RESIDENTIAL INVESTMENT SECURITIES - Portfolio (Details) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Agency Securities
Mortgage-Backed Securities Portfolio [Line Items]
Principal / Notional	$ 92,353,733	$ 92,600,794
Remaining Premium	6,161,411	6,128,353
Remaining Discount	(4,126)	(4,043)
Amortized Cost	91,854,888	91,920,389
Unrealized Gains	77,707	157,875
Unrealized Losses	(3,353,498)	(1,526,501)
Estimated Fair Value	88,579,097	90,551,763
Agency Securities | Fixed-rate pass-through
Mortgage-Backed Securities Portfolio [Line Items]
Principal / Notional	78,830,362	78,509,335
Remaining Premium	4,563,295	4,514,815
Remaining Discount	(1,624)	(1,750)
Amortized Cost	83,392,033	83,022,400
Unrealized Gains	62,602	140,115
Unrealized Losses	(2,906,925)	(1,178,673)
Estimated Fair Value	80,547,710	81,983,842
Agency Securities | Adjustable-rate pass-through
Mortgage-Backed Securities Portfolio [Line Items]
Principal / Notional	6,321,714	6,760,991
Remaining Premium	292,681	277,212
Remaining Discount	(1,313)	(1,952)
Amortized Cost	6,613,082	7,036,251
Unrealized Gains	12,301	15,776
Unrealized Losses	(152,751)	(103,121)
Estimated Fair Value	6,472,632	6,948,906
Agency Securities | Interest-only
Mortgage-Backed Securities Portfolio [Line Items]
Principal / Notional	6,656,130	6,804,715
Remaining Premium	1,296,294	1,326,761
Remaining Discount	0	0
Amortized Cost	1,296,294	1,326,761
Unrealized Gains	2,593	1,863
Unrealized Losses	(278,896)	(242,862)
Estimated Fair Value	1,019,991	1,085,762
Agency Securities | Multifamily
Mortgage-Backed Securities Portfolio [Line Items]
Principal / Notional	510,063	490,753
Remaining Premium	4,730	5,038
Remaining Discount	(1,189)	(341)
Amortized Cost	513,604	495,450
Unrealized Gains	211	84
Unrealized Losses	(14,757)	(1,845)
Estimated Fair Value	499,058	493,689
Agency Securities | Reverse mortgages
Mortgage-Backed Securities Portfolio [Line Items]
Principal / Notional	35,464	35,000
Remaining Premium	4,411	4,527
Remaining Discount	0	0
Amortized Cost	39,875	39,527
Unrealized Gains	0	37
Unrealized Losses	(169)	0
Estimated Fair Value	39,706	39,564
Residential Credit Securities Mortgage Backed Securities
Mortgage-Backed Securities Portfolio [Line Items]
Principal / Notional	2,623,300	2,711,128
Remaining Premium	45,284	44,285
Remaining Discount	(138,960)	(144,248)
Amortized Cost	1,589,997	1,622,113
Unrealized Gains	108,341	128,949
Unrealized Losses	(3,053)	(2,004)
Estimated Fair Value	1,695,285	1,749,058
Residential Credit Securities Mortgage Backed Securities | CRT
Mortgage-Backed Securities Portfolio [Line Items]
Principal / Notional	579,077	593,027
Remaining Premium	27,299	25,463
Remaining Discount	(1,215)	(3,456)
Amortized Cost	605,161	615,034
Unrealized Gains	24,195	36,730
Unrealized Losses	(414)	0
Estimated Fair Value	628,942	651,764
Residential Credit Securities Mortgage Backed Securities | Alt-A
Mortgage-Backed Securities Portfolio [Line Items]
Principal / Notional	200,819	204,213
Remaining Premium	498	499
Remaining Discount	(33,609)	(34,000)
Amortized Cost	167,708	170,712
Unrealized Gains	12,542	13,976
Unrealized Losses	(211)	(802)
Estimated Fair Value	180,039	183,886
Residential Credit Securities Mortgage Backed Securities | Prime
Mortgage-Backed Securities Portfolio [Line Items]
Principal / Notional	222,366	197,756
Remaining Premium	345	358
Remaining Discount	(24,870)	(24,158)
Amortized Cost	197,841	173,956
Unrealized Gains	17,041	18,804
Unrealized Losses	(54)	0
Estimated Fair Value	214,828	192,760
Residential Credit Securities Mortgage Backed Securities | Subprime
Mortgage-Backed Securities Portfolio [Line Items]
Principal / Notional	516,223	554,470
Remaining Premium	1,998	2,037
Remaining Discount	(75,243)	(78,561)
Amortized Cost	442,978	477,946
Unrealized Gains	50,320	56,024
Unrealized Losses	(792)	(90)
Estimated Fair Value	492,506	533,880
Residential Credit Securities Mortgage Backed Securities | NPL/RPL
Mortgage-Backed Securities Portfolio [Line Items]
Principal / Notional	39,286	42,585
Remaining Premium	14	14
Remaining Discount	(94)	(117)
Amortized Cost	39,206	42,482
Unrealized Gains	366	506
Unrealized Losses	0	0
Estimated Fair Value	39,572	42,988
Residential Credit Securities Mortgage Backed Securities | Prime Jumbo ( 2010 Vintage)
Mortgage-Backed Securities Portfolio [Line Items]
Principal / Notional	125,902	130,025
Remaining Premium	614	627
Remaining Discount	(3,929)	(3,956)
Amortized Cost	122,587	126,696
Unrealized Gains	195	1,038
Unrealized Losses	(1,582)	(1,112)
Estimated Fair Value	121,200	126,622
Residential Credit Securities Mortgage Backed Securities | Prime Jumbo ( 2010 Vintage) Interest-Only
Mortgage-Backed Securities Portfolio [Line Items]
Principal / Notional	939,627	989,052
Remaining Premium	14,516	15,287
Remaining Discount	0	0
Amortized Cost	14,516	15,287
Unrealized Gains	3,682	1,871
Unrealized Losses	0	0
Estimated Fair Value	18,198	17,158
Residential Investments
Mortgage-Backed Securities Portfolio [Line Items]
Principal / Notional	94,977,033	95,311,922
Remaining Premium	6,206,695	6,172,638
Remaining Discount	(143,086)	(148,291)
Amortized Cost	93,444,885	93,542,502
Unrealized Gains	186,048	286,824
Unrealized Losses	(3,356,551)	(1,528,505)
Estimated Fair Value	$ 90,274,382	$ 92,300,821