Fair Value - Quantitative Summary of Key Assumptions Used in Valuation of MSRs as of Dates Presented, and Effect on Estimated Fair Value from Adverse Changes in Those Inputs (Detail) $ in Thousands	9 Months Ended	12 Months Ended
	Sep. 30, 2024 USD ($) UsdPerLoan	Dec. 31, 2023 USD ($) UsdPerLoan	Jun. 30, 2024 USD ($)	Sep. 30, 2023 USD ($)	Jun. 30, 2023 USD ($)	Dec. 31, 2022 USD ($)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Balance at end of period	$ 3,809,047	$ 3,919,107	$ 3,941,861	$ 4,108,661	$ 3,977,938	$ 4,012,737
Unpaid principal balance of underlying loans, Fair Value	$ 228,127,324	$ 230,294,583
Weighted-average annual servicing fee rate (in basis points), Fair value input	28.00%	28.00%
Weighted-average note interest rate, Fair value	3.80%	3.70%
Pricing Spread [Member] | Effect On Value Of Five Percentage Adverse Change
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Effect on value of percentage adverse change, Fair value input	$ (46,966)	$ (48,362)
Pricing Spread [Member] | Effect On Value Of Ten Percentage Adverse Change
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Effect on value of percentage adverse change, Fair value input	(92,816)	(95,575)
Pricing Spread [Member] | Effect On Value Of Twenty Percentage Adverse Change
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Effect on value of percentage adverse change, Fair value input	(181,313)	(186,699)
Prepayment Speed [Member] | Effect On Value Of Five Percentage Adverse Change
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Effect on value of percentage adverse change, Fair value input	(57,286)	(53,964)
Prepayment Speed [Member] | Effect On Value Of Ten Percentage Adverse Change
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Effect on value of percentage adverse change, Fair value input	(112,596)	(106,144)
Prepayment Speed [Member] | Effect On Value Of Twenty Percentage Adverse Change
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Effect on value of percentage adverse change, Fair value input	(217,698)	(205,509)
Cost of Servicing [Member] | Effect On Value Of Five Percentage Adverse Change
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Effect on value of percentage adverse change, Fair value input	(16,607)	(17,276)
Cost of Servicing [Member] | Effect On Value Of Ten Percentage Adverse Change
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Effect on value of percentage adverse change, Fair value input	(33,214)	(34,551)
Cost of Servicing [Member] | Effect On Value Of Twenty Percentage Adverse Change
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Effect on value of percentage adverse change, Fair value input	$ (66,428)	$ (69,103)
Minimum [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Estimated fair value inputs, Pricing spread	7.40%	6.50%
Estimated fair value inputs, Prepayment speed	5.40%	5.50%
Estimated fair value inputs, Annual per-loan cost of servicing | UsdPerLoan	68	70
Minimum [Member] | Mortgage service rights [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Estimated fair value inputs, Life (in years)	2 years 6 months	2 years 8 months 12 days
Maximum [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Estimated fair value inputs, Pricing spread	18.80%	17.90%
Estimated fair value inputs, Prepayment speed	8.10%	8.50%
Estimated fair value inputs, Annual per-loan cost of servicing | UsdPerLoan	89	89
Maximum [Member] | Mortgage service rights [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Estimated fair value inputs, Life (in years)	8 years 6 months	9 years 4 months 24 days
Weighted Average [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Estimated fair value inputs, Pricing spread	5.40%	5.50%
Estimated fair value inputs, Prepayment speed	7.50%	7.00%
Estimated fair value inputs, Annual per-loan cost of servicing | UsdPerLoan	68	70
Weighted Average [Member] | Mortgage service rights [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Estimated fair value inputs, Life (in years)	8 years 2 months 12 days	8 years 6 months