Summary of Significant Accounting Policies Allowance for Uncollectible Accounts (Details) $ in Millions |
9 Months Ended | |
|---|---|---|
|
Jun. 30, 2025
USD ($)
Units
|
Sep. 30, 2024
USD ($)
|
|
| Accounts, Notes, Loans and Financing Receivable [Line Items] | ||
| Allowance for uncollectible accounts | $ 10 | $ 1 |
| Financing Receivable, after Allowance for Credit Loss | 108 | 99 |
| Accounts and Financing Receivable, Allowance for Credit Loss | 2 | 2 |
| Holly Springs | ||
| Accounts, Notes, Loans and Financing Receivable [Line Items] | ||
| Allowance for uncollectible accounts | $ 8 | |
| Number of customers with allowance for uncollectible accounts | Units | 1 | |
| Reported Value Measurement | ||
| Accounts, Notes, Loans and Financing Receivable [Line Items] | ||
| Financing Receivable, after Allowance for Credit Loss | $ 114 | $ 105 |