Variable Interest Entities - Summary of Residential Mortgage Loans Held-for-Investment (Detail) - Residential Mortgage Backed Securities $ in Thousands	9 Months Ended	12 Months Ended
	Sep. 30, 2019 USD ($) loan	Dec. 31, 2018 USD ($) loan
Portfolio Characteristics:
Number of loans | loan	263	22
Current principal balance	$ 125,565	$ 11,281
Average loan balance	$ 477	$ 513
Weighted average FICO score	746	722
Current Performance:
Current	$ 125,565	$ 11,281
Total	$ 125,565	$ 11,281
Variable Interest Entities Primary Beneficiary
Portfolio Characteristics:
Number of loans | loan	751	829
Current principal balance	$ 481,712	$ 545,881
Average loan balance	$ 641	$ 658
Net weighted average coupon rate	3.87%	3.85%
Weighted average maturity (years)	24 years 7 months 6 days	25 years 3 months 18 days
Weighted average FICO score	762	761
Current Performance:
Current	$ 477,803	$ 543,328
Bankruptcy/foreclosure		591
Total	481,712	545,881
Variable Interest Entities Primary Beneficiary | 30 days delinquent
Current Performance:
Delinquent	644
Variable Interest Entities Primary Beneficiary | 60 days delinquent
Current Performance:
Delinquent		896
Variable Interest Entities Primary Beneficiary | 90+ days delinquent
Current Performance:
Delinquent	$ 3,265	$ 1,066