VARIABLE INTEREST ENTITIES - Additional Information (Details) - USD ($)		1 Months Ended		3 Months Ended
		Nov. 30, 2019	Aug. 31, 2017	Mar. 31, 2020	Mar. 31, 2019	Feb. 29, 2020	Dec. 31, 2019		Oct. 31, 2019	Jul. 31, 2019	Jun. 30, 2019	Apr. 30, 2019	Feb. 28, 2019	Jan. 31, 2019	Dec. 31, 2018	Oct. 31, 2018	Aug. 31, 2018	Mar. 31, 2018
Variable Interest Entity [Line Items]
Securitized debt of consolidated VIEs				$ 6,364,949,000			$ 5,622,801,000	[1]
Estimated fair value	[2]			79,357,596,000			114,833,580,000	[1]
Costs incurred in connection with securitization				77,629,000	$ 83,737,000
Principal receivable	[3]			4,068,189,000			4,462,350,000	[1]
Other secured financings				1,805,428,000			4,455,700,000	[1]
Corporate Loans
Variable Interest Entity [Line Items]
Loans				2,150,263,000			2,144,850,000								$ 1,887,182,000
Pingora
Variable Interest Entity [Line Items]
Variable interest entity, ownership percentage			100.00%
Consolidated VIEs
Variable Interest Entity [Line Items]
Exposure to obligations of VIEs				2,900,000,000
Consolidated VIEs | Commercial Trusts
Variable Interest Entity [Line Items]
Mortgage loans, unpaid principal balance				2,300,000,000			2,300,000,000
Gain (Loss) attributable to instrument- specific credit risk				0	0
Loans				0			0
Other secured financings				0			0
Consolidated VIEs | Residential Trusts
Variable Interest Entity [Line Items]
Loans				0			0
Contractual principal amount of debt held by third parties				50,100,000			57,300,000
Other secured financings				0			0
Consolidated VIEs | Borrower
Variable Interest Entity [Line Items]
Credit facility, maximum borrowing capacity				625,000,000.0
Other secured financings				421,500,000
Consolidated VIEs | Consolidation, Eliminations | Borrower
Variable Interest Entity [Line Items]
Principal receivable				421,500,000
Consolidated VIEs | Corporate Loans | Borrower
Variable Interest Entity [Line Items]
Transferred loans pledged as collateral for credit facility				732,500,000
Consolidated VIEs | NLY 2019-FL2
Variable Interest Entity [Line Items]
Mortgage loans, unpaid principal balance				857,300,000
Loans													$ 857,300,000
Securitized debt of consolidated VIEs				552,200,000
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				167,600,000
Consolidated VIEs | OBX Trust
Variable Interest Entity [Line Items]
Loans				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				2,500,000,000
Costs incurred in connection with securitization				3,700,000	$ 1,700,000
Contractual principal amount of debt held by third parties				2,500,000,000
Consolidated VIEs | OBX Trust | Consolidation, Eliminations
Variable Interest Entity [Line Items]
Estimated fair value				513,000,000.0
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
Consolidated VIEs | July 2017 Credit Facility | Corporate Loans | Borrower
Variable Interest Entity [Line Items]
Transferred loans pledged as collateral for credit facility				490,500,000
Other secured financings				289,500,000
Consolidated VIEs | January 2019 Credit Facility | Borrower
Variable Interest Entity [Line Items]
Credit facility, maximum borrowing capacity														$ 300,000,000.0
Other secured financings							$ 326,700,000
Multifamily | Consolidated VIEs
Variable Interest Entity [Line Items]
Cut-off Date Principal Balance		$ 1,000,000,000.0		500,000,000
Costs incurred		1,100,000
Retained Interest | Multifamily | Consolidated VIEs
Variable Interest Entity [Line Items]
Retained interest notional balance		1,000,000,000.0		500,000,000
Senior Loans | Multifamily | Consolidated VIEs
Variable Interest Entity [Line Items]
Mortgage loans, unpaid principal balance		$ 28,500,000
Fund Entities | VIE, Not Primary Beneficiary | Corporate Loans
Variable Interest Entity [Line Items]
Loans				$ 109,800,000

[1] Derived from the audited consolidated financial statements at December 31, 2019.

[2] Excludes $127.7 million and $102.5 million at March 31, 2020 and December 31, 2019, respectively, of Agency mortgage-backed securities, $412.0 million and $468.0 million at March 31, 2020 and December 31, 2019, respectively, of non-Agency mortgage-backed securities and $367.6 million and $500.3 million at March 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.

[3] Includes $57.8 million and $66.7 million of residential mortgage loans held for sale at March 31, 2020 and December 31, 2019, respectively.