Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents (Details) - USD ($) $ in Millions	Mar. 31, 2020	Sep. 30, 2019	Mar. 31, 2019	Sep. 30, 2018
Restricted Cash and Cash Equivalents Items [Line Items]
Cash and cash equivalents	$ 9,740	$ 7,838
Cash, cash equivalents, restricted cash and restricted cash equivalents	12,816	10,832	$ 10,282	$ 10,977
U.S. litigation escrow
Restricted Cash and Cash Equivalents Items [Line Items]
Restricted cash and restricted cash equivalents	1,264	1,205
Customer collateral
Restricted Cash and Cash Equivalents Items [Line Items]
Restricted cash and restricted cash equivalents	1,698	1,648
Prepaid expenses and other current assets
Restricted Cash and Cash Equivalents Items [Line Items]
Restricted cash and restricted cash equivalents	$ 114	$ 141