Variable Interest Entities - Summary of Residential Mortgage Loans Held-for-Investment (Detail) - Residential Mortgage Backed Securities $ in Thousands	6 Months Ended	12 Months Ended
	Jun. 30, 2020 USD ($) loan	Dec. 31, 2019 USD ($) loan
Portfolio Characteristics:
Number of loans | loan	261	300
Current principal balance	$ 127,782	$ 148,908
Average loan balance	$ 490	$ 496
Weighted average FICO score	742	744
Current Performance:
Current	$ 99,974	$ 146,999
Total	127,782	148,908
30 days delinquent
Current Performance:
Delinquent	1,547	$ 1,909
60 days delinquent
Current Performance:
Delinquent	13,252
90+ days delinquent
Current Performance:
Delinquent	$ 13,009
Variable Interest Entities Primary Beneficiary
Portfolio Characteristics:
Number of loans | loan	580	704
Current principal balance	$ 366,584	$ 456,768
Average loan balance	$ 632	$ 649
Net weighted average coupon rate	3.86%	3.87%
Weighted average maturity (years)	23 years 9 months 18 days	24 years 3 months 18 days
Weighted average FICO score	761	762
Current Performance:
Current	$ 361,114	$ 452,875
Total	366,584	456,768
Variable Interest Entities Primary Beneficiary | 30 days delinquent
Current Performance:
Delinquent	3,494	2,122
Variable Interest Entities Primary Beneficiary | 60 days delinquent
Current Performance:
Delinquent	1,250	726
Variable Interest Entities Primary Beneficiary | 90+ days delinquent
Current Performance:
Delinquent	$ 726	$ 1,045