Summary of Significant Accounting Policies - Leases (Details) - USD ($) $ in Millions		12 Months Ended
	Oct. 01, 2019	Sep. 30, 2018	Sep. 30, 2020
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Accounting Standards Update [Extensible List]	us-gaap:AccountingStandardsUpdate201602Member	us-gaap:AccountingStandardsUpdate201409Member
Operating lease ROU assets			$ 415
Lease liabilities			$ 421
ASU 2016-02
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Operating lease ROU assets	$ 452
Lease liabilities	$ 452