VARIABLE INTEREST ENTITIES - Narrative (Details) - USD ($)		1 Months Ended	3 Months Ended
		Aug. 31, 2017	Mar. 31, 2021	Mar. 31, 2020	Dec. 31, 2020		Nov. 30, 2019	Jan. 31, 2019
Variable Interest Entity [Line Items]
Principal receivable	[1]		$ 2,603,343,000		$ 3,083,821,000	[2]
Securitized debt of consolidated VIEs			3,044,725,000		5,652,982,000	[2]
Mortgage-backed securities	[3]		71,849,437,000		75,652,396,000	[2]
Costs incurred in connection with securitization			49,695,000	$ 77,629,000
Other secured financing			922,605,000		917,876,000	[2]
Participations issued			180,527,000		39,198,000	[2]
MSR Silo
Variable Interest Entity [Line Items]
Variable interest entity, ownership percentage		100.00%
Consolidated VIEs
Variable Interest Entity [Line Items]
Exposure to obligations of VIEs			2,500,000,000
Consolidated VIEs | OBX Trust
Variable Interest Entity [Line Items]
Contractual principal amount of debt held by third parties			2,400,000,000
Securitized debt of consolidated VIEs			2,500,000,000
Costs incurred in connection with securitization			700,000	3,700,000
Consolidated VIEs | Consolidation, Eliminations | OBX Trust
Variable Interest Entity [Line Items]
Mortgage-backed securities			660,400,000
Consolidated VIEs | Residential Trusts
Variable Interest Entity [Line Items]
Contractual principal amount of debt held by third parties			20,000,000.0		23,000,000.0
Other secured financing					0
Consolidated VIEs | Borrower
Variable Interest Entity [Line Items]
Credit facility, maximum borrowing capacity			625,000,000.0
Other secured financing			461,400,000
Consolidated VIEs | Borrower | Corporate Loans
Variable Interest Entity [Line Items]
Transferred loans pledged as collateral for credit facility			766,100,000
Consolidated VIEs | Borrower | July 2017 Credit Facility
Variable Interest Entity [Line Items]
Credit facility, maximum borrowing capacity			320,000,000.0
Consolidated VIEs | Borrower | July 2017 Credit Facility | Corporate Loans
Variable Interest Entity [Line Items]
Transferred loans pledged as collateral for credit facility			397,500,000
Other secured financing			206,800,000
Consolidated VIEs | Borrower | January 2019 Credit Facility
Variable Interest Entity [Line Items]
Credit facility, maximum borrowing capacity			400,000,000.0					$ 300,000,000.0
Consolidated VIEs | Borrower | January 2019 Credit Facility | Corporate Loans
Variable Interest Entity [Line Items]
Transferred loans pledged as collateral for credit facility			400,500,000
Other secured financing			254,400,000
Consolidated VIEs | Borrower | Consolidation, Eliminations
Variable Interest Entity [Line Items]
Principal receivable			461,400,000
VIE, Not Primary Beneficiary
Variable Interest Entity [Line Items]
Loans transferred for cash			15,100,000
VIE, Not Primary Beneficiary | Residential mortgage loans
Variable Interest Entity [Line Items]
Participations issued			141,300,000		$ 39,200,000
Multifamily | Consolidated VIEs
Variable Interest Entity [Line Items]
Cut-off date principal balance				500,000,000			$ 1,000,000,000.0
Costs incurred			$ 1,100,000
Multifamily | Retained Interest | Consolidated VIEs
Variable Interest Entity [Line Items]
Retained interest notional balance				$ 500,000,000			1,000,000,000.0
Multifamily | Senior Loans | Consolidated VIEs
Variable Interest Entity [Line Items]
Principal receivable							$ 28,500,000

[1] Includes $46.0 million and $47.0 million of residential mortgage loans held for sale at March 31, 2021 and December 31, 2020, respectively.

[2] Derived from the audited consolidated financial statements at December 31, 2020.

[3] Excludes $78.3 million and $81.5 million at March 31, 2021 and December 31, 2020, respectively, of Agency mortgage-backed securities, $357.6 million and $576.6 million at March 31, 2021 and December 31, 2020, respectively, of non-Agency mortgage-backed securities and $0 and $391.0 million at March 31, 2021 and December 31, 2020, respectively, of commercial mortgage-backed securities in consolidated VIEs pledged as collateral and eliminated from the Company’s Consolidated Statements of Financial Condition.