Leases - Additional Information (Details) - USD ($) $ in Millions	12 Months Ended
	Sep. 30, 2025	Sep. 30, 2024	Sep. 30, 2023
Leases [Abstract]
Renewal term (in years)	10 years
Operating Lease, Right-of-Use Asset, Statement of Financial Position [Extensible Enumeration]	Other assets	Other assets
ROU assets	$ 954	$ 873
Operating Lease, Liability, Current, Statement of Financial Position [Extensible List]	Unsettled repurchases	Unsettled repurchases
Current portion of lease liabilities	$ 150	$ 150
Operating Lease, Liability, Noncurrent, Statement of Financial Position [Extensible List]	Other liabilities	Other liabilities
Noncurrent portion of lease liabilities	$ 763	$ 685
Operating lease cost	$ 195	$ 179	$ 129
Weighted average remaining lease term (in years)	9 years	8 years
Weighted average discount rate	4.11%	3.51%
Right-of-use asset obtained in exchange for operating lease liability	$ 231	$ 410	$ 82