Financial Instruments and Fair Value Measurements - Additional Information (Detail) - USD ($) $ in Millions	3 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Sep. 30, 2017
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Remaining cash equivalents	$ 1,120		$ 12,153
Fair value of long-term debt	22,700		19,200
Fair value of debt reclassified from long term to short term	703		$ 206
Transfer of assets in and out of level 1, 2 and 3 measurements during the period	0	$ 0
Transfer of liabilities in and out of level 1, 2 and 3 measurements during the period	$ 0	$ 0
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