VARIABLE INTEREST ENTITIES - Additional Information (Details) - USD ($)		1 Months Ended		3 Months Ended		9 Months Ended		12 Months Ended
		Aug. 31, 2019	Aug. 31, 2017	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018	Dec. 31, 2018		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	[1]			$ 3,946,614,000		$ 3,946,614,000		$ 4,585,975,000	[2]
Securitized debt of consolidated VIEs				3,856,082,000		3,856,082,000		3,347,062,000	[2]
Estimated Fair Value	[3]			116,094,061,000		116,094,061,000		92,623,788,000	[2]
Costs incurred in connection with securitization				66,138,000	$ 126,509,000	228,283,000	$ 252,800,000
Other secured financings				4,466,030,000		4,466,030,000		4,183,311,000	[2]
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				2,700,000,000		2,700,000,000
Consolidated VIEs | Consolidation, Eliminations
Variable Interest Entity [Line Items]
Estimated Fair Value				182,300,000		182,300,000		224,300,000
Consolidated VIEs | NLY 2019-FL2
Variable Interest Entity [Line Items]
Mortgage loans, unpaid principal balance				814,400,000		814,400,000
Loans													$ 857,300,000
Securitized debt of consolidated VIEs				635,900,000		635,900,000
Costs incurred in connection with securitization						8,300,000
Contractual principal amount of debt held by third parties				633,900,000		633,900,000
Consolidated VIEs | NLY 2019-FL2 | Consolidation, Eliminations
Variable Interest Entity [Line Items]
Estimated Fair Value				223,900,000		223,900,000
Consolidated VIEs | OBX Trust
Variable Interest Entity [Line Items]
Loans										$ 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				1,800,000,000		1,800,000,000
Costs incurred in connection with securitization				2,600,000	$ 1,800,000	7,300,000	$ 3,300,000
Contractual principal amount of debt held by third parties				1,800,000,000		1,800,000,000
Consolidated VIEs | OBX Trust | Consolidation, Eliminations
Variable Interest Entity [Line Items]
Estimated Fair Value				512,600,000		512,600,000
CMBS Securitization Trust | Consolidated VIEs | Collateralized Mortgage Backed Securities
Variable Interest Entity [Line Items]
Commitments to purchase subordinated tranche in securitization trust		$ 20,300,000						94,000,000
Commercial Trusts | Consolidated VIEs
Variable Interest Entity [Line Items]
Mortgage loans, unpaid principal balance				1,500,000,000		1,500,000,000
Gain (Loss) attributable to instrument- specific credit risk						0
Loans				0		0		0
Other secured financings				0		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		0
Contractual principal amount of debt held by third parties				66,200,000		66,200,000
Other secured financings				0		0		$ 0
Borrower
Variable Interest Entity [Line Items]
Credit facility, maximum borrowing capacity				500,000,000.0		500,000,000.0
Other secured financings				434,700,000		434,700,000
Borrower | Consolidation, Eliminations
Variable Interest Entity [Line Items]
Intercompany receivable				434,700,000		434,700,000
Borrower | Corporate debt held for sale, net
Variable Interest Entity [Line Items]
Transferred loans pledged as collateral for credit facility				748,800,000		748,800,000
Borrower | July 2017 Credit Facility | Consolidated VIEs
Variable Interest Entity [Line Items]
Credit facility, maximum borrowing capacity				250,000,000.0		250,000,000.0
Borrower | July 2017 Credit Facility | Consolidated VIEs | Corporate debt held for sale, net
Variable Interest Entity [Line Items]
Transferred loans pledged as collateral for credit facility				435,500,000		435,500,000
Other secured financings				246,700,000		246,700,000
Borrower | January 2019 Credit Facility | Consolidated VIEs
Variable Interest Entity [Line Items]
Credit facility, maximum borrowing capacity														$ 200,000,000.0
Other secured financings				$ 152,200,000		$ 152,200,000

[1] ⁽⁴⁾  Includes $76.3 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 September 30, 2019 and December 31, 2018, respectively.

[2] ⁽¹⁾  Derived from the audited consolidated financial statements at December 31, 2018.

[3] ⁽³⁾  Excludes $429.1 million and $83.6 million at September 30, 2019 and December 31, 2018, respectively, of non-Agency mortgage-backed securities and $182.3 million and $224.3 million at September 30, 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.