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Summary of Significant Accounting Policies Carrying Amounts and Classification of Assets and Liabilities for Consolidated VIE's (Detail) - USD ($)
$ in Millions
Sep. 30, 2017
Sep. 30, 2016
Sep. 30, 2015
Variable Interest Entity [Line Items]      
Current assets $ 12,292 $ 17,109  
Total assets 51,884 63,179 $ 29,590
Liabilities, Current 11,854 16,331  
Long-term liabilities 18,452 21,524  
Variable Interest Entity, Primary Beneficiary, Aggregated Disclosure [Member]      
Variable Interest Entity [Line Items]      
Current assets 2 284  
Assets, Noncurrent 53 98  
Total assets 55 382  
Liabilities, Current 6 230  
Long-term liabilities 42 29  
Liabilities $ 48 $ 259