Variable Interest Entities - Summary of Residential Mortgage Loans Held-for-Investment (Detail) - Residential Mortgage Backed Securities $ in Thousands	3 Months Ended	12 Months Ended
	Mar. 31, 2020 USD ($) loan	Dec. 31, 2019 USD ($) loan
Portfolio Characteristics:
Number of loans | loan	279	300
Current principal balance	$ 137,911	$ 148,908
Average loan balance	$ 494	$ 496
Weighted average FICO score	742	744
Current Performance:
Current	$ 135,385	$ 146,999
Total	137,911	148,908
30 days delinquent
Current Performance:
Delinquent	2,115	$ 1,909
60 days delinquent
Current Performance:
Delinquent	$ 411
Variable Interest Entities Primary Beneficiary
Portfolio Characteristics:
Number of loans | loan	659	704
Current principal balance	$ 415,320	$ 456,768
Average loan balance	$ 630	$ 649
Net weighted average coupon rate	3.86%	3.87%
Weighted average maturity (years)	24 years 1 month 6 days	24 years 3 months 18 days
Weighted average FICO score	761	762
Current Performance:
Current	$ 410,959	$ 452,875
Total	415,320	456,768
Variable Interest Entities Primary Beneficiary | 30 days delinquent
Current Performance:
Delinquent	2,439	2,122
Variable Interest Entities Primary Beneficiary | 60 days delinquent
Current Performance:
Delinquent	605	726
Variable Interest Entities Primary Beneficiary | 90+ days delinquent
Current Performance:
Delinquent	$ 1,317	$ 1,045