VARIABLE INTEREST ENTITIES - Additional Information (Details) - USD ($)		1 Months Ended		12 Months Ended
		Nov. 30, 2019	Aug. 31, 2017	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017	Oct. 31, 2019	Jul. 31, 2019	Jun. 30, 2019	Apr. 30, 2019	Feb. 28, 2019	Jan. 31, 2019	Oct. 31, 2018	Jul. 31, 2018	Mar. 31, 2018
Variable Interest Entity [Line Items]
Loans				$ 669,713,000
Securitized debt of consolidated VIEs				5,622,801,000	$ 3,347,062,000
Estimated fair value	[1]			114,833,580,000	92,623,788,000
Costs incurred in connection with securitization				301,634,000	329,873,000	$ 224,124,000
Principal receivable	[2]			4,462,350,000	4,585,975,000
Other secured financings				4,455,700,000	4,183,311,000
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				3,300,000,000
Consolidated VIEs | Multifamily
Variable Interest Entity [Line Items]
Cut-off Date Principal Balance		$ 1,000,000,000.0
Costs incurred		1,900,000
Consolidated VIEs | Multifamily | Retained Interest
Variable Interest Entity [Line Items]
Retained interest notional balance		1,000,000,000.0
Consolidated VIEs | Multifamily | Senior Securities
Variable Interest Entity [Line Items]
Mortgage loans, unpaid principal balance		$ 28,500,000
Consolidated VIEs | Consolidation, Eliminations
Variable Interest Entity [Line Items]
Estimated fair value				500,300,000	224,300,000
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				635,700,000
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				223,900,000
Consolidated VIEs | OBX Trust
Variable Interest Entity [Line Items]
Loans							$ 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				2,000,000,000.0	766,500,000
Costs incurred in connection with securitization				9,000,000.0	5,400,000
Contractual principal amount of debt held by third parties				1,900,000,000	769,000,000.0
Consolidated VIEs | OBX Trust | Consolidation, Eliminations
Variable Interest Entity [Line Items]
Estimated fair value				565,700,000	221,300,000
Commercial Trusts | Consolidated VIEs
Variable Interest Entity [Line Items]
Mortgage loans, unpaid principal balance				2,300,000,000	2,700,000,000
Gain (loss) attributable to instrument- specific credit risk				0	0
Loans				0	0
Other secured financings				0	0
Commercial Trusts | Consolidated VIEs | Financing Receivables, Equal to Greater than 90 Days Past Due
Variable Interest Entity [Line Items]
Loans 90 days or more past due or on nonaccrual status				0	0
Residential Trusts | Consolidated VIEs
Variable Interest Entity [Line Items]
Loans				0	0
Contractual principal amount of debt held by third parties				57,300,000	72,100,000
Other secured financings				0	0
Borrower
Variable Interest Entity [Line Items]
Credit facility, maximum borrowing capacity				625,000,000.0	400,000,000.0
Other secured financings				426,600,000	376,600,000
Borrower | Consolidation, Eliminations
Variable Interest Entity [Line Items]
Principal receivable				426,600,000	376,600,000
Borrower | Corporate Loans
Variable Interest Entity [Line Items]
Transferred loans pledged as collateral for credit facility				741,300,000	568,700,000
Borrower | July 2017 Credit Facility | Consolidated VIEs
Variable Interest Entity [Line Items]
Credit facility, maximum borrowing capacity				320,000,000.0	150,000,000.0
Borrower | July 2017 Credit Facility | Consolidated VIEs | Corporate Loans
Variable Interest Entity [Line Items]
Transferred loans pledged as collateral for credit facility				413,700,000	234,800,000
Other secured financings				244,200,000	$ 150,000,000.0
Borrower | January 2019 Credit Facility | Consolidated VIEs
Variable Interest Entity [Line Items]
Credit facility, maximum borrowing capacity												$ 300,000,000.0
Other secured financings				$ 157,500,000

[1] Excludes $102.5 million and $0 at December 31, 2019 and 2018, respectively, of agency mortgage-backed securities, $468.0 million and $83.6 million at December 31, 2019 and 2018, respectively, of non-Agency mortgage-backed securities and $500.3 million and $224.3 million at December 31, 2019 and December 31, 2018, respectively, of commercial mortgage-backed securities in consolidated VIEs pledged as collateral and eliminated from the Company’s Consolidated Statements of Financial Condition.

[2] Includes $66.7 million and $97.5 million of residential mortgage loans held for sale and $0 and $42.2 million of commercial mortgage loans held for sale at December 31, 2019 and 2018, respectively.