Real Estate Securities - Trading Securities by Collateral Type (Details) - USD ($) $ in Thousands	Sep. 30, 2019	Dec. 31, 2018
Investment Holdings [Line Items]
Trading securities	$ 1,013,785	$ 1,118,612
Senior IO Securities
Investment Holdings [Line Items]
Trading securities	149,634	158,670
Subordinate Securities
Investment Holdings [Line Items]
Trading securities	864,000	960,000
Subordinate Securities | Mezzanine
Investment Holdings [Line Items]
Trading securities	644,571	610,819
Subordinate Securities | Subordinate
Investment Holdings [Line Items]
Trading securities	$ 219,580	$ 349,123