RESIDENTIAL INVESTMENT SECURITIES - Portfolio (Details) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017
Agency Securities
Mortgage-Backed Securities Portfolio [Line Items]
Principal / Notional	$ 90,883,519	$ 92,600,794
Remaining Premium	6,002,587	6,128,353
Remaining Discount	(9,893)	(4,043)
Amortized Cost	90,339,162	91,920,389
Unrealized Gains	57,118	157,875
Unrealized Losses	(3,803,222)	(1,526,501)
Estimated Fair Value	86,593,058	90,551,763
Agency Securities | Fixed-rate pass-through
Mortgage-Backed Securities Portfolio [Line Items]
Principal / Notional	77,646,763	78,509,335
Remaining Premium	4,442,937	4,514,815
Remaining Discount	(1,556)	(1,750)
Amortized Cost	82,088,144	83,022,400
Unrealized Gains	43,095	140,115
Unrealized Losses	(3,309,253)	(1,178,673)
Estimated Fair Value	78,821,986	81,983,842
Agency Securities | Adjustable-rate pass-through
Mortgage-Backed Securities Portfolio [Line Items]
Principal / Notional	5,847,337	6,760,991
Remaining Premium	277,049	277,212
Remaining Discount	(1,100)	(1,952)
Amortized Cost	6,123,286	7,036,251
Unrealized Gains	10,478	15,776
Unrealized Losses	(156,983)	(103,121)
Estimated Fair Value	5,976,781	6,948,906
Agency Securities | Interest-only
Mortgage-Backed Securities Portfolio [Line Items]
Principal / Notional	6,537,051	6,804,715
Remaining Premium	1,273,388	1,326,761
Remaining Discount	0	0
Amortized Cost	1,273,388	1,326,761
Unrealized Gains	2,447	1,863
Unrealized Losses	(315,482)	(242,862)
Estimated Fair Value	960,353	1,085,762
Agency Securities | Multifamily
Mortgage-Backed Securities Portfolio [Line Items]
Principal / Notional	816,976	490,753
Remaining Premium	4,872	5,038
Remaining Discount	(7,237)	(341)
Amortized Cost	814,611	495,450
Unrealized Gains	1,065	84
Unrealized Losses	(21,285)	(1,845)
Estimated Fair Value	794,391	493,689
Agency Securities | Reverse mortgages
Mortgage-Backed Securities Portfolio [Line Items]
Principal / Notional	35,392	35,000
Remaining Premium	4,341	4,527
Remaining Discount	0	0
Amortized Cost	39,733	39,527
Unrealized Gains	33	37
Unrealized Losses	(219)	0
Estimated Fair Value	39,547	39,564
Residential Credit Securities Mortgage Backed Securities
Mortgage-Backed Securities Portfolio [Line Items]
Principal / Notional	2,484,718	2,711,128
Remaining Premium	36,694	44,285
Remaining Discount	(132,633)	(144,248)
Amortized Cost	1,478,714	1,622,113
Unrealized Gains	95,850	128,949
Unrealized Losses	(3,983)	(2,004)
Estimated Fair Value	1,570,581	1,749,058
Residential Credit Securities Mortgage Backed Securities | CRT
Mortgage-Backed Securities Portfolio [Line Items]
Principal / Notional	528,869	593,027
Remaining Premium	18,234	25,463
Remaining Discount	(1,079)	(3,456)
Amortized Cost	546,024	615,034
Unrealized Gains	18,429	36,730
Unrealized Losses	(657)	0
Estimated Fair Value	563,796	651,764
Residential Credit Securities Mortgage Backed Securities | Alt-A
Mortgage-Backed Securities Portfolio [Line Items]
Principal / Notional	191,939	204,213
Remaining Premium	378	499
Remaining Discount	(32,845)	(34,000)
Amortized Cost	159,472	170,712
Unrealized Gains	11,539	13,976
Unrealized Losses	(89)	(802)
Estimated Fair Value	170,922	183,886
Residential Credit Securities Mortgage Backed Securities | Prime
Mortgage-Backed Securities Portfolio [Line Items]
Principal / Notional	270,226	197,756
Remaining Premium	1,926	358
Remaining Discount	(23,127)	(24,158)
Amortized Cost	249,025	173,956
Unrealized Gains	15,588	18,804
Unrealized Losses	(122)	0
Estimated Fair Value	264,491	192,760
Residential Credit Securities Mortgage Backed Securities | Subprime
Mortgage-Backed Securities Portfolio [Line Items]
Principal / Notional	449,644	554,470
Remaining Premium	1,827	2,037
Remaining Discount	(71,425)	(78,561)
Amortized Cost	380,046	477,946
Unrealized Gains	46,511	56,024
Unrealized Losses	(108)	(90)
Estimated Fair Value	426,449	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	(44)	(117)
Amortized Cost	3,387	42,482
Unrealized Gains	60	506
Unrealized Losses	0	0
Estimated Fair Value	3,447	42,988
Residential Credit Securities Mortgage Backed Securities | Prime Jumbo ( 2010 Vintage)
Mortgage-Backed Securities Portfolio [Line Items]
Principal / Notional	130,544	130,025
Remaining Premium	598	627
Remaining Discount	(4,113)	(3,956)
Amortized Cost	127,029	126,696
Unrealized Gains	107	1,038
Unrealized Losses	(2,995)	(1,112)
Estimated Fair Value	124,141	126,622
Residential Credit Securities Mortgage Backed Securities | Prime Jumbo ( 2010 Vintage) Interest-Only
Mortgage-Backed Securities Portfolio [Line Items]
Principal / Notional	910,065	989,052
Remaining Premium	13,731	15,287
Remaining Discount	0	0
Amortized Cost	13,731	15,287
Unrealized Gains	3,616	1,871
Unrealized Losses	(12)	0
Estimated Fair Value	17,335	17,158
Residential Investments
Mortgage-Backed Securities Portfolio [Line Items]
Principal / Notional	93,368,237	95,311,922
Remaining Premium	6,039,281	6,172,638
Remaining Discount	(142,526)	(148,291)
Amortized Cost	91,817,876	93,542,502
Unrealized Gains	152,968	286,824
Unrealized Losses	(3,807,205)	(1,528,505)
Estimated Fair Value	$ 88,163,639	$ 92,300,821