Available-for-Sale Securities - Details of Carrying and Fair Values Of CMBS And CRE CLO Investment Portfolio (Details) - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018
Schedule Of Available For Sale Securities [Line Items]
Estimated Fair Value	$ 308,294	$ 74,381
CMBS and CRE CLO Investments
Schedule Of Available For Sale Securities [Line Items]
Face Amount	310,192	76,404
Unamortized Premium (Discount), net	(19)	(38)
Gross Unrealized (Loss)	(1,879)	(1,985)
Estimated Fair Value	$ 308,294	$ 74,381