Fair Value Measurements - Unobservable Inputs Assumptions - Securitized Debt (Detail) - Variable Interest Entities, Primary Beneficiary, Aggregated Disclosure - Loans held for investment - Level 3	Mar. 31, 2020	Dec. 31, 2019
Discount Rate | Minimum
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Significant Inputs	0.00	0.00
Discount Rate | Maximum
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Significant Inputs	0.35	0.10
Discount Rate | Weighted Average
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Significant Inputs	0.052	0.033
CPR | Minimum
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Significant Inputs	0.05	0.06
CPR | Maximum
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Significant Inputs	0.25	0.20
CPR | Weighted Average
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Significant Inputs	0.088	0.091
CDR | Minimum
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Significant Inputs	0.00	0.00
CDR | Maximum
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Significant Inputs	0.03	0.03
CDR | Weighted Average
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Significant Inputs	0.015	0.014
Loss Severity | Minimum
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Significant Inputs	0.30	0.30
Loss Severity | Maximum
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Significant Inputs	0.75	0.75
Loss Severity | Weighted Average
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Significant Inputs	0.611	0.599