Summary of Significant Accounting Policies Carrying Amounts and Classification of Assets and Liabilities for Consolidated VIE's (Detail) - USD ($) $ in Millions	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2016
Variable Interest Entity [Line Items]
Current assets	$ 11,823	$ 12,292
Total assets	48,797	51,884	$ 63,179
Liabilities, Current	11,250	11,854
Long-term liabilities	$ 15,089	18,452
Variable Interest Entity, Primary Beneficiary, Aggregated Disclosure [Member]
Variable Interest Entity [Line Items]
Current assets		2
Assets, Noncurrent		53
Total assets		55
Liabilities, Current		6
Long-term liabilities		42
Liabilities		$ 48