Fair Value Measurements - Summary of Restricted Investment Portfolio (Details) - USD ($) $ in Millions |
Jun. 30, 2026 |
Sep. 30, 2025 |
|---|---|---|
| Demand Deposit [Member] | ||
| Schedule of Available-for-sale Securities [Line Items] | ||
| Adjusted cost | $ 2 | $ 3 |
| Gross unrealized gain | 0 | 0 |
| Gross unrealized loss | 0 | 0 |
| Fair value | 2 | 3 |
| Equity Mutual Fund [Member] | ||
| Schedule of Available-for-sale Securities [Line Items] | ||
| Adjusted cost | 86 | 103 |
| Gross unrealized gain | 76 | 67 |
| Gross unrealized loss | 0 | 0 |
| Fair value | 162 | 170 |
| Fixed Income Mutual Fund [Member] | ||
| Schedule of Available-for-sale Securities [Line Items] | ||
| Adjusted cost | 201 | 205 |
| Gross unrealized gain | 0 | 0 |
| Gross unrealized loss | (33) | (31) |
| Fair value | 168 | 174 |
| Restricted Securities [Member] | ||
| Schedule of Available-for-sale Securities [Line Items] | ||
| Adjusted cost | 289 | 311 |
| Gross unrealized gain | 76 | 67 |
| Gross unrealized loss | (33) | (31) |
| Fair value | $ 332 | $ 347 |
| X | ||||||||||
- Definition 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). Reference 1: http://www.xbrl.org/2003/role/disclosureRef
|
| X | ||||||||||
- Definition Amount, before tax, of unrealized loss 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). Reference 1: http://www.xbrl.org/2003/role/disclosureRef
|
| X | ||||||||||
- Definition Amortized cost of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale). Reference 1: http://www.xbrl.org/2003/role/disclosureRef
|
| X | ||||||||||
- Definition Amount of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale). Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
|
| X | ||||||||||
- Definition Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table. Reference 1: http://www.xbrl.org/2003/role/disclosureRef
|
| X | ||||||||||
- Details
|
| X | ||||||||||
- Details
|
| X | ||||||||||
- Details
|
| X | ||||||||||
- Details
|