Fair Value Measurements - Summary of Restricted Investment Portfolio (Details) - USD ($) $ in Millions	Mar. 31, 2023	Sep. 30, 2022
Restricted Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Adjusted cost	$ 423	$ 426
Gross Unrealized Gain	27	20
Gross Unrealized Loss	(40)	(72)
Fair Value	410	374
Fixed Income Mutual Fund [Member]
Schedule of Available-for-sale Securities [Line Items]
Adjusted cost	234	234
Gross Unrealized Gain	0	0
Gross Unrealized Loss	(36)	(47)
Fair Value	198	187
Equity Mutual Fund [Member]
Schedule of Available-for-sale Securities [Line Items]
Adjusted cost	177	186
Gross Unrealized Gain	27	20
Gross Unrealized Loss	(4)	(25)
Fair Value	200	181
Demand Deposit [Member]
Schedule of Available-for-sale Securities [Line Items]
Adjusted cost	12	6
Gross Unrealized Gain	0	0
Gross Unrealized Loss	0	0
Fair Value	$ 12	$ 6

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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