Residential Mortgage Loans Held-For-Securitization - Summary of Residential Mortgage Loans Held-for-Investment (Detail) - Residential Mortgage Backed Securities $ in Thousands	12 Months Ended
	Dec. 31, 2019 USD ($) loan	Dec. 31, 2019 USD ($) item	Dec. 31, 2019 USD ($)	Dec. 31, 2018 USD ($) loan
Portfolio Characteristics:
Number of loans	300	300		22
Current principal balance | $	$ 148,908	$ 148,908	$ 148,908	$ 11,281
Average loan balance | $	$ 496	$ 496	$ 496	$ 513
Net weighted average coupon rate			5.40%	5.95%
Weighted average FICO score	744	744	744	722
Weighted average LTV (loan-to-value)			70.00%	77.00%
Weighted average DTI (debt-to-income)			38.00%	40.00%
Performance:
Current | $	$ 146,999	$ 146,999	$ 146,999	$ 11,281
Total | $	148,908	148,908	148,908	$ 11,281
30 days delinquent
Performance:
Delinquent | $	$ 1,909	$ 1,909	$ 1,909
12-months bank statements
Portfolio Characteristics:
Number of loans | loan	17			2
24-months bank statements
Portfolio Characteristics:
Number of loans | loan	56			18
Alt documentation
Portfolio Characteristics:
Number of loans | loan	97
Full documentation
Portfolio Characteristics:
Number of loans | loan	15			2
Written Verification of Employment
Portfolio Characteristics:
Number of loans | loan	115