Fair Value Measurements and Investments - Additional Information (Detail) - USD ($)		3 Months Ended		6 Months Ended
	Feb. 01, 2020	Mar. 31, 2020	Mar. 31, 2019	Mar. 31, 2020	Mar. 31, 2019	Sep. 30, 2019
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Transfers between Level 1 and Level 2 assets		$ 0		$ 0
U.S. securities		3,400,000,000		3,400,000,000		$ 6,300,000,000
Upward adjustments		0		9,000,000
Downward adjustments				0
Impairment charges of non-marketable equity securities		0	$ 0	0	$ 0
Impairment of indefinite-lived intangible assets and goodwill	$ 0
Carrying value of long-term debt		$ 17,892,000,000		$ 17,892,000,000		16,729,000,000
Minimum
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale investment securities, stated maturities		1 year		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		5 years
Senior Notes
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Carrying value of long-term debt		$ 16,900,000,000		$ 16,900,000,000		16,700,000,000
Estimated Fair Value | Senior Notes
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Estimated fair value of long-term deb		$ 18,600,000,000		$ 18,600,000,000		$ 18,400,000,000