Fair Value - Carrying and Fair Values of Notes Payable Secured by Credit Risk Transfer and Mortgage Servicing Rights and Exchangeable Senior Notes (Detail) - USD ($) $ in Thousands	Sep. 30, 2024	Dec. 31, 2023
Notes Payable Secured by Credit Risk Transfer and Mortgage Servicing Assets [Member] | Carrying Value [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis Valuation Techniques [Line Items]
Notes payable	$ 2,830,108	$ 2,910,605
Notes Payable Secured by Credit Risk Transfer and Mortgage Servicing Assets [Member] | Fair Value [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis Valuation Techniques [Line Items]
Notes payable	2,846,037	2,904,678
Unsecured Senior Notes [Member] | Carrying Value [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis Valuation Techniques [Line Items]
Notes payable	814,915	600,458
Unsecured Senior Notes [Member] | Fair Value [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis Valuation Techniques [Line Items]
Notes payable	$ 818,647	$ 580,090