Fair Value Measurements and Investments - Additional Information (Detail) - USD ($)	3 Months Ended	6 Months Ended
	Mar. 31, 2019	Mar. 31, 2019	Sep. 30, 2018
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale Securities	$ 7,200,000,000	$ 7,200,000,000	$ 7,500,000,000
Non-marketable equity investments	438,000,000	438,000,000	137,000,000
Equity Securities without Readily Determinable Fair Value, Upward Price Adjustment, Annual Amount	66,000,000	66,000,000
Equity Securities without Readily Determinable Fair Value, Downward Price Adjustment, Annual Amount	0	0
Realized Gain (Loss) on Marketable Securities, Cost Method Investments, and Other Investments	15,000,000	15,000,000
Equity Securities, FV-NI, Unrealized Gain (Loss)	79,000,000	59,000,000
Long-term Debt	16,630,000,000	$ 16,630,000,000	16,630,000,000
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	16,600,000,000	$ 16,600,000,000	16,600,000,000
Estimated Fair Value | Senior Notes
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Long-term Debt, Fair Value	$ 17,300,000,000	$ 17,300,000,000	$ 16,600,000,000