SECURED FINANCING - Repurchase Agreements - Remaining Maturities, Collateral Types and Weighted Average Rate (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Repurchase Agreements:
Repurchase agreements	$ 64,825,239	$ 101,740,728
Weighted average rate	0.32%	2.03%
Percentage of total repurchase agreements with remaining maturity of 1 year (less than)	1.00%	0.00%
CRTs
Repurchase Agreements:
Repurchase agreements	$ 245,686	$ 268,738
Non-Agency Mortgage-Backed Securities
Repurchase Agreements:
Repurchase agreements	1,050,439	1,024,528
Residential mortgage loans
Repurchase Agreements:
Repurchase agreements	184,723
Commercial Loans
Repurchase Agreements:
Repurchase agreements	271,801	303,078
Agency Mortgage-Backed Securities
Repurchase Agreements:
Repurchase agreements	62,744,910	99,591,465
Commercial Mortgage-Backed Securities
Repurchase Agreements:
Repurchase agreements	327,680	552,919
U.S. Treasury Securities
Repurchase Agreements:
Repurchase agreements		0
1 day
Repurchase Agreements:
Repurchase agreements	$ 0	$ 0
Weighted average rate	0.00%	0.00%
1 day | CRTs
Repurchase Agreements:
Repurchase agreements	$ 0	$ 0
1 day | Non-Agency Mortgage-Backed Securities
Repurchase Agreements:
Repurchase agreements	0	0
1 day | Residential mortgage loans
Repurchase Agreements:
Repurchase agreements	0
1 day | Commercial Loans
Repurchase Agreements:
Repurchase agreements	0	0
1 day | Agency Mortgage-Backed Securities
Repurchase Agreements:
Repurchase agreements	0	0
1 day | Commercial Mortgage-Backed Securities
Repurchase Agreements:
Repurchase agreements	0	0
1 day | U.S. Treasury Securities
Repurchase Agreements:
Repurchase agreements		0
2 to 29 days
Repurchase Agreements:
Repurchase agreements	$ 30,841,838	$ 37,382,531
Weighted average rate	0.29%	2.15%
2 to 29 days | CRTs
Repurchase Agreements:
Repurchase agreements	$ 129,993	$ 237,897
2 to 29 days | Non-Agency Mortgage-Backed Securities
Repurchase Agreements:
Repurchase agreements	354,904	698,091
2 to 29 days | Residential mortgage loans
Repurchase Agreements:
Repurchase agreements	76,799
2 to 29 days | Commercial Loans
Repurchase Agreements:
Repurchase agreements	0	0
2 to 29 days | Agency Mortgage-Backed Securities
Repurchase Agreements:
Repurchase agreements	30,151,875	36,030,104
2 to 29 days | Commercial Mortgage-Backed Securities
Repurchase Agreements:
Repurchase agreements	128,267	416,439
2 to 29 days | U.S. Treasury Securities
Repurchase Agreements:
Repurchase agreements		0
30 to 59 days
Repurchase Agreements:
Repurchase agreements	$ 10,567,655	$ 15,300,157
Weighted average rate	0.42%	2.00%
30 to 59 days | CRTs
Repurchase Agreements:
Repurchase agreements	$ 16,073	$ 0
30 to 59 days | Non-Agency Mortgage-Backed Securities
Repurchase Agreements:
Repurchase agreements	161,274	115,805
30 to 59 days | Residential mortgage loans
Repurchase Agreements:
Repurchase agreements	0
30 to 59 days | Commercial Loans
Repurchase Agreements:
Repurchase agreements	0	0
30 to 59 days | Agency Mortgage-Backed Securities
Repurchase Agreements:
Repurchase agreements	10,247,972	15,079,989
30 to 59 days | Commercial Mortgage-Backed Securities
Repurchase Agreements:
Repurchase agreements	142,336	104,363
30 to 59 days | U.S. Treasury Securities
Repurchase Agreements:
Repurchase agreements		0
60 to 89 days
Repurchase Agreements:
Repurchase agreements	$ 8,568,837	$ 22,117,735
Weighted average rate	0.30%	1.97%
60 to 89 days | CRTs
Repurchase Agreements:
Repurchase agreements	$ 99,620	$ 30,841
60 to 89 days | Non-Agency Mortgage-Backed Securities
Repurchase Agreements:
Repurchase agreements	259,401	151,920
60 to 89 days | Residential mortgage loans
Repurchase Agreements:
Repurchase agreements	0
60 to 89 days | Commercial Loans
Repurchase Agreements:
Repurchase agreements	0	0
60 to 89 days | Agency Mortgage-Backed Securities
Repurchase Agreements:
Repurchase agreements	8,181,410	21,931,335
60 to 89 days | Commercial Mortgage-Backed Securities
Repurchase Agreements:
Repurchase agreements	28,406	3,639
60 to 89 days | U.S. Treasury Securities
Repurchase Agreements:
Repurchase agreements		0
90 to 119 days
Repurchase Agreements:
Repurchase agreements	$ 2,154,733	$ 9,992,914
Weighted average rate	0.23%	1.97%
90 to 119 days | CRTs
Repurchase Agreements:
Repurchase agreements	$ 0	$ 0
90 to 119 days | Non-Agency Mortgage-Backed Securities
Repurchase Agreements:
Repurchase agreements	0	0
90 to 119 days | Residential mortgage loans
Repurchase Agreements:
Repurchase agreements	0
90 to 119 days | Commercial Loans
Repurchase Agreements:
Repurchase agreements	0	0
90 to 119 days | Agency Mortgage-Backed Securities
Repurchase Agreements:
Repurchase agreements	2,154,733	9,992,914
90 to 119 days | Commercial Mortgage-Backed Securities
Repurchase Agreements:
Repurchase agreements	0	0
90 to 119 days | U.S. Treasury Securities
Repurchase Agreements:
Repurchase agreements		0
Over 119 days
Repurchase Agreements:
Repurchase agreements	$ 12,692,176	$ 16,947,391
Weighted average rate	0.36%	1.90%
Over 119 days | CRTs
Repurchase Agreements:
Repurchase agreements	$ 0	$ 0
Over 119 days | Non-Agency Mortgage-Backed Securities
Repurchase Agreements:
Repurchase agreements	274,860	58,712
Over 119 days | Residential mortgage loans
Repurchase Agreements:
Repurchase agreements	107,924
Over 119 days | Commercial Loans
Repurchase Agreements:
Repurchase agreements	271,801	303,078
Over 119 days | Agency Mortgage-Backed Securities
Repurchase Agreements:
Repurchase agreements	12,008,920	16,557,123
Over 119 days | Commercial Mortgage-Backed Securities
Repurchase Agreements:
Repurchase agreements	$ 28,671	28,478
Over 119 days | U.S. Treasury Securities
Repurchase Agreements:
Repurchase agreements		$ 0