Cost	1,226,033	1,326,761
Unrealized Gains	2,500	1,863
Unrealized Losses	(340,766)	(242,862)
Estimated Fair Value	887,767	1,085,762
Agency Securities | Multifamily
Mortgage-Backed Securities Portfolio [Line Items]
Principal / Notional	1,204,773	490,753
Remaining Premium	8,596	5,038
Remaining Discount	(8,242)	(341)
Amortized Cost	1,205,127	495,450
Unrealized Gains	306	84
Unrealized Losses	(31,135)	(1,845)
Estimated Fair Value	1,174,298	493,689
Agency Securities | Reverse mortgages
Mortgage-Backed Securities Portfolio [Line Items]
Principal / Notional	34,552	35,000
Remaining Premium	4,272	4,527
Remaining Discount	0	0
Amortized Cost	38,824	39,527
Unrealized Gains	0	37
Unrealized Losses	(339)	0
Estimated Fair Value	38,485	39,564
Residential Credit Securities Mortgage Backed Securities
Mortgage-Backed Securities Portfolio [Line Items]
Principal / Notional	2,752,677	2,711,128
Remaining Premium	51,991	44,285
Remaining Discount	(149,847)	(144,248)
Amortized Cost	1,770,496	1,622,113
Unrealized Gains	96,355	128,949
Unrealized Losses	(4,863)	(2,004)
Estimated Fair Value	1,861,988	1,749,058
Residential Credit Securities Mortgage Backed Securities | CRT
Mortgage-Backed Securities Portfolio [Line Items]
Principal / Notional	651,360	593,027
Remaining Premium	33,670	25,463
Remaining Discount	(14,996)	(3,456)
Amortized Cost	670,034	615,034
Unrealized Gains	18,537	36,730
Unrealized Losses	(50)	0
Estimated Fair Value	688,521	651,764
Residential Credit Securities Mortgage Backed Securities | Alt-A
Mortgage-Backed Securities Portfolio [Line Items]
Principal / Notional	239,990	204,213
Remaining Premium	385	499
Remaining Discount	(36,910)	(34,000)
Amortized Cost	203,465	170,712
Unrealized Gains	12,730	13,976
Unrealized Losses	(131)	(802)
Estimated Fair Value	216,064	183,886
Residential Credit Securities Mortgage Backed Securities | Prime
Mortgage-Backed Securities Portfolio [Line Items]
Principal / Notional	386,193	197,756
Remaining Premium	2,111	358
Remaining Discount	(25,419)	(24,158)
Amortized Cost	362,885	173,956
Unrealized Gains	15,341	18,804
Unrealized Losses	(596)	0
Estimated Fair Value	377,630	192,760
Residential Credit Securities Mortgage Backed Securities | Subprime
Mortgage-Backed Securities Portfolio [Line Items]
Principal / Notional	449,425	554,470
Remaining Premium	1,973	2,037
Remaining Discount	(67,841)	(78,561)
Amortized Cost	383,557	477,946
Unrealized Gains	45,238	56,024
Unrealized Losses	(109)	(90)
Estimated Fair Value	428,686	533,880
Residential Credit Securities Mortgage Backed Securities | NPL/RPL
Mortgage-Backed Securities Portfolio [Line Items]
Principal / Notional	3,431	42,585
Remaining Premium	0	14
Remaining Discount	(37)	(117)
Amortized Cost	3,394	42,482
Unrealized Gains	45	506
Unrealized Losses	0	0
Estimated Fair Value	3,439	42,988
Residential Credit Securities Mortgage Backed Securities | Prime Jumbo ( 2010 Vintage)
Mortgage-Backed Securities Portfolio [Line Items]
Principal / Notional	137,953	130,025
Remaining Premium	587	627
Remaining Discount	(4,644)	(3,956)
Amortized Cost	133,896	126,696
Unrealized Gains	49	1,038
Unrealized Losses	(3,977)	(1,112)
Estimated Fair Value	129,968	126,622
Residential Credit Securities Mortgage Backed Securities | Prime Jumbo ( 2010 Vintage) Interest-Only
Mortgage-Backed Securities Portfolio [Line Items]
Principal / Notional	884,325	989,052
Remaining Premium	13,265	15,287
Remaining Discount	0	0
Amortized Cost	13,265	15,287
Unrealized Gains	4,415	1,871
Unrealized Losses	0	0
Estimated Fair Value	17,680	17,158
Residential Investments
Mortgage-Backed Securities Portfolio [Line Items]
Principal / Notional	96,585,652	95,311,922
Remaining Premium	5,936,726	6,172,638
Remaining Discount	(167,300)	(148,291)
Amortized Cost	95,221,868	93,542,502
Unrealized Gains	136,415	286,824
Unrealized Losses	(4,206,167)	(1,528,505)
Estimated Fair Value	91,152,116	92,300,821
Commercial Securities
Mortgage-Backed Securities Portfolio [Line Items]
Principal / Notional	196,407	270,288
Remaining Premium	339	680
Remaining Discount	(9,903)	(9,731)
Amortized Cost	186,843	261,237
Unrealized Gains	575	1,843
Unrealized Losses	(923)	(329)
Estimated Fair Value	$ 186,495	$ 262,751