Fair Value Measurements - Summary of Available-for-sale Securities Portfolio (Details) - USD ($) $ in Millions		Jun. 30, 2022	Sep. 30, 2021
Schedule of Available-for-sale Securities [Line Items]
Adjusted cost	[1]	$ 372	$ 285
Fair Value		407	421
Restricted Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Adjusted cost		432	372
Gross Unrealized Gain		23	51
Gross Unrealized Loss		(48)	(2)
Fair Value		407	421
Fixed Income Mutual Fund [Member]
Schedule of Available-for-sale Securities [Line Items]
Adjusted cost		247	223
Gross Unrealized Gain		0	7
Gross Unrealized Loss		(35)	(1)
Fair Value		212	229
Equity Mutual Fund [Member]
Schedule of Available-for-sale Securities [Line Items]
Adjusted cost		178	143
Gross Unrealized Gain		23	44
Gross Unrealized Loss		(13)	(1)
Fair Value		188	186
Demand Deposit [Member]
Schedule of Available-for-sale Securities [Line Items]
Adjusted cost		7	6
Gross Unrealized Gain		0	0
Gross Unrealized Loss		0	0
Fair Value		$ 7	$ 6

[1] The adjusted cost of the demand deposits includes accumulated investment income, realized gains, additional funds restricted for specific transactions and disbursements recorded in previous periods. The adjusted cost as of June 30, 2022 includes the $90 million funding to establish the Environmental trust