SECURED FINANCING - Repurchase Agreements - Remaining Maturities, Collateral Types and Weighted Average Rate (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2019
Repurchase Agreements:
Repurchase agreements	$ 81,115,874	$ 101,740,728
Weighted average rate	2.97%	2.03%
Percentage of total repurchase agreements with remaining maturity of 1 year	1.00%
CRTs
Repurchase Agreements:
Repurchase agreements	$ 373,536	$ 268,738
Non-Agency Mortgage-Backed Securities
Repurchase Agreements:
Repurchase agreements	721,304	1,024,528
Commercial Loans
Repurchase Agreements:
Repurchase agreements	693,939	303,078
Agency Mortgage-Backed Securities
Repurchase Agreements:
Repurchase agreements	78,482,373	99,591,465
Commercial Mortgage-Backed Securities
Repurchase Agreements:
Repurchase agreements	204,257	552,919
U.S. Treasury Securities
Repurchase Agreements:
Repurchase agreements	640,465	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 | 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	0
2 to 29 days
Repurchase Agreements:
Repurchase agreements	$ 32,012,641	$ 37,382,531
Weighted average rate	3.50%	2.15%
2 to 29 days | CRTs
Repurchase Agreements:
Repurchase agreements	$ 284,906	$ 237,897
2 to 29 days | Non-Agency Mortgage-Backed Securities
Repurchase Agreements:
Repurchase agreements	353,429	698,091
2 to 29 days | Commercial Loans
Repurchase Agreements:
Repurchase agreements	0	0
2 to 29 days | Agency Mortgage-Backed Securities
Repurchase Agreements:
Repurchase agreements	30,661,001	36,030,104
2 to 29 days | Commercial Mortgage-Backed Securities
Repurchase Agreements:
Repurchase agreements	72,840	416,439
2 to 29 days | U.S. Treasury Securities
Repurchase Agreements:
Repurchase agreements	640,465	0
30 to 59 days
Repurchase Agreements:
Repurchase agreements	$ 8,164,165	$ 15,300,157
Weighted average rate	2.33%	2.00%
30 to 59 days | CRTs
Repurchase Agreements:
Repurchase agreements	$ 0	$ 0
30 to 59 days | Non-Agency Mortgage-Backed Securities
Repurchase Agreements:
Repurchase agreements	0	115,805
30 to 59 days | Commercial Loans
Repurchase Agreements:
Repurchase agreements	0	0
30 to 59 days | Agency Mortgage-Backed Securities
Repurchase Agreements:
Repurchase agreements	8,164,165	15,079,989
30 to 59 days | Commercial Mortgage-Backed Securities
Repurchase Agreements:
Repurchase agreements	0	104,363
30 to 59 days | U.S. Treasury Securities
Repurchase Agreements:
Repurchase agreements	0	0
60 to 89 days
Repurchase Agreements:
Repurchase agreements	$ 18,689,772	$ 22,117,735
Weighted average rate	2.62%	1.97%
60 to 89 days | CRTs
Repurchase Agreements:
Repurchase agreements	$ 88,630	$ 30,841
60 to 89 days | Non-Agency Mortgage-Backed Securities
Repurchase Agreements:
Repurchase agreements	251,441	151,920
60 to 89 days | Commercial Loans
Repurchase Agreements:
Repurchase agreements	0	0
60 to 89 days | Agency Mortgage-Backed Securities
Repurchase Agreements:
Repurchase agreements	18,326,399	21,931,335
60 to 89 days | Commercial Mortgage-Backed Securities
Repurchase Agreements:
Repurchase agreements	23,302	3,639
60 to 89 days | U.S. Treasury Securities
Repurchase Agreements:
Repurchase agreements	0	0
90 to 119 days
Repurchase Agreements:
Repurchase agreements	$ 10,067,183	$ 9,992,914
Weighted average rate	2.54%	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 | Commercial Loans
Repurchase Agreements:
Repurchase agreements	0	0
90 to 119 days | Agency Mortgage-Backed Securities
Repurchase Agreements:
Repurchase agreements	10,067,183	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	0
Over 120 days
Repurchase Agreements:
Repurchase agreements	$ 12,182,113	$ 16,947,391
Weighted average rate	2.92%	1.90%
Over 120 days | CRTs
Repurchase Agreements:
Repurchase agreements	$ 0	$ 0
Over 120 days | Non-Agency Mortgage-Backed Securities
Repurchase Agreements:
Repurchase agreements	116,434	58,712
Over 120 days | Commercial Loans
Repurchase Agreements:
Repurchase agreements	693,939	303,078
Over 120 days | Agency Mortgage-Backed Securities
Repurchase Agreements:
Repurchase agreements	11,263,625	16,557,123
Over 120 days | Commercial Mortgage-Backed Securities
Repurchase Agreements:
Repurchase agreements	108,115	28,478
Over 120 days | U.S. Treasury Securities
Repurchase Agreements:
Repurchase agreements	$ 0	$ 0