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Fair Value Measurements and Investments - Additional Information (Detail) - USD ($)
$ in Millions
12 Months Ended
Feb. 01, 2018
Sep. 30, 2018
Sep. 30, 2017
Sep. 30, 2016
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]        
Impairment losses, investments   $ 0 $ 0 $ 0
Fair value, assets, level 1 to level 2 transfers, amount   0    
Non-marketable equity investments   137 94  
Goodwill and intangible asset impairment $ 0      
Long-term debt (Note 6)   16,630 16,618  
Trading assets, mutual fund investments related to various employee compensation plans   98 82  
Senior Notes        
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]        
Long-term debt (Note 6)   16,600 18,400  
Estimate of Fair Value Measurement | Senior Notes        
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]        
Estimated Fair Value   $ 16,600 $ 19,200