Fair Value Measurements - Unobservable Inputs Assumptions - Securitized Debt (Detail) - Loans held for investment - Variable Interest Entities, Primary Beneficiary, Aggregated Disclosure - Level 3	Jun. 30, 2019	Dec. 31, 2018
Discount Rate | Weighted Average
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Significant Inputs	0.035	0.043
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	0.10	0.10
CPR | Weighted Average
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Significant Inputs	0.090	0.093
CPR | Minimum
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Significant Inputs	0.06	0.06
CPR | Maximum
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Significant Inputs	0.15	0.18
CDR | Weighted Average
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Significant Inputs	0.014	0.014
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	0.17	0.23
Loss Severity | Weighted Average
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Significant Inputs	0.608	0.587
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	0.65