Fair Value - Summary of Key Unobservable Inputs Used in Valuation of Firm Commitment to Purchase CRT Securities (Detail) - Firm commitment to purchase CRT securities [Member]	Jun. 30, 2019	Dec. 31, 2018
Discount Rate [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Key inputs used in valuation	0.067	0.079
Voluntary Prepayment Speed [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Key inputs used in valuation	0.164	0.124
Involuntary Prepayment Speed [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Key inputs used in valuation	0.001	0.001
Remaining Loss Expectation [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Key inputs used in valuation	0.001	0.001