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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.

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