VARIABLE INTEREST ENTITIES - Additional Information (Details) - USD ($)		1 Months Ended	12 Months Ended
		Aug. 31, 2017	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018	Sep. 30, 2020	Jul. 31, 2020	Mar. 31, 2020	Feb. 29, 2020	Jan. 31, 2020	Nov. 30, 2019	Oct. 31, 2019	Jul. 31, 2019	Jun. 30, 2019	Apr. 30, 2019	Feb. 28, 2019	Jan. 31, 2019	Oct. 31, 2018	Aug. 31, 2018	Mar. 31, 2018
Variable Interest Entity [Line Items]
Principal balance	[1]		$ 3,083,821,000	$ 4,462,350,000
Loans			498,081,000
Securitized debt of consolidated VIEs			5,652,982,000	5,622,801,000
Estimated fair value	[2]		75,652,396,000	114,833,580,000
Costs incurred in connection with securitization			239,198,000	301,634,000	$ 329,873,000
Net gains (losses) on disposal of investments			661,513,000	(47,944,000)	$ (1,124,448,000)
Other secured financing			917,876,000	4,455,700,000
Participations issued			39,198,000	0
Corporate Loans
Variable Interest Entity [Line Items]
Loans 90 days or more past due or on nonaccrual status			0.0	12,200,000
Loans			2,239,930,000	2,144,850,000
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 | Commercial Trusts
Variable Interest Entity [Line Items]
Principal balance			2,300,000,000
Gain (loss) attributable to instrument- specific credit risk			0	0
Loans			0	0
Other secured financing			0	0
Consolidated VIEs | Residential Trusts
Variable Interest Entity [Line Items]
Loans			0	0
Contractual principal amount of debt held by third parties			23,000,000.0	57,300,000
Other secured financing			0	0
Consolidated VIEs | Borrower
Variable Interest Entity [Line Items]
Credit facility, maximum borrowing capacity			625,000,000.0	625,000,000.0
Other secured financing			441,100,000	426,600,000
Consolidated VIEs | Multifamily
Variable Interest Entity [Line Items]
Cut-off date principal balance								$ 500,000,000			$ 1,000,000,000.0
Derecognized securities			1,200,000,000
Derecognized securitization vehicles			1,100,000,000
Net gains (losses) on disposal of investments			104,800,000
Costs incurred			1,100,000
Consolidated VIEs | Multifamily | Retained Interest
Variable Interest Entity [Line Items]
Retained interest notional balance								$ 500,000,000			1,000,000,000.0
Consolidated VIEs | Multifamily | Senior Securities
Variable Interest Entity [Line Items]
Principal balance											$ 28,500,000
Consolidated VIEs | Consolidation, Eliminations | Borrower
Variable Interest Entity [Line Items]
Principal balance			441,100,000	426,600,000
Consolidated VIEs | Corporate Loans | Borrower
Variable Interest Entity [Line Items]
Transferred loans pledged as collateral for credit facility			786,900,000	741,300,000
Consolidated VIEs | NLY 2019-FL2
Variable Interest Entity [Line Items]
Principal balance			856,900,000
Loans																$ 857,300,000
Securitized debt of consolidated VIEs			625,800,000
Costs incurred in connection with securitization				8,300,000
Contractual principal amount of debt held by third parties			633,900,000
Consolidated VIEs | NLY 2019-FL2 | Consolidation, Eliminations
Variable Interest Entity [Line Items]
Estimated fair value			202,400,000
Consolidated VIEs | OBX Trust
Variable Interest Entity [Line Items]
Loans						$ 514,609,000	$ 489,352,000		$ 467,511,000	$ 374,609,000		$ 465,492,000	$ 463,405,000	$ 383,760,000	$ 388,156,000		$ 393,961,000	$ 384,027,000	$ 383,451,000	$ 327,162,000
Securitized debt of consolidated VIEs			2,600,000,000	2,000,000,000.0
Costs incurred in connection with securitization			7,200,000	9,000,000.0
Contractual principal amount of debt held by third parties			2,500,000,000	1,900,000,000
Consolidated VIEs | OBX Trust | Consolidation, Eliminations
Variable Interest Entity [Line Items]
Estimated fair value			653,000,000.0	565,700,000
Consolidated VIEs | Financing Receivables, Equal to Greater than 90 Days Past Due | Commercial Trusts
Variable Interest Entity [Line Items]
Loans 90 days or more past due or on nonaccrual status			0	0
Consolidated VIEs | July 2017 Credit Facility | Borrower
Variable Interest Entity [Line Items]
Credit facility, maximum borrowing capacity			320,000,000.0	320,000,000.0
Consolidated VIEs | July 2017 Credit Facility | Corporate Loans | Borrower
Variable Interest Entity [Line Items]
Transferred loans pledged as collateral for credit facility			400,400,000	413,700,000
Other secured financing			209,700,000	244,200,000
Consolidated VIEs | January 2019 Credit Facility | Borrower
Variable Interest Entity [Line Items]
Credit facility, maximum borrowing capacity																	$ 300,000,000.0
Consolidated VIEs | January 2019 Credit Facility | Corporate Loans | Borrower
Variable Interest Entity [Line Items]
Other secured financing			236,600,000	$ 157,500,000
VIE, Not Primary Beneficiary
Variable Interest Entity [Line Items]
Loans transferred for cash			159,300,000
VIE, Not Primary Beneficiary | Residential mortgage loans
Variable Interest Entity [Line Items]
Participations issued			$ 39,200,000

[1] Includes $47.0 million and $66.7 million of residential mortgage loans held for sale.

[2] Excludes $81.5 million and $102.5 million at December 31, 2020 and 2019, respectively, of agency mortgage-backed securities, $576.6 million and $468.0 million at December 31, 2020 and 2019, respectively, of non-Agency mortgage-backed securities and $391.0 million and $500.3 million at December 31, 2020 and December 31, 2019, respectively, of commercial mortgage-backed securities in consolidated VIEs pledged as collateral and eliminated from the Company’s Consolidated Statements of Financial Condition.