Mortgage-Backed Securities - Schedule of Available-for-sale Securities Reconciliation (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Outstanding Face Amount	$ 2,000
Fair Value	$ 1,987
Weighted Average Coupon	7.28%	5.63%
CMBS [Member]
Outstanding Face Amount	$ 2,000
Amortized Cost Basis	1,997
Gross Unrealized Losses	(10)
Fair Value	$ 1,987
Weighted Average Coupon	3.60%
Weighted Average Remaining Duration (years)	1 year 9 months 18 days