Fair Value Measurements and Investments - Schedule of Non-Marketable Equity Securities (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Mar. 31, 2025	Mar. 31, 2024	Mar. 31, 2025	Mar. 31, 2024	Sep. 30, 2024
Fair Value Disclosures [Abstract]
Initial cost basis	$ 702		$ 702		$ 711
Adjustments:
Upward adjustments	850		850		910
Downward adjustments, including impairment	(494)		(494)		(465)
Carrying amount	1,058		1,058		$ 1,156
Upward adjustments	7	$ 0	7	$ 9
Downward adjustments, including impairment	$ (31)	$ (15)	$ (49)	$ (15)