Fair Value Measurements - Summary of Investment Portfolio (Details) - USD ($) $ in Millions		3 Months Ended		9 Months Ended			12 Months Ended
		Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020		Jun. 30, 2019	Sep. 30, 2019
Schedule of Available-for-sale Securities [Line Items]
Original cost		$ 335		$ 335			$ 335
Accumulated adjustments, net	[1]	(30)		(30)			(47)
Adjusted cost		305		305			288
Investment income		2	$ 2	8	[2]	$ 7	10	[2]
Net unrealized gain	[3]			37			29
Realized gains	[3]			0			32
Settlement funds				3			7
Disbursements		(7)	$ (5)	(26)		$ (25)	(32)
Fair value		327		327			334
Available-for-sale Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Adjusted cost		292		292			305
Fair value		327		327			334
Gross Unrealized Gain		39		39			31
Gross Unrealized Loss		(4)		(4)			(2)
Fixed Income Mutual Fund [Member]
Schedule of Available-for-sale Securities [Line Items]
Adjusted cost		166		166			176
Fair value		181		181			183
Gross Unrealized Gain		15		15			7
Gross Unrealized Loss		0		0			0
Equity Mutual Fund [Member]
Schedule of Available-for-sale Securities [Line Items]
Adjusted cost		116		116			120
Fair value		136		136			142
Gross Unrealized Gain		24		24			24
Gross Unrealized Loss		(4)		(4)			(2)
Demand Deposit [Member]
Schedule of Available-for-sale Securities [Line Items]
Adjusted cost		10		10			9
Fair value		$ 10		$ 10			$ 9

[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.