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Real Estate Securities - Trading Securities by Collateral Type (Detail) - USD ($) $ in Thousands | Sep. 30, 2015 | Dec. 31, 2014
--- | --- | ---
Investment Holdings [Line Items] |  |
Trading securities | $ 114,211 | $ 111,606
Senior Securities |  |
Investment Holdings [Line Items] |  |
Trading securities | 57,822 | 101,753
Senior Securities | Prime |  |
Investment Holdings [Line Items] |  |
Trading securities | 51,308 | 93,802
Senior Securities | Non-prime |  |
Investment Holdings [Line Items] |  |
Trading securities | 6,514 | 7,951
Subordinate Securities | Prime |  |
Investment Holdings [Line Items] |  |
Trading securities | 56,389 | 9,853
Subordinate Securities | Prime mezzanine |  |
Investment Holdings [Line Items] |  |
Trading securities | 38,750 | 0
Subordinate Securities | Prime subordinate |  |
Investment Holdings [Line Items] |  |
Trading securities | $ 17,639 | $ 9,853

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