Leases - Component of Lease Expense (Details) $ in Millions	12 Months Ended
	Sep. 30, 2020 USD ($)
Leases [Abstract]
Operating lease cost	$ 102
Finance lease cost:
Amortization of ROU assets	5
Interest on lease liabilities	3
Variable lease cost	5
Short-term lease cost	3
Total lease cost	118
Cash paid related to lease liabilities:
Operating cash flows — operating leases	103
Operating cash flows — finance leases	3
Financing cash flows — finance leases	4
Non-cash lease liability activities:
ROU assets obtained in exchange for operating lease liabilities (including the impact upon adoption)	506
ROU assets obtained in exchange for finance lease liabilities	$ 22
Weighted-average remaining lease term
Operating leases	6 years 2 months 12 days
Finance leases	2 years 4 months 24 days
Weighted-average discount rate
Operating leases	3.80%
Finance leases	1.90%