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Debt - Additional Information (Detail) - USD ($)
1 Months Ended 12 Months Ended
Sep. 30, 2018
Oct. 11, 2017
Jan. 27, 2017
Sep. 30, 2017
Sep. 30, 2018
Sep. 30, 2017
Debt Instrument [Line Items]            
Net aggregate proceeds $ 16,630,000,000     $ 16,618,000,000 $ 16,630,000,000 $ 16,618,000,000
Revolving Credit Facility            
Debt Instrument [Line Items]            
Maturity period for senior notes     5 years      
Maximum borrowing capacity     $ 4,000,000,000      
Credit facility amount outstanding 0     0 0 0
Commercial Paper            
Debt Instrument [Line Items]            
Commercial paper program, amount available $ 3,000,000,000       3,000,000,000  
Commercial paper program, maturity period P397D          
Commercial paper program, amount outstanding $ 0     0 0 0
Senior Notes            
Debt Instrument [Line Items]            
Net aggregate proceeds 16,600,000,000     18,400,000,000 16,600,000,000 18,400,000,000
long-term debt, current maturities, gross 0       0  
Interest expense         550,000,000 505,000,000
Senior Notes | September 2017 Notes            
Debt Instrument [Line Items]            
Principal amount       2,500,000,000   2,500,000,000
Net aggregate proceeds       2,500,000,000   2,500,000,000
Senior Notes | 2017 Notes            
Debt Instrument [Line Items]            
long-term debt, current maturities, gross $ 0     $ 1,750,000,000 $ 0 $ 1,750,000,000
Repayments of debt   $ 1,750,000,000        
Senior Notes | Minimum | September 2017 Notes            
Debt Instrument [Line Items]            
Maturity period for senior notes       5 years    
Senior Notes | Maximum | September 2017 Notes            
Debt Instrument [Line Items]            
Maturity period for senior notes       30 years