Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents (Details) - USD ($) $ in Millions	Mar. 31, 2021	Sep. 30, 2020	Mar. 31, 2020	Sep. 30, 2019
Restricted Cash and Cash Equivalents Items [Line Items]
Cash and cash equivalents	$ 16,494	$ 16,289
Cash, cash equivalents, restricted cash and restricted cash equivalents	19,558	19,171	$ 12,816	$ 10,832
U.S. litigation escrow
Restricted Cash and Cash Equivalents Items [Line Items]
Restricted cash and restricted cash equivalents	894	901
Customer collateral
Restricted Cash and Cash Equivalents Items [Line Items]
Restricted cash and restricted cash equivalents	2,057	1,850
Prepaid expenses and other current assets
Restricted Cash and Cash Equivalents Items [Line Items]
Restricted cash and restricted cash equivalents	$ 113	$ 131