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Real Estate Securities - Additional Information (Details) | 3 Months Ended | 9 Months Ended |  |
--- | --- | --- | --- | ---
Sep. 30, 2020 USD ($) investment | Sep. 30, 2019 USD ($) | Sep. 30, 2020 USD ($) investment | Sep. 30, 2019 USD ($) | Jun. 30, 2020 USD ($) | Dec. 31, 2019 USD ($) investment
Investment Holdings [Line Items] |  |  |  |  |  |
Real estate securities | $ 144,162,000 |  | $ 144,162,000 |  |  | $ 860,540,000
Trading securities acquired | 11,000,000 | $ 66,000,000 | 78,000,000 | $ 335,000,000 |  |
Trading securities sold | 16,000,000 | 236,000,000 | 721,000,000 | 397,000,000 |  |
Increase (decrease) in valuation of trading securities | (4,000,000) | 15,000,000 | (225,000,000) | 56,000,000 |  |
Available-for-sale securities | 207,173,000 |  | 207,173,000 |  |  | $ 239,334,000
Available-for-sale securities purchased | 25,000,000 | 12,000,000 | 57,000,000 | 21,000,000 |  |
Available-for-sale securities sold | 0 | 15,000,000 | 55,000,000 | 82,000,000 |  |
Net realized gains on AFS securities | $ 0 | $ 4,000,000 | $ 5,000,000 | $ 13,000,000 |  |
Number of AFS securities (in investments) | investment | 96 |  | 96 |  |  | 107
Number of securities in unrealized loss position | investment | 7 |  | 7 |  |  | 1
Number of securities in a continuous unrealized loss position for twelve consecutive months or longer (in investments) | investment | 0 |  | 0 |  |  | 1
Gross unrealized losses | $ 2,507,000 |  | $ 2,507,000 |  |  | $ 29,000
CECL credit allowance | 1,027,000 |  | 1,027,000 |  | $ 1,471,000 | 0
Interest Only Senior Trading Securities |  |  |  |  |  |
Investment Holdings [Line Items] |  |  |  |  |  |
Debt securities, trading | 28,000,000 |  | 28,000,000 |  |  | 64,000,000
Unpaid principal balance | 0 |  | 0 |  |  | 84,000,000
Certificated Servicing Strips |  |  |  |  |  |
Investment Holdings [Line Items] |  |  |  |  |  |
Debt securities, trading | 12,000,000 |  | 12,000,000 |  |  | 36,000,000
Re-preforming Loans Securities |  |  |  |  |  |
Investment Holdings [Line Items] |  |  |  |  |  |
Debt securities, trading | 60,000,000 |  | 60,000,000 |  |  | 161,000,000
Third Party Multifamily Mortgage-backed Securities |  |  |  |  |  |
Investment Holdings [Line Items] |  |  |  |  |  |
Available-for-sale securities | 48,000,000 |  | 48,000,000 |  |  | 9,000,000
Residential |  |  |  |  |  |
Investment Holdings [Line Items] |  |  |  |  |  |
Available-for-sale securities | 160,000,000 |  | 160,000,000 |  |  | 230,000,000
AFS securities, contractual maturities | 45,000,000 |  | 45,000,000 |  |  |
Marketable securities, due from five to ten years | 3,000,000 |  | 3,000,000 |  |  |
CECL credit allowance | 43,186,000 |  | 43,186,000 |  | $ 37,785,000 | 32,940,000
Residential | Sequoia Choice |  |  |  |  |  |
Investment Holdings [Line Items] |  |  |  |  |  |
Available-for-sale securities | 124,000,000 |  | 124,000,000 |  |  | 141,000,000
Residential | Other Third Party Securities |  |  |  |  |  |
Investment Holdings [Line Items] |  |  |  |  |  |
Available-for-sale securities | 35,000,000 |  | 35,000,000 |  |  | 90,000,000
Mezzanine |  |  |  |  |  |
Investment Holdings [Line Items] |  |  |  |  |  |
Available-for-sale securities | 2,016,000 |  | 2,016,000 |  |  | 13,687,000
Gross unrealized losses | 0 |  | 0 |  |  | 0
CECL credit allowance | 0 |  | 0 |  |  |
Mezzanine | Re-preforming Loans Securities |  |  |  |  |  |
Investment Holdings [Line Items] |  |  |  |  |  |
Real estate securities | 0 |  | 0 |  |  | 30,000,000
Mezzanine | Trading securities |  |  |  |  |  |
Investment Holdings [Line Items] |  |  |  |  |  |
Unpaid principal balance | 4,000,000 |  | 4,000,000 |  |  | 537,000,000
Mezzanine | Sequoia securities |  |  |  |  |  |
Investment Holdings [Line Items] |  |  |  |  |  |
Real estate securities | 4,000,000 |  | 4,000,000 |  |  | 39,000,000
Mezzanine | Subordinate Securities |  |  |  |  |  |
Investment Holdings [Line Items] |  |  |  |  |  |
Real estate securities | 3,651,000 |  | 3,651,000 |  |  | 538,489,000
Available-for-sale securities | 2,016,000 |  | 2,016,000 |  |  | 13,687,000
Mezzanine | Third Party Multifamily Mortgage-backed Securities |  |  |  |  |  |
Investment Holdings [Line Items] |  |  |  |  |  |
Real estate securities | 0 |  | 0 |  |  | 395,000,000
Mezzanine | Other Third Party Securities |  |  |  |  |  |
Investment Holdings [Line Items] |  |  |  |  |  |
Real estate securities | 0 |  | 0 |  |  | 104,000,000
Subordinate Securities |  |  |  |  |  |
Investment Holdings [Line Items] |  |  |  |  |  |
Available-for-sale securities | 205,157,000 |  | 205,157,000 |  |  | 199,855,000
Gross unrealized losses | 2,507,000 |  | 2,507,000 |  |  | 0
CECL credit allowance | 1,027,000 |  | 1,027,000 |  |  |
Subordinate Securities | Re-preforming Loans Securities |  |  |  |  |  |
Investment Holdings [Line Items] |  |  |  |  |  |
Real estate securities | 51,000,000 |  | 51,000,000 |  |  | 76,000,000
Subordinate Securities | Other Third Party Securities |  |  |  |  |  |
Investment Holdings [Line Items] |  |  |  |  |  |
Real estate securities | 55,000,000 |  | 55,000,000 |  |  | 82,000,000
Subordinate Securities | Credit Risk Transfer (CRT) Securities |  |  |  |  |  |
Investment Holdings [Line Items] |  |  |  |  |  |
Real estate securities | 50,000,000 |  | 50,000,000 |  |  | 90,000,000
Residential Subordinate Securities | Trading securities |  |  |  |  |  |
Investment Holdings [Line Items] |  |  |  |  |  |
Unpaid principal balance | 266,000,000 |  | 266,000,000 |  |  | 302,000,000
Subordinate | Subordinate Securities |  |  |  |  |  |
Investment Holdings [Line Items] |  |  |  |  |  |
Real estate securities | 112,420,000 |  | 112,420,000 |  |  | 171,984,000
Available-for-sale securities | $ 205,157,000 |  | $ 205,157,000 |  |  | $ 199,855,000

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