Summary of Significant Accounting Policies Revenues (Details) - USD ($) $ in Millions |
3 Months Ended | 9 Months Ended | |||
|---|---|---|---|---|---|
Jun. 30, 2025 |
Jun. 30, 2024 |
Jun. 30, 2025 |
Jun. 30, 2024 |
Sep. 30, 2024 |
|
| Property, Plant and Equipment [Line Items] | |||||
| Pre-commercial Ops - capitalized revenue | $ 1 | $ 0 | $ 3 | $ (3) | |
| Accounts and Financing Receivable, Allowance for Credit Loss | 2 | 2 | $ 2 | ||
| Paradise CT Units 5-7 | |||||
| Property, Plant and Equipment [Line Items] | |||||
| Pre-commercial Ops - capitalized revenue | 3 | ||||
| Pre-commercial Ops - Capitalized Fuel | $ 3 | ||||
| Johnsonville Aeroderivative CT Units 25-28 | |||||
| Property, Plant and Equipment [Line Items] | |||||
| Pre-commercial Ops - capitalized revenue | 1 | 3 | |||
| Pre-commercial Ops - Capitalized Fuel | $ 2 | $ 6 | |||