Fair Value Measurements - Unobservable Inputs Assumptions - Securitized Debt (Detail) - Loans held for investment - Variable Interest Entities, Primary Beneficiary, Aggregated Disclosure	Jun. 30, 2018	Dec. 31, 2017
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Significant Inputs	0.044	0.041
CPR | Minimum | Level 3
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Significant Inputs	0.06	0.06
CPR | Maximum | Level 3
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Significant Inputs	0.15	0.15
CDR | Minimum | Level 3
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Significant Inputs	0.00	0.00
CDR | Maximum | Level 3
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Significant Inputs	0.08	0.06
Loss Severity | Minimum | Level 3
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Significant Inputs	0.35	0.35
Loss Severity | Maximum | Level 3
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Significant Inputs	0.60	0.60