VARIABLE INTEREST ENTITIES - Narrative (Details) - USD ($)		1 Months Ended	3 Months Ended		9 Months Ended
		Aug. 31, 2017	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020	Dec. 31, 2020		Mar. 31, 2020	Nov. 30, 2019
Variable Interest Entity [Line Items]
Principal receivable	[1]		$ 3,580,521,000		$ 3,580,521,000		$ 3,083,821,000	[2]
Securitized debt of consolidated VIEs			3,935,410,000		3,935,410,000		5,652,982,000	[2]
Mortgage-backed securities	[3]		65,622,352,000		65,622,352,000		75,652,396,000	[2]
Costs incurred in connection with securitization			43,882,000	$ 44,587,000	145,313,000	$ 179,222,000
Other secured financing			729,555,000		729,555,000		917,876,000	[2]
Participations issued			641,006,000		641,006,000		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			2,200,000,000		2,200,000,000
Consolidated VIEs | Residential mortgage loans
Variable Interest Entity [Line Items]
Securitized debt of consolidated VIEs			10,700,000		10,700,000
Consolidated VIEs | OBX Trust
Variable Interest Entity [Line Items]
Contractual principal amount of debt held by third parties			3,300,000,000		3,300,000,000
Securitized debt of consolidated VIEs			3,400,000,000		3,400,000,000
Costs incurred in connection with securitization			2,200,000	$ 2,800,000	4,000,000	7,100,000
Consolidated VIEs | Consolidation, Eliminations | Residential mortgage loans
Variable Interest Entity [Line Items]
Mortgage-backed securities			12,100,000		12,100,000
Consolidated VIEs | Consolidation, Eliminations | OBX Trust
Variable Interest Entity [Line Items]
Mortgage-backed securities			678,400,000		678,400,000
Consolidated VIEs | Residential Trusts
Variable Interest Entity [Line Items]
Contractual principal amount of debt held by third parties			10,700,000		10,700,000		23,000,000
Other secured financing							0
Consolidated VIEs | Borrower | June 2016 Credit Facility
Variable Interest Entity [Line Items]
Credit facility, maximum borrowing capacity			675,000,000		675,000,000
Other secured financing			359,000,000		359,000,000
Consolidated VIEs | Borrower | June 2016 Credit Facility | Corporate Loans
Variable Interest Entity [Line Items]
Transferred loans pledged as collateral for credit facility			595,400,000		595,400,000
Consolidated VIEs | Borrower | July 2017 Credit Facility
Variable Interest Entity [Line Items]
Credit facility, maximum borrowing capacity			400,000,000		400,000,000
Consolidated VIEs | Borrower | July 2017 Credit Facility | Corporate Loans
Variable Interest Entity [Line Items]
Transferred loans pledged as collateral for credit facility			371,200,000		371,200,000
Other secured financing			199,200,000		199,200,000
Consolidated VIEs | Borrower | January 2019 Credit Facility
Variable Interest Entity [Line Items]
Credit facility, maximum borrowing capacity			400,000,000		400,000,000
Consolidated VIEs | Borrower | January 2019 Credit Facility | Corporate Loans
Variable Interest Entity [Line Items]
Transferred loans pledged as collateral for credit facility			301,700,000		301,700,000
Other secured financing			171,400,000		171,400,000
Consolidated VIEs | Borrower | Consolidation, Eliminations | June 2016 Credit Facility
Variable Interest Entity [Line Items]
Principal receivable			359,000,000		359,000,000
VIE, Not Primary Beneficiary
Variable Interest Entity [Line Items]
Loans transferred for cash			209,900,000		284,900,000
VIE, Not Primary Beneficiary | Residential mortgage loans
Variable Interest Entity [Line Items]
Participations issued			$ 641,000,000		$ 641,000,000		$ 39,200,000
Multifamily | Consolidated VIEs
Variable Interest Entity [Line Items]
Cut-off date principal balance									$ 500,000,000	$ 1,000,000,000
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
Multifamily | Senior Loans | Consolidated VIEs
Variable Interest Entity [Line Items]
Principal receivable										$ 28,500,000

[1] (4)Includes $3.7 million and $47.0 million of residential mortgage loans held for sale at September 30, 2021 and December 31, 2020, respectively, and $2.1 million and $0 of corporate loans held for sale at September 30, 2021 and December 31, 2020, respectively.

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

[3] (3)Excludes $46.4 million and $81.5 million at September 30, 2021 and December 31, 2020, respectively, of Agency mortgage-backed securities, $245.8 million and $576.6 million at September 30, 2021 and December 31, 2020, respectively, of non-Agency mortgage-backed securities and $0 and $391.0 million at September 30, 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.