Fair Value - Summary of Financial Statement Items Measured at Fair Value on Recurring Basis (Detail) - USD ($) $ in Thousands		Sep. 30, 2030	Sep. 30, 2029	Sep. 30, 2028	Sep. 30, 2027	Sep. 30, 2026	Sep. 30, 2025	Sep. 30, 2024	Jun. 30, 2024	Dec. 31, 2023	Sep. 30, 2023	Jun. 30, 2023	Dec. 31, 2022
Assets:
Short-term investments								$ 102,787		$ 128,338
Mortgage-backed securities at fair value								4,182,382		4,836,292
Loans acquired for sale at fair value								1,665,796		669,018
Derivative assets								66,103		89,591
Derivative assets, Netting								15,741		88,393
Total derivative assets after netting								81,844		177,984
Mortgage servicing rights at fair value								3,809,047	$ 3,941,861	3,919,107	$ 4,108,661	$ 3,977,938	$ 4,012,737
Liabilities:
Interest-only security payable at fair value		$ 35,098	$ 0	$ 0	$ 0	$ 0	$ 0	35,098		32,667
Derivative and credit risk transfer strip liabilities:
Derivative liabilities								20,556		54,250
Derivative liabilities, Netting								(17,880)		(49,561)
Total derivative liabilities after netting								2,676		4,689
Total derivative and credit risk transfer strip liabilities								16,151		51,381
CRT Derivatives [Member]
Assets:
Derivative assets	[1]							29,690		16,160
Total derivative assets after netting								29,690		16,160
Derivative and credit risk transfer strip liabilities:
Derivative liabilities	[1]							0		0
Interest Rate Lock Commitments [Member]
Assets:
Derivative assets	[1]							7,476		7,596
Total derivative assets after netting								7,476		7,596
Derivative and credit risk transfer strip liabilities:
Derivative liabilities	[1]							1,574		64
Forward Purchase Contracts [Member]
Assets:
Derivative assets	[1]							635		15,905
Derivative and credit risk transfer strip liabilities:
Derivative liabilities	[1]							7,452		490
Forward Sales Contracts [Member]
Assets:
Derivative assets	[1]							11,997		671
Derivative and credit risk transfer strip liabilities:
Derivative liabilities	[1]							11,530		50,363
Call Options on Interest Rate Futures Purchase Contracts [Member]
Assets:
Derivative assets	[1]							12,844		41,712
Derivative and credit risk transfer strip liabilities:
Derivative liabilities	[1]							0		2,005
Put Options On Interest Rate Futures Purchase Contracts
Assets:
Derivative assets	[1]							1,953		4,324
Derivative and credit risk transfer strip liabilities:
Derivative liabilities	[1]							0		0
MBS Call Options [Member]
Assets:
Derivative assets	[1]							0		3,218
Derivative and credit risk transfer strip liabilities:
Derivative liabilities	[1]							0		0
MBS Put Options [Member]
Assets:
Derivative assets	[1]							1,508		5
Derivative and credit risk transfer strip liabilities:
Derivative liabilities	[1]							0		0
Call Options On Interest Rate Futures Sell Contracts [Member]
Assets:
Derivative assets	[1]							0		0
Derivative and credit risk transfer strip liabilities:
Derivative liabilities	[1]							0		1,328
Recurring [Member]
Assets:
Short-term investments								102,787		128,338
Mortgage-backed securities at fair value								4,182,382		4,836,292
Loans acquired for sale at fair value								1,665,796		669,018
Loans at fair value								1,429,525		1,433,820
Total Assets								11,271,381		11,164,559
Derivative assets								66,103		89,591
Derivative assets, Netting								15,741		88,393
Total derivative assets after netting								81,844		177,984
Mortgage servicing rights at fair value								3,809,047		3,919,107
Liabilities:
Asset-backed financings of the variable interest entities at fair value								1,334,797		1,336,731
Interest-only security payable at fair value								35,098		32,667
Derivative and credit risk transfer strip liabilities:
Derivative liabilities								20,556		54,250
Derivative liabilities, Netting								(17,880)		(49,561)
Total derivative liabilities after netting								2,676		4,689
Total derivative and credit risk transfer strip liabilities								16,151		51,381
Total liabilities								1,386,046		1,420,779
Recurring [Member] | Credit Risk Transfer Strips [Member]
Derivative and credit risk transfer strip liabilities:
Derivative liabilities								13,475		46,692
Recurring [Member] | CRT Derivatives [Member]
Assets:
Derivative assets								29,690		16,160
Recurring [Member] | Level 1 [Member]
Assets:
Short-term investments								102,787		128,338
Mortgage-backed securities at fair value								0		0
Loans acquired for sale at fair value								0		0
Loans at fair value								0		0
Total Assets								117,584		174,374
Derivative assets								14,797		46,036
Derivative assets, Netting								0		0
Total derivative assets after netting								14,797		46,036
Mortgage servicing rights at fair value								0		0
Liabilities:
Asset-backed financings of the variable interest entities at fair value								0		0
Interest-only security payable at fair value								0		0
Derivative and credit risk transfer strip liabilities:
Derivative liabilities								0		3,333
Derivative liabilities, Netting								0		0
Total derivative liabilities after netting								0		3,333
Total derivative and credit risk transfer strip liabilities								0		3,333
Total liabilities								0		3,333
Recurring [Member] | Level 1 [Member] | Credit Risk Transfer Strips [Member]
Derivative and credit risk transfer strip liabilities:
Derivative liabilities								0		0
Recurring [Member] | Level 1 [Member] | CRT Derivatives [Member]
Assets:
Derivative assets								0		0
Recurring [Member] | Level 2 [Member]
Assets:
Short-term investments								0		0
Mortgage-backed securities at fair value								4,101,126		4,742,061
Loans acquired for sale at fair value								1,659,727		662,700
Loans at fair value								1,427,575		1,431,689
Total Assets								7,202,568		6,856,249
Derivative assets								14,140		19,799
Derivative assets, Netting								0		0
Total derivative assets after netting								14,140		19,799
Mortgage servicing rights at fair value								0		0
Liabilities:
Asset-backed financings of the variable interest entities at fair value								1,334,797		1,336,731
Interest-only security payable at fair value								0		0
Derivative and credit risk transfer strip liabilities:
Derivative liabilities								18,982		50,853
Derivative liabilities, Netting								0		0
Total derivative liabilities after netting								18,982		50,853
Total derivative and credit risk transfer strip liabilities								18,982		50,853
Total liabilities								1,353,779		1,387,584
Recurring [Member] | Level 2 [Member] | Credit Risk Transfer Strips [Member]
Derivative and credit risk transfer strip liabilities:
Derivative liabilities								0		0
Recurring [Member] | Level 2 [Member] | CRT Derivatives [Member]
Assets:
Derivative assets								0		0
Recurring [Member] | Level 3 [Member]
Assets:
Short-term investments								0		0
Mortgage-backed securities at fair value								81,256		94,231
Loans acquired for sale at fair value								6,069		6,318
Loans at fair value								1,950		2,131
Total Assets								3,935,488		4,045,543
Derivative assets								37,166		23,756
Derivative assets, Netting								0		0
Total derivative assets after netting								37,166		23,756
Mortgage servicing rights at fair value								3,809,047		3,919,107
Liabilities:
Asset-backed financings of the variable interest entities at fair value								0		0
Interest-only security payable at fair value								35,098		32,667
Derivative and credit risk transfer strip liabilities:
Derivative liabilities								1,574		64
Derivative liabilities, Netting								0		0
Total derivative liabilities after netting								1,574		64
Total derivative and credit risk transfer strip liabilities								15,049		46,756
Total liabilities								50,147		79,423
Recurring [Member] | Level 3 [Member] | Credit Risk Transfer Strips [Member]
Derivative and credit risk transfer strip liabilities:
Derivative liabilities								13,475		46,692
Recurring [Member] | Level 3 [Member] | CRT Derivatives [Member]
Assets:
Derivative assets								29,690		16,160
Recurring [Member] | Interest Rate Lock Commitments [Member]
Assets:
Derivative assets								7,476		7,596
Derivative and credit risk transfer strip liabilities:
Derivative liabilities								1,574		64
Recurring [Member] | Interest Rate Lock Commitments [Member] | Level 1 [Member]
Assets:
Derivative assets								0		0
Derivative and credit risk transfer strip liabilities:
Derivative liabilities								0		0
Recurring [Member] | Interest Rate Lock Commitments [Member] | Level 2 [Member]
Assets:
Derivative assets								0		0
Derivative and credit risk transfer strip liabilities:
Derivative liabilities								0		0
Recurring [Member] | Interest Rate Lock Commitments [Member] | Level 3 [Member]
Assets:
Derivative assets								7,476		7,596
Derivative and credit risk transfer strip liabilities:
Derivative liabilities								1,574		64
Recurring [Member] | Forward Purchase Contracts [Member]
Assets:
Derivative assets								635		15,905
Derivative and credit risk transfer strip liabilities:
Derivative liabilities								7,452		490
Recurring [Member] | Forward Purchase Contracts [Member] | Level 1 [Member]
Assets:
Derivative assets								0		0
Derivative and credit risk transfer strip liabilities:
Derivative liabilities								0		0
Recurring [Member] | Forward Purchase Contracts [Member] | Level 2 [Member]
Assets:
Derivative assets								635		15,905
Derivative and credit risk transfer strip liabilities:
Derivative liabilities								7,452		490
Recurring [Member] | Forward Purchase Contracts [Member] | Level 3 [Member]
Assets:
Derivative assets								0		0
Derivative and credit risk transfer strip liabilities:
Derivative liabilities								0		0
Recurring [Member] | Forward Sales Contracts [Member]
Assets:
Derivative assets								11,997		671
Derivative and credit risk transfer strip liabilities:
Derivative liabilities								11,530		50,363
Recurring [Member] | Forward Sales Contracts [Member] | Level 1 [Member]
Assets:
Derivative assets								0		0
Derivative and credit risk transfer strip liabilities:
Derivative liabilities								0		0
Recurring [Member] | Forward Sales Contracts [Member] | Level 2 [Member]
Assets:
Derivative assets								11,997		671
Derivative and credit risk transfer strip liabilities:
Derivative liabilities								11,530		50,363
Recurring [Member] | Forward Sales Contracts [Member] | Level 3 [Member]
Assets:
Derivative assets								0		0
Derivative and credit risk transfer strip liabilities:
Derivative liabilities								0		0
Recurring [Member] | Call Options on Interest Rate Futures Purchase Contracts [Member]
Assets:
Derivative assets								12,844		41,712
Derivative and credit risk transfer strip liabilities:
Derivative liabilities										2,005
Recurring [Member] | Call Options on Interest Rate Futures Purchase Contracts [Member] | Level 1 [Member]
Assets:
Derivative assets								12,844		41,712
Derivative and credit risk transfer strip liabilities:
Derivative liabilities										2,005
Recurring [Member] | Call Options on Interest Rate Futures Purchase Contracts [Member] | Level 2 [Member]
Assets:
Derivative assets								0		0
Derivative and credit risk transfer strip liabilities:
Derivative liabilities										0
Recurring [Member] | Call Options on Interest Rate Futures Purchase Contracts [Member] | Level 3 [Member]
Assets:
Derivative assets								0		0
Derivative and credit risk transfer strip liabilities:
Derivative liabilities										0
Recurring [Member] | Put Options On Interest Rate Futures Purchase Contracts
Assets:
Derivative assets								1,953		4,324
Recurring [Member] | Put Options On Interest Rate Futures Purchase Contracts | Level 1 [Member]
Assets:
Derivative assets								1,953		4,324
Recurring [Member] | Put Options On Interest Rate Futures Purchase Contracts | Level 2 [Member]
Assets:
Derivative assets								0		0
Recurring [Member] | Put Options On Interest Rate Futures Purchase Contracts | Level 3 [Member]
Assets:
Derivative assets								0		0
Recurring [Member] | MBS Call Options [Member]
Assets:
Derivative assets										3,218
Recurring [Member] | MBS Call Options [Member] | Level 1 [Member]
Assets:
Derivative assets										0
Recurring [Member] | MBS Call Options [Member] | Level 2 [Member]
Assets:
Derivative assets										3,218
Recurring [Member] | MBS Call Options [Member] | Level 3 [Member]
Assets:
Derivative assets										0
Recurring [Member] | MBS Put Options [Member]
Assets:
Derivative assets								1,508		5
Recurring [Member] | MBS Put Options [Member] | Level 1 [Member]
Assets:
Derivative assets								0		0
Recurring [Member] | MBS Put Options [Member] | Level 2 [Member]
Assets:
Derivative assets								1,508		5
Recurring [Member] | MBS Put Options [Member] | Level 3 [Member]
Assets:
Derivative assets								$ 0		0
Recurring [Member] | Call Options On Interest Rate Futures Sell Contracts [Member]
Derivative and credit risk transfer strip liabilities:
Derivative liabilities										1,328
Recurring [Member] | Call Options On Interest Rate Futures Sell Contracts [Member] | Level 1 [Member]
Derivative and credit risk transfer strip liabilities:
Derivative liabilities										1,328
Recurring [Member] | Call Options On Interest Rate Futures Sell Contracts [Member] | Level 2 [Member]
Derivative and credit risk transfer strip liabilities:
Derivative liabilities										0
Recurring [Member] | Call Options On Interest Rate Futures Sell Contracts [Member] | Level 3 [Member]
Derivative and credit risk transfer strip liabilities:
Derivative liabilities										$ 0

[1] All hedging derivatives are interest rate derivatives that are used as economic hedges.