Fair Value Measurements and Investments - Additional Information (Detail) - USD ($) $ in Millions	3 Months Ended
	Dec. 31, 2018	Sep. 30, 2018
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale Securities	$ 7,500	$ 7,500
Non-marketable equity investments	159	137
Equity Securities, FV-NI, Unrealized Gain (Loss)	(20)
Realized Gain (Loss) on Marketable Securities, Cost Method Investments, and Other Investments	0
Long-term Debt	$ 16,633	16,630
Minimum
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale investment securities, stated maturities	1 year
Maximum
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale investment securities, stated maturities	5 years
Senior Notes
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Long-term Debt	$ 16,600	16,600
Estimated Fair Value | Senior Notes
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Long-term Debt, Fair Value	$ 16,700	$ 16,600