Debt (Short-term Debt) (Details) - USD ($)
$ in Millions
 3 Months Ended 6 Months Ended
Mar. 31, 2016
Sep. 30, 2015
Mar. 31, 2016
Short-term Debt [Line Items]      
Current portion of long-term debt $ 300 $ 0 $ 300
Short-term debt 1,120 448 1,120
Short-term commercial paper borrowings [Member]      
Short-term Debt [Line Items]      
Short-term commercial paper borrowings outstanding $ 820 [1] $ 448 $ 820 [1]
Weighted Average Interest Rate of commercial paper borrowings 0.76% 0.52% 0.76%
Weighted average maturity period of commercial paper borrowings (days) 17 days 25 days  
Maximum Amount of Commercial Paper Outstanding     $ 929
Line of credit facility, maximum borrowing capacity $ 1,200   $ 1,200
[1] The maximum amount of short-term commercial paper borrowings outstanding during the six months ended March 31, 2016 was $929 million