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Rate And Regulatory Matters (Narrative-Illinois) (Details) - Ameren Illinois Company - USD ($) $ in Millions | 3 Months Ended | 6 Months Ended | 9 Months Ended
--- | --- | --- | ---
Dec. 31, 2025 | Jun. 30, 2028 | Jun. 30, 2027 | Jun. 30, 2026 | Jun. 30, 2025 | Sep. 30, 2025
Rate And Regulatory Matters [Line Items] |  |  |  |  |  |
Annual investment in energy-efficiency programs |  |  |  |  |  | $ 126
IETL | Electric Distribution |  |  |  |  |  |
Rate And Regulatory Matters [Line Items] |  |  |  |  |  |
Multi-year rate plan approved revenue requirement |  |  |  |  | $ 1,206 |
Public Utilities, Approved Rate Increase (Decrease), Amount |  |  |  |  | $ 308 |
IETL | Electric Distribution | Forecast |  |  |  |  |  |
Rate And Regulatory Matters [Line Items] |  |  |  |  |  |
Multi-year rate plan approved revenue requirement |  | $ 1,421 | $ 1,367 | $ 1,287 |  |
Pending Rate Case | Electric Distribution |  |  |  |  |  |
Rate And Regulatory Matters [Line Items] |  |  |  |  |  |
Public Utilities, Requested Rate Increase (Decrease), Amount |  |  |  |  |  | $ 60
Public Utilities, Requested Equity Capital Structure, Percentage |  |  |  |  |  | 50.00%
ICC Staff's recommended rate base |  |  |  |  |  | $ 47
Pending Rate Case | Electric Energy-Efficiency |  |  |  |  |  |
Rate And Regulatory Matters [Line Items] |  |  |  |  |  |
Public Utilities, Requested Rate Increase (Decrease), Amount |  |  |  |  |  | 12
Pending Rate Case | Natural gas |  |  |  |  |  |
Rate And Regulatory Matters [Line Items] |  |  |  |  |  |
Public Utilities, Requested Rate Increase (Decrease), Amount |  |  |  |  |  | $ 135
Public Utilities, Requested Equity Capital Structure, Percentage |  |  |  |  |  | 52.00%
Public Utilities, Requested Return on Equity, Percentage |  |  |  |  |  | 10.70%
Rate Base |  |  |  |  |  | $ 3,300
ICC staff recommended rate increase |  |  |  |  |  | $ 104
ICC recommended ROE |  |  |  |  |  | 9.93%
ICC Staff's recommended capital structure |  |  |  |  |  | 50.00%
ICC Staff's recommended rate base |  |  |  |  |  | $ 3,200
Illinois Attorney General's office's recommended rate increase |  |  |  |  |  | $ 55
Illinois Attorney General's office's recommended ROE |  |  |  |  |  | 9.45%
Illinois Attorney General's office's recommended capital structure |  |  |  |  |  | 50.00%
Illinois Attorney General's office's recommended rate base |  |  |  |  |  | $ 3,000
Pending Rate Case | Natural gas | Subsequent Event |  |  |  |  |  |
Rate And Regulatory Matters [Line Items] |  |  |  |  |  |
Administrative law judge's recommended rate increase | $ 91 |  |  |  |  |

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