Financial Statements - Carrying Amounts and Classification of Assets and Liabilities for Consolidated VIEs (Detail) - USD ($) $ in Millions	Mar. 31, 2018	Sep. 30, 2017
Variable Interest Entity [Line Items]
Current assets	$ 12,271	$ 12,292
Current liabilities	10,992	11,854
Long-term liabilities	$ 16,845	18,452
Variable Interest Entity, Primary Beneficiary [Member]
Variable Interest Entity [Line Items]
Current assets		2
Noncurrent assets		53
Total assets		55
Current liabilities		6
Long-term liabilities		42
Total liabilities		$ 48