v3.26.3
Fair Value - Summary of Key Unobservable Inputs Used in Valuation Credit Risk Transfer Strip Liabilities (Detail) - Credit Risk Transfer Strips [Member] - USD ($)
$ in Thousands
Sep. 30, 2026
Dec. 31, 2025
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Fair value of CRT derivatives liabilities $ 5,231 $ 5,999
Unpaid principal balance of loans in the reference pools $ 13,971,976 $ 14,961,848
Maximum [Member] | Discount Rate [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Key inputs used in valuation 9.80% 8.60%
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% 7.50%
Maximum [Member] | Involuntary Prepayment Speed [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Key inputs used in valuation 0.30% 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 1.40% 1.40%
Minimum [Member] | Discount Rate [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Key inputs used in valuation 6.10% 5.00%
Minimum [Member] | Voluntary Prepayment Speed [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Key inputs used in valuation 7.10% 7.00%
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% 0.10%
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% 0.40%
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% 8.10%
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 7.10% 7.10%
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 0.20% 0.10%
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% 0.50%