Fair Value - Quantitative Summary of Key Unobservable Inputs Used in Review and Approval of Broker-provided Fair Values for CRT Derivatives (Detail) - CRT Derivatives [Member] - USD ($) $ in Thousands	9 Months Ended	12 Months Ended
	Sep. 30, 2024	Dec. 31, 2023
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value	$ 29,690	$ 16,160
UPB of loans in reference pools	$ 5,071,449	$ 5,437,551
Discount Rate [Member] | Minimum [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Key inputs used in fair value for CRT agreements	8.40%	9.00%
Discount Rate [Member] | Maximum [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Key inputs used in fair value for CRT agreements	11.70%	9.70%
Discount Rate [Member] | Weighted Average [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Key inputs used in fair value for CRT agreements	8.70%	9.60%
Voluntary Prepayment Speed [Member] | Minimum [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Key inputs used in fair value for CRT agreements	6.80%	6.90%
Voluntary Prepayment Speed [Member] | Maximum [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Key inputs used in fair value for CRT agreements	7.10%	7.60%
Voluntary Prepayment Speed [Member] | Weighted Average [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Key inputs used in fair value for CRT agreements	7.10%	7.40%
Involuntary Prepayment Speed [Member] | Minimum [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Key inputs used in fair value for CRT agreements	0.10%	0.20%
Involuntary Prepayment Speed [Member] | Maximum [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Key inputs used in fair value for CRT agreements	0.10%	0.80%
Involuntary Prepayment Speed [Member] | Weighted Average [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Key inputs used in fair value for CRT agreements	0.10%	0.30%
Measurement Input, Loss Severity | Minimum [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Key inputs used in fair value for CRT agreements	0.00%	0.20%
Measurement Input, Loss Severity | Maximum [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Key inputs used in fair value for CRT agreements	0.20%	0.30%
Measurement Input, Loss Severity | Weighted Average [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Key inputs used in fair value for CRT agreements	0.10%	0.30%