Debt - Additional Information (Detail) - USD ($)	3 Months Ended	12 Months Ended
	Sep. 30, 2017	Jun. 30, 2017	Oct. 31, 2017
Debt Instrument [Line Items]
Commercial paper	$ 8,200,000,000	$ 9,100,000,000
Credit facility, financial covenant	As of September 30, 2017, we were in compliance with the only financial covenant in both credit agreements, which requires us to maintain a coverage ratio of at least three times earnings before interest, taxes, depreciation, and amortization to interest expense, as defined in the credit agreements.
Credit facility, drawn	$ 0	0
Long-term debt	77,300,000,000	77,100,000,000
Long-term debt fair value	81,300,000,000	80,300,000,000
Debt issuance costs and unamortized discount	702,000,000	$ 715,000,000
Credit Facility Expire on October 31st 2017
Debt Instrument [Line Items]
Credit facility	$ 5,000,000,000
Credit facility, expiration date	Oct. 31, 2017
Credit Facility Expire on November 14th 2018
Debt Instrument [Line Items]
Credit facility	$ 5,000,000,000
Credit facility, expiration date	Nov. 14, 2018
New Credit Facility One Effective from October 2017 | Scenario, Plan
Debt Instrument [Line Items]
Credit facility			$ 5,000,000,000
New Credit Facility Two Effective from October 2017 | Scenario, Plan
Debt Instrument [Line Items]
Credit facility			$ 5,000,000,000
Commercial paper
Debt Instrument [Line Items]
Commercial paper, weighted average interest rate	1.20%	1.01%
Commercial paper | Lower Limit
Debt Instrument [Line Items]
Commercial paper, maturity term	34 days	25 days
Commercial paper | Upper Limit
Debt Instrument [Line Items]
Commercial paper, maturity term	196 days	264 days