Fair Value Measurements - Unobservable Inputs Assumptions - Non-Agency RMBS Held for Investment (Detail) - Level 3	Jun. 30, 2018	Dec. 31, 2017
Weighted Average Discount Rate | Non-Agency RMBS - Senior
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Significant Inputs	0.046	0.044
Weighted Average Discount Rate | Non-Agency RMBS - Senior interest-only
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Significant Inputs	0.100	0.112
Weighted Average Discount Rate | Non-Agency RMBS - Subordinated
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Significant Inputs	0.054	0.058
Weighted Average Discount Rate | Non-Agency RMBS - Subordinated interest-only
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Significant Inputs	0.104	0.120
CPR | Minimum | Non-Agency RMBS - Senior
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Significant Inputs	0.02	0.01
CPR | Minimum | Non-Agency RMBS - Senior interest-only
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Significant Inputs	0.04	0.03
CPR | Minimum | Non-Agency RMBS - Subordinated
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Significant Inputs	0.01	0.01
CPR | Minimum | Non-Agency RMBS - Subordinated interest-only
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Significant Inputs	0.04	0.06
CPR | Maximum | Non-Agency RMBS - Senior
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Significant Inputs	0.22	0.45
CPR | Maximum | Non-Agency RMBS - Senior interest-only
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Significant Inputs	0.31	0.25
CPR | Maximum | Non-Agency RMBS - Subordinated
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Significant Inputs	0.23	0.20
CPR | Maximum | Non-Agency RMBS - Subordinated interest-only
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Significant Inputs	0.25	0.22
CDR | Minimum | Non-Agency RMBS - Senior
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Significant Inputs	0.00	0.00
CDR | Minimum | Non-Agency RMBS - Senior interest-only
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Significant Inputs	0.00	0.00
CDR | Minimum | Non-Agency RMBS - Subordinated
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Significant Inputs	0.00	0.00
CDR | Minimum | Non-Agency RMBS - Subordinated interest-only
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Significant Inputs	0.00	0.00
CDR | Maximum | Non-Agency RMBS - Senior
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Significant Inputs	0.52	0.55
CDR | Maximum | Non-Agency RMBS - Senior interest-only
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Significant Inputs	0.59	0.50
CDR | Maximum | Non-Agency RMBS - Subordinated
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Significant Inputs	0.45	0.33
CDR | Maximum | Non-Agency RMBS - Subordinated interest-only
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Significant Inputs	0.15	0.09
Loss Severity | Minimum | Non-Agency RMBS - Senior
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Significant Inputs	0.35	0.35
Loss Severity | Minimum | Non-Agency RMBS - Senior interest-only
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Significant Inputs	0.35	0.35
Loss Severity | Minimum | Non-Agency RMBS - Subordinated
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Significant Inputs	0.10	0.35
Loss Severity | Minimum | Non-Agency RMBS - Subordinated interest-only
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Significant Inputs	0.35	0.35
Loss Severity | Maximum | Non-Agency RMBS - Senior
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Significant Inputs	0.95	0.89
Loss Severity | Maximum | Non-Agency RMBS - Senior interest-only
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Significant Inputs	0.95	0.95
Loss Severity | Maximum | Non-Agency RMBS - Subordinated
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Significant Inputs	0.71	0.69
Loss Severity | Maximum | Non-Agency RMBS - Subordinated interest-only
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Significant Inputs	0.58	0.63