Fair Value - Summary of Key Unobservable Inputs Used in Valuation Credit Risk Transfer Strip Liabilities (Detail) - Credit Risk Transfer Strips [Member] - USD ($) $ in Thousands	Jun. 30, 2024	Dec. 31, 2023
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value of CRT derivatives liabilities	$ 16,974	$ 46,692
Unpaid principal balance of loans in the reference pools	$ 17,006,523	$ 17,714,679
Maximum [Member] | Discount Rate [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Key inputs used in valuation	9.40%	9.60%
Maximum [Member] | Voluntary Prepayment Speed [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Key inputs used in valuation	7.40%	8.20%
Maximum [Member] | Involuntary Prepayment Speed [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Key inputs used in valuation	0.20%	0.30%
Maximum [Member] | Remaining Loss Expectation [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Key inputs used in valuation	1.50%	1.60%
Minimum [Member] | Discount Rate [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Key inputs used in valuation	8.20%	7.90%
Minimum [Member] | Voluntary Prepayment Speed [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Key inputs used in valuation	7.30%	6.60%
Minimum [Member] | Involuntary Prepayment Speed [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Key inputs used in valuation	0.10%	0.20%
Minimum [Member] | Remaining Loss Expectation [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Key inputs used in valuation	0.40%	0.50%
Weighted Average [Member] | Discount Rate [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Key inputs used in valuation	9.30%	9.40%
Weighted Average [Member] | Voluntary Prepayment Speed [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Key inputs used in valuation	7.30%	6.80%
Weighted Average [Member] | Involuntary Prepayment Speed [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Key inputs used in valuation	0.10%	0.20%
Weighted Average [Member] | Remaining Loss Expectation [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Key inputs used in valuation	0.50%	0.60%