Real Estate Securities - Available for Sale Securities by Collateral Type (Detail) - USD ($) $ in Thousands	Sep. 30, 2015	Dec. 31, 2014
Investment Holdings [Line Items]
Available-for-sale securities	$ 971,013	$ 1,267,624
Senior Securities
Investment Holdings [Line Items]
Available-for-sale securities	409,313	487,557
Senior Securities | Prime
Investment Holdings [Line Items]
Available-for-sale securities	251,273	307,813
Senior Securities | Non-prime
Investment Holdings [Line Items]
Available-for-sale securities	158,040	179,744
Re-REMIC
Investment Holdings [Line Items]
Available-for-sale securities	167,639	168,347
Subordinate Securities
Investment Holdings [Line Items]
Available-for-sale securities	394,061	611,720
Subordinate Securities | Prime
Investment Holdings [Line Items]
Available-for-sale securities	233,458	448,838
Subordinate Securities | Non-prime
Investment Holdings [Line Items]
Available-for-sale securities	1,000	1,000
Subordinate Securities | Subordinate
Investment Holdings [Line Items]
Available-for-sale securities	$ 160,603	$ 162,882