Commercial Mortgage-Backed Securities - Available-for-Sale Commercial Mortgage-Backed Securities (Details) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Schedule Of Available For Sale Securities [Line Items]
Estimated Fair Value	$ 75,414	$ 85,895
Commercial Mortgage-Backed Securities
Schedule Of Available For Sale Securities [Line Items]
Face Amount	76,603	85,661
Unamortized Premium (Discount), net	(40)	268
Gross Unrealized Loss	(1,149)	(34)
Estimated Fair Value	$ 75,414	$ 85,895