Fair Value Measurements - Summary of Investment Portfolio (Details) - USD ($) $ in Millions		3 Months Ended		9 Months Ended			12 Months Ended
		Jun. 30, 2021	Jun. 30, 2020	Jun. 30, 2021		Jun. 30, 2020	Sep. 30, 2020
Fair Value Disclosures [Abstract]
Original cost		$ 335		$ 335			$ 335
Accumulated adjustments, net	[1]	(50)		(50)			(30)
Adjusted cost		285		285			305
Investment income		2	$ 2	10	[2]	$ 8	10	[2]
Net unrealized gain	[3]			55			46
Realized gains	[3]			16			2
Settlement funds				1			3
Disbursements		(6)	$ (7)	(25)		$ (26)	(35)
Fair value		$ 342		$ 342			$ 331

[1] The accumulated adjustments include investment income, realized gains, disbursements and settlements recorded in previous periods.

[2] Investment income for the demand deposit includes interest income as well as dividend income transferred from the equity and fixed income mutual funds.

[3] Presented under the original cost method.