Leases - Additional Information (Details) $ in Millions	3 Months Ended	6 Months Ended
	Mar. 31, 2020 USD ($)	Mar. 31, 2020 USD ($) option_to_renew
Lessee, Lease, Description [Line Items]
Operating lease cost	$ 29	$ 55
Weighted average remaining lease term (in years)	7 years	7 years
Weighted average discount rate	2.28%	2.28%
Operating leases, not yet commenced, lease obligations	$ 465	$ 465
Minimum
Lessee, Lease, Description [Line Items]
Number of renewal options | option_to_renew		1
Operating leases, non-cancellable lease terms	3 years	3 years
Maximum
Lessee, Lease, Description [Line Items]
Operating leases, non-cancellable lease terms	15 years	15 years