MORTGAGE SERVICING RIGHTS AND MSR FINANCING RECEIVABLES - Summary of the Geographic Distribution of the Underlying Residential Mortgage Loans (Details) - MSRs - Mortgage Loans	Mar. 31, 2021	Dec. 31, 2020
Schedule of MSRs [Line Items]
Percentage of Total Outstanding Unpaid Principal Amount	100.00%	100.00%
California
Schedule of MSRs [Line Items]
Percentage of Total Outstanding Unpaid Principal Amount	20.60%	21.20%
Florida
Schedule of MSRs [Line Items]
Percentage of Total Outstanding Unpaid Principal Amount	7.60%	7.40%
New York
Schedule of MSRs [Line Items]
Percentage of Total Outstanding Unpaid Principal Amount	7.10%	7.00%
Texas
Schedule of MSRs [Line Items]
Percentage of Total Outstanding Unpaid Principal Amount	5.80%	5.60%
New Jersey
Schedule of MSRs [Line Items]
Percentage of Total Outstanding Unpaid Principal Amount	4.80%	4.80%
Illinois
Schedule of MSRs [Line Items]
Percentage of Total Outstanding Unpaid Principal Amount	3.60%	3.60%
Massachusetts
Schedule of MSRs [Line Items]
Percentage of Total Outstanding Unpaid Principal Amount	3.40%	3.40%
Georgia
Schedule of MSRs [Line Items]
Percentage of Total Outstanding Unpaid Principal Amount	3.40%	3.30%
Pennsylvania
Schedule of MSRs [Line Items]
Percentage of Total Outstanding Unpaid Principal Amount	3.20%	3.10%
MARYLAND
Schedule of MSRs [Line Items]
Percentage of Total Outstanding Unpaid Principal Amount	3.10%	3.10%
Other U.S.
Schedule of MSRs [Line Items]
Percentage of Total Outstanding Unpaid Principal Amount	37.40%	37.50%