Income Taxes - Additional Information (Detail) - USD ($)		12 Months Ended
	Jul. 22, 2020	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2017
Tax Credit Carryforward [Line Items]
US income before taxes		$ 9,178,000,000	$ 9,536,000,000	$ 8,088,000,000
Non-recurring, non-cash tax expense related to the remeasurement of UK deferred tax liabilities	$ 329,000,000
Deferred tax assets, net		$ 798,000,000	$ 698,000,000
Income tax provision, percent		21.00%	19.00%	20.00%
U.S. federal income tax at statutory rate, percent		21.00%	21.00%	24.50%
Tax Cuts and Jobs Act, transition tax				$ 1,100,000,000
Tax Cuts and Jobs Act, change in tax rate benefit				1,100,000,000
Tax benefits due to various non-recurring audit settlements				161,000,000
Income taxes receivable included in prepaid and other current assets		$ 93,000,000	$ 130,000,000
Income taxes payable included in accrued taxes as part of accrued liabilities		134,000,000	327,000,000
Accrued income taxes included in other long-term liabilities		2,800,000,000	2,500,000,000
Decreased Singapore tax as a result of the tax incentive agreement		$ (280,000,000)	$ (324,000,000)	$ (295,000,000)
Benefit of the tax incentive agreement on diluted net income per share (in dollars per share)		$ 0.13	$ 0.14	$ 0.13
Total unrecognized tax benefits exclusive of interest and penalties		$ 2,579,000,000	$ 2,234,000,000	$ 1,658,000,000	$ 1,353,000,000
Unrecognized tax benefits, if recognized, would reduce the effective tax rate in a future period		1,600,000,000	1,400,000,000	1,200,000,000
Interest expense included in interest expense and administrative and other		68,000,000	66,000,000	15,000,000
Accrued penalties related to uncertain tax positions		4,000,000	5,000,000	0
Accrued interest related to uncertain tax positions in other long term liabilities		233,000,000	165,000,000
Accrued penalties related to uncertain tax positions in other long term liabilities		31,000,000	26,000,000
Domestic Tax Authority
Tax Credit Carryforward [Line Items]
Net operating loss carryforwards		13,000,000
State and Local Jurisdiction
Tax Credit Carryforward [Line Items]
Net operating loss carryforwards		16,000,000
Foreign Country
Tax Credit Carryforward [Line Items]
Net operating loss carryforwards		367,000,000
Other Assets
Tax Credit Carryforward [Line Items]
Deferred tax assets, net		63,000,000	24,000,000
Non-current income tax receivable		988,000,000	771,000,000
Non United States Customers
Tax Credit Carryforward [Line Items]
US income before taxes		$ 3,000,000,000.0	$ 3,000,000,000.0	$ 2,700,000,000