Leases - Additional Information (Details) - USD ($) $ in Millions	12 Months Ended
	Sep. 30, 2024	Sep. 30, 2023	Sep. 30, 2022
Lessee, Lease, Description [Line Items]
Renewal term (in years)	10 years
Operating Lease, Right-of-Use Asset, Statement of Financial Position [Extensible Enumeration]	Other assets	Other assets
ROU assets	$ 873	$ 488
Operating Lease, Liability, Current, Statement of Financial Position [Extensible List]	Unsettled repurchases	Unsettled repurchases
Current portion of lease liabilities	$ 150	$ 106
Operating Lease, Liability, Noncurrent, Statement of Financial Position [Extensible List]	Other liabilities	Other liabilities
Noncurrent portion of lease liabilities	$ 685	$ 412
Operating lease cost	$ 179	$ 129	$ 117
Weighted average remaining lease term (in years)	8 years	8 years
Weighted average discount rate	3.51%	2.43%
Right-of-use asset obtained in exchange for operating lease liability	$ 410	$ 82	$ 74