Fair Value Measurements - Summary of Investment Income, Net Gains and Losses Realized and Disbursements Related to Investments (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended			12 Months Ended
	Jun. 30, 2022	Jun. 30, 2021	Jun. 30, 2022		Jun. 30, 2021	Sep. 30, 2021
Fair Value Disclosures [Abstract]
Investment income	$ 3	$ 2	$ 13	[1]	$ 10	$ 12	[1]
Net gains (losses)	(48)	15	(72)		26
Disbursements	$ 0	$ (6)	$ (28)		$ (25)	$ (33)

[1] Investment income relates to the demand deposit and includes interest income as well as dividend income transferred from the equity and fixed income mutual funds