RESIDENTIAL INVESTMENT SECURITIES - Portfolio (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Agency Securities
Mortgage-Backed Securities Portfolio [Line Items]
Principal / Notional	$ 92,600,794	$ 79,546,231
Remaining Premium	6,128,353	5,460,813
Remaining Discount	(4,043)	(6,037)
Amortized Cost	91,920,389	76,867,202
Unrealized Gains	157,875	279,905
Unrealized Losses	(1,526,501)	(1,557,234)
Estimated Fair Value	90,551,763	75,589,873
Agency Securities | Fixed-rate pass-through
Mortgage-Backed Securities Portfolio [Line Items]
Principal / Notional	78,509,335	60,759,317
Remaining Premium	4,514,815	3,633,354
Remaining Discount	(1,750)	(1,956)
Amortized Cost	83,022,400	64,390,715
Unrealized Gains	140,115	228,430
Unrealized Losses	(1,178,673)	(1,307,771)
Estimated Fair Value	81,983,842	63,311,373
Agency Securities | Adjustable-rate pass-through
Mortgage-Backed Securities Portfolio [Line Items]
Principal / Notional	6,760,991	10,653,109
Remaining Premium	277,212	391,267
Remaining Discount	(1,952)	(4,081)
Amortized Cost	7,036,251	11,040,295
Unrealized Gains	15,776	47,250
Unrealized Losses	(103,121)	(53,795)
Estimated Fair Value	6,948,906	11,033,751
Agency Securities | Interest-only
Mortgage-Backed Securities Portfolio [Line Items]
Principal / Notional	6,804,715	8,133,805
Remaining Premium	1,326,761	1,436,192
Remaining Discount	0	0
Amortized Cost	1,326,761	1,436,192
Unrealized Gains	1,863	4,225
Unrealized Losses	(242,862)	(195,668)
Estimated Fair Value	1,085,762	1,244,749
Agency Securities | Multifamily
Mortgage-Backed Securities Portfolio [Line Items]
Principal / Notional	490,753
Remaining Premium	5,038
Remaining Discount	(341)
Amortized Cost	495,450
Unrealized Gains	84
Unrealized Losses	(1,845)
Estimated Fair Value	493,689
Agency Securities | Reverse mortgages
Mortgage-Backed Securities Portfolio [Line Items]
Principal / Notional	35,000
Remaining Premium	4,527
Remaining Discount	0
Amortized Cost	39,527
Unrealized Gains	37
Unrealized Losses	0
Estimated Fair Value	39,564
Residential Credit Securities Mortgage Backed Securities
Mortgage-Backed Securities Portfolio [Line Items]
Principal / Notional	2,711,128	3,072,383
Remaining Premium	44,285	29,499
Remaining Discount	(144,248)	(165,899)
Amortized Cost	1,622,113	2,072,613
Unrealized Gains	128,949	58,666
Unrealized Losses	(2,004)	(5,250)
Estimated Fair Value	1,749,058	2,126,029
Residential Credit Securities Mortgage Backed Securities | CRT
Mortgage-Backed Securities Portfolio [Line Items]
Principal / Notional	593,027	690,491
Remaining Premium	25,463	11,113
Remaining Discount	(3,456)	(10,907)
Amortized Cost	615,034	690,697
Unrealized Gains	36,730	34,046
Unrealized Losses	0	(21)
Estimated Fair Value	651,764	724,722
Residential Credit Securities Mortgage Backed Securities | Alt-A
Mortgage-Backed Securities Portfolio [Line Items]
Principal / Notional	204,213	173,108
Remaining Premium	499	1,068
Remaining Discount	(34,000)	(23,039)
Amortized Cost	170,712	151,137
Unrealized Gains	13,976	3,721
Unrealized Losses	(802)	(685)
Estimated Fair Value	183,886	154,173
Residential Credit Securities Mortgage Backed Securities | Prime
Mortgage-Backed Securities Portfolio [Line Items]
Principal / Notional	197,756	248,176
Remaining Premium	358	287
Remaining Discount	(24,158)	(35,068)
Amortized Cost	173,956	213,395
Unrealized Gains	18,804	7,050
Unrealized Losses	0	(253)
Estimated Fair Value	192,760	220,192
Residential Credit Securities Mortgage Backed Securities | Subprime
Mortgage-Backed Securities Portfolio [Line Items]
Principal / Notional	554,470	697,983
Remaining Premium	2,037	380
Remaining Discount	(78,561)	(96,331)
Amortized Cost	477,946	602,032
Unrealized Gains	56,024	12,578
Unrealized Losses	(90)	(1,061)
Estimated Fair Value	533,880	613,549
Residential Credit Securities Mortgage Backed Securities | NPL/RPL
Mortgage-Backed Securities Portfolio [Line Items]
Principal / Notional	42,585	269,802
Remaining Premium	14	670
Remaining Discount	(117)	(209)
Amortized Cost	42,482	270,263
Unrealized Gains	506	1,004
Unrealized Losses	0	(429)
Estimated Fair Value	42,988	270,838
Residential Credit Securities Mortgage Backed Securities | Prime Jumbo (2010 Vintage)
Mortgage-Backed Securities Portfolio [Line Items]
Principal / Notional	130,025	129,453
Remaining Premium	627	852
Remaining Discount	(3,956)	(345)
Amortized Cost	126,696	129,960
Unrealized Gains	1,038	267
Unrealized Losses	(1,112)	(308)
Estimated Fair Value	126,622	129,919
Residential Credit Securities Mortgage Backed Securities | Prime Jumbo (2010 Vintage) Interest-Only
Mortgage-Backed Securities Portfolio [Line Items]
Principal / Notional	989,052	863,370
Remaining Premium	15,287	15,129
Remaining Discount	0	0
Amortized Cost	15,287	15,129
Unrealized Gains	1,871	0
Unrealized Losses	0	(2,493)
Estimated Fair Value	17,158	12,636
Residential Investments
Mortgage-Backed Securities Portfolio [Line Items]
Principal / Notional	95,311,922	82,618,614
Remaining Premium	6,172,638	5,490,312
Remaining Discount	(148,291)	(171,936)
Amortized Cost	93,542,502	78,939,815
Unrealized Gains	286,824	338,571
Unrealized Losses	(1,528,505)	(1,562,484)
Estimated Fair Value	$ 92,300,821	$ 77,715,902