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Fair Value Measurements - Summary of Restricted Investment Portfolio (Details) - USD ($) $ in Millions	Mar. 31, 2025	Sep. 30, 2024
Restricted Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Adjusted cost	$ 337	$ 340
Gross Unrealized Gain	44	56
Gross Unrealized Loss	(36)	(28)
Fair Value	345	368
Fixed Income Mutual Fund [Member]
Schedule of Available-for-sale Securities [Line Items]
Adjusted cost	211	211
Gross Unrealized Gain	0	0
Gross Unrealized Loss	(36)	(28)
Fair Value	175	183
Equity Mutual Fund [Member]
Schedule of Available-for-sale Securities [Line Items]
Adjusted cost	121	124
Gross Unrealized Gain	44	56
Gross Unrealized Loss	0	0
Fair Value	165	180
Demand Deposit [Member]
Schedule of Available-for-sale Securities [Line Items]
Adjusted cost	5	5
Gross Unrealized Gain	0	0
Gross Unrealized Loss	0	0
Fair Value	$ 5	$ 5

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