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Real Estate Securities - Available for Sale Securities by Collateral Type (Details) - USD ($) $ in Thousands | Jun. 30, 2019 | Dec. 31, 2018
--- | --- | ---
Investment Holdings [Line Items] |  |
Available-for-sale | $ 272,097 | $ 333,882
Senior IO Securities |  |
Investment Holdings [Line Items] |  |
Available-for-sale | 44,467 | 87,615
Subordinate Securities | Mezzanine |  |
Investment Holdings [Line Items] |  |
Available-for-sale | 8,976 | 36,407
Subordinate Securities | Subordinate |  |
Investment Holdings [Line Items] |  |
Available-for-sale | $ 218,654 | $ 209,860

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