Financial Instruments and Fair Value Measurements - Additional Information (Detail) - USD ($) $ in Millions	6 Months Ended
	Mar. 31, 2019	Sep. 30, 2018
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash and cash equivalents, fair value	$ 25	$ 228
Remaining cash equivalents	661	913
Fair value of long-term debt	18,200	18,800
Fair value of debt reclassified from long term to short term	$ 3,000	$ 1,900
Minimum
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Maturity period of short-term investments at the time of purchase	3 months
Maximum
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Maturity period of short-term investments at the time of purchase	1 year