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Fair Value Measurements - Summary of Available-for-sale Securities Portfolio (Details) - USD ($) $ in Millions		Mar. 31, 2022	Sep. 30, 2021
Schedule of Available-for-sale Securities [Line Items]
Adjusted cost	[1]	$ 372	$ 285
Fair Value		423	421
Restricted Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Adjusted cost		399	372
Gross Unrealized Gain		41	51
Gross Unrealized Loss		(17)	(2)
Fair Value		423	421
Fixed Income Mutual Fund [Member]
Schedule of Available-for-sale Securities [Line Items]
Adjusted cost		235	223
Gross Unrealized Gain		0	7
Gross Unrealized Loss		(16)	(1)
Fair Value		219	229
Equity Mutual Fund [Member]
Schedule of Available-for-sale Securities [Line Items]
Adjusted cost		160	143
Gross Unrealized Gain		41	44
Gross Unrealized Loss		(1)	(1)
Fair Value		200	186
Demand Deposit [Member]
Schedule of Available-for-sale Securities [Line Items]
Adjusted cost		4	6
Gross Unrealized Gain		0	0
Gross Unrealized Loss		0	0
Fair Value		$ 4	$ 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 March 31, 2022 includes the $90 million funding to establish the Environmental trust

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Amount, before tax, of unrealized gain in accumulated other comprehensive income (AOCI) on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

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