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Real Estate Securities - Additional Information (Details) | 3 Months Ended | 6 Months Ended |  |
--- | --- | --- | --- | ---
Jun. 30, 2020 USD ($) investment | Jun. 30, 2019 USD ($) | Jun. 30, 2020 USD ($) investment | Jun. 30, 2019 USD ($) | Mar. 31, 2020 USD ($) | Dec. 31, 2019 USD ($) investment
Investment Holdings [Line Items] |  |  |  |  |  |
Available-for-sale securities | $ 173,737,000 |  | $ 173,737,000 |  |  | $ 239,334,000
Trading securities | 142,699,000 |  | 142,699,000 |  |  | $ 860,540,000
Trading securities acquired | 10,000,000 | $ 115,000,000 | 67,000,000 | $ 269,000,000 |  |
Trading securities sold | 86,000,000 | 132,000,000 | 705,000,000 | 161,000,000 |  |
Increase (decrease) in valuation of trading securities | 42,000,000 | 18,000,000 | (221,000,000) | 40,000,000 |  |
Available-for-sale securities purchased | 0 | 4,000,000 | 31,000,000 | 9,000,000 |  |
Available-for-sale securities sold | 9,000,000 | 25,000,000 | 55,000,000 | 67,000,000 |  |
Net realized gains on AFS securities | $ 1,000,000 | $ 3,000,000 | $ 5,000,000 | $ 9,000,000 |  |
Number of AFS securities (in investments) | investment | 94 |  | 94 |  |  | 107
Number of securities in unrealized loss position | investment | 13 |  | 13 |  |  | 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 | $ 3,846,000 |  | $ 3,846,000 |  |  | $ 29,000
CECL credit allowance | 1,471,000 |  | 1,471,000 |  | $ 1,525,000 | 0
Interest Only Senior Trading Securities |  |  |  |  |  |
Investment Holdings [Line Items] |  |  |  |  |  |
Debt securities, trading | 33,000,000 |  | 33,000,000 |  |  | 64,000,000
Unpaid principal balance | 0 |  | 0 |  |  | 84,000,000
Certificated Servicing Strips |  |  |  |  |  |
Investment Holdings [Line Items] |  |  |  |  |  |
Debt securities, trading | 14,000,000 |  | 14,000,000 |  |  | 36,000,000
Re-preforming Loans Securities [Member] |  |  |  |  |  |
Investment Holdings [Line Items] |  |  |  |  |  |
Debt securities, trading | 11,000,000 |  | 11,000,000 |  |  | 55,000,000
Residential |  |  |  |  |  |
Investment Holdings [Line Items] |  |  |  |  |  |
Available-for-sale securities | 150,000,000 |  | 150,000,000 |  |  | 230,000,000
AFS securities, contractual maturities | 21,000,000 |  | 21,000,000 |  |  |
Marketable securities, due from five to ten years | 2,000,000 |  | 2,000,000 |  |  |
CECL credit allowance | 37,785,000 |  | 37,785,000 |  | $ 37,717,000 | 32,940,000
Residential | Sequoia Choice |  |  |  |  |  |
Investment Holdings [Line Items] |  |  |  |  |  |
Available-for-sale securities | 118,000,000 |  | 118,000,000 |  |  | 141,000,000
Residential | Other Third Party Securities |  |  |  |  |  |
Investment Holdings [Line Items] |  |  |  |  |  |
Available-for-sale securities | 33,000,000 |  | 33,000,000 |  |  | 90,000,000
Third Party Multifamily Mortgage-backed Securities |  |  |  |  |  |
Investment Holdings [Line Items] |  |  |  |  |  |
Available-for-sale securities | 23,000,000 |  | 23,000,000 |  |  | 9,000,000
Mezzanine |  |  |  |  |  |
Investment Holdings [Line Items] |  |  |  |  |  |
Available-for-sale securities | 0 |  | 0 |  |  | 13,687,000
Gross unrealized losses | 0 |  | 0 |  |  | 0
CECL credit allowance | 0 |  | 0 |  |  |
Mezzanine | Subordinate Securities |  |  |  |  |  |
Investment Holdings [Line Items] |  |  |  |  |  |
Available-for-sale securities | 0 |  | 0 |  |  | 13,687,000
Trading securities | 3,514,000 |  | 3,514,000 |  |  | 538,489,000
Mezzanine | Sequoia securities |  |  |  |  |  |
Investment Holdings [Line Items] |  |  |  |  |  |
Trading securities | 4,000,000 |  | 4,000,000 |  |  | 39,000,000
Mezzanine | Re-preforming Loans Securities [Member] |  |  |  |  |  |
Investment Holdings [Line Items] |  |  |  |  |  |
Trading securities | 0 |  | 0 |  |  | 30,000,000
Mezzanine | Other Third Party Securities |  |  |  |  |  |
Investment Holdings [Line Items] |  |  |  |  |  |
Trading securities | 0 |  | 0 |  |  | 104,000,000
Mezzanine | Third Party Multifamily Mortgage-backed Securities |  |  |  |  |  |
Investment Holdings [Line Items] |  |  |  |  |  |
Trading securities | 0 |  | 0 |  |  | 395,000,000
Mezzanine | Trading securities |  |  |  |  |  |
Investment Holdings [Line Items] |  |  |  |  |  |
Unpaid principal balance | 4,000,000 |  | 4,000,000 |  |  | 537,000,000
Residential Subordinate Securities | Trading securities |  |  |  |  |  |
Investment Holdings [Line Items] |  |  |  |  |  |
Unpaid principal balance | 272,000,000 |  | 272,000,000 |  |  | 302,000,000
Subordinate Securities |  |  |  |  |  |
Investment Holdings [Line Items] |  |  |  |  |  |
Available-for-sale securities | 173,737,000 |  | 173,737,000 |  |  | 199,855,000
Gross unrealized losses | 3,846,000 |  | 3,846,000 |  |  | 0
CECL credit allowance | 1,471,000 |  | 1,471,000 |  |  |
Subordinate Securities | Credit Risk Transfer (CRT) Securities |  |  |  |  |  |
Investment Holdings [Line Items] |  |  |  |  |  |
Trading securities | 50,000,000 |  | 50,000,000 |  |  | 90,000,000
Subordinate Securities | Other Third Party Securities |  |  |  |  |  |
Investment Holdings [Line Items] |  |  |  |  |  |
Trading securities | 52,000,000 |  | 52,000,000 |  |  | 82,000,000
Subordinate Securities | Third Party Multifamily Mortgage-backed Securities |  |  |  |  |  |
Investment Holdings [Line Items] |  |  |  |  |  |
Trading securities | 49,000,000 |  | 49,000,000 |  |  | 76,000,000
Subordinate | Subordinate Securities |  |  |  |  |  |
Investment Holdings [Line Items] |  |  |  |  |  |
Available-for-sale securities | 173,737,000 |  | 173,737,000 |  |  | 199,855,000
Trading securities | $ 106,325,000 |  | $ 106,325,000 |  |  | $ 171,984,000

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