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Fair Value - Summary of Reverse Mortgage Loans Held for Investment, Subject to HMBS Related Obligations (Detail)

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Fair Value - Summary of Reverse Mortgage Loans Held for Investment, Subject to HMBS Related Obligations (Detail) - Reverse Mortgage Loans Held for Investment, Subject to HMBS Related Obligations [Member]
9 Months Ended 12 Months Ended
Sep. 30, 2021
Dec. 31, 2020
Conditional repayment rate [Member] | Weighted Average [Member]    
Fair Value Reverse Mortgage Loans Held For Investment Measured On Recurring And Nonrecurring Basis Fair Value Measurement [Line Items]    
Conditional repayment rate 20.80% 20.00%
Loss frequency [Member] | Weighted Average [Member]    
Fair Value Reverse Mortgage Loans Held For Investment Measured On Recurring And Nonrecurring Basis Fair Value Measurement [Line Items]    
Loss frequency 4.80% 4.40%
Loss severity [Member] | Minimum [Member]    
Fair Value Reverse Mortgage Loans Held For Investment Measured On Recurring And Nonrecurring Basis Fair Value Measurement [Line Items]    
Loss severity 4.10% 5.10%
Loss severity [Member] | Maximum [Member]    
Fair Value Reverse Mortgage Loans Held For Investment Measured On Recurring And Nonrecurring Basis Fair Value Measurement [Line Items]    
Loss severity 11.80% 13.30%
Loss severity [Member] | Weighted Average [Member]    
Fair Value Reverse Mortgage Loans Held For Investment Measured On Recurring And Nonrecurring Basis Fair Value Measurement [Line Items]    
Loss severity 4.40% 5.40%
Discount rate [Member] | Weighted Average [Member]    
Fair Value Reverse Mortgage Loans Held For Investment Measured On Recurring And Nonrecurring Basis Fair Value Measurement [Line Items]    
Discount rate 2.20% 1.60%
Average draw rate 1.10% 1.10%