Variable Interest Entities - Summary of Residential Mortgage Loans Held-for-Investment (Detail) - Residential Mortgage Backed Securities $ in Thousands	12 Months Ended
	Dec. 31, 2019 USD ($)	Dec. 31, 2019 USD ($) loan	Dec. 31, 2019 USD ($) item	Dec. 31, 2019 USD ($)	Dec. 31, 2018 USD ($) loan item
Portfolio Characteristics:
Number of loans		300	300		22
Current principal balance	$ 148,908	$ 148,908	$ 148,908	$ 148,908	$ 11,281
Average loan balance	$ 496	$ 496	$ 496	$ 496	$ 513
Weighted average FICO score	744	744	744	744	722
Current Performance:
Current	$ 146,999	$ 146,999	$ 146,999	$ 146,999	$ 11,281
Total	148,908	148,908	148,908	148,908	$ 11,281
30 days delinquent
Current Performance:
Delinquent	1,909	$ 1,909	$ 1,909	1,909
Variable Interest Entities Primary Beneficiary
Portfolio Characteristics:
Number of loans		704	704		829
Current principal balance	456,768	$ 456,768	$ 456,768	456,768	$ 545,881
Average loan balance	$ 649	$ 649	$ 649	$ 649	$ 658
Net weighted average coupon rate				3.87%	3.85%
Weighted average maturity (years)	24 years 3 months 18 days				25 years 3 months 18 days
Weighted average FICO score	762	762	762	762	761
Current Performance:
Current	$ 452,875	$ 452,875	$ 452,875	$ 452,875	$ 543,328
Bankruptcy/foreclosure					591
Total	456,768	456,768	456,768	456,768	545,881
Variable Interest Entities Primary Beneficiary | 30 days delinquent
Current Performance:
Delinquent	2,122	2,122	2,122	2,122
Variable Interest Entities Primary Beneficiary | 60 days delinquent
Current Performance:
Delinquent	726	726	726	726	896
Variable Interest Entities Primary Beneficiary | 90+ days delinquent
Current Performance:
Delinquent	$ 1,045	$ 1,045	$ 1,045	$ 1,045	$ 1,066