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Derivative Financial Instruments (Open Gross Derivative Volumes By Commodity Type) (Detail) gal in Millions, MWh in Millions, MMBTU in Millions | 9 Months Ended | 12 Months Ended
--- | --- | ---
Sep. 30, 2025 MWh MMBTU gal | Dec. 31, 2024 MMBTU MWh gal
Fuel oils |  |
Derivative [Line Items] |  |
Derivative, Nonmonetary Notional Amount, Volume | gal | 23 | 23
Natural gas |  |
Derivative [Line Items] |  |
Derivative, Nonmonetary Notional Amount, Energy Measure | MMBTU | 261 | 258
Power |  |
Derivative [Line Items] |  |
Derivative, Nonmonetary Notional Amount, Energy Measure | MWh | 4 | 4
Union Electric Company | Fuel oils |  |
Derivative [Line Items] |  |
Derivative, Nonmonetary Notional Amount, Volume | gal | 23 | 23
Union Electric Company | Natural gas |  |
Derivative [Line Items] |  |
Derivative, Nonmonetary Notional Amount, Energy Measure | MMBTU | 50 | 45
Union Electric Company | Power |  |
Derivative [Line Items] |  |
Derivative, Nonmonetary Notional Amount, Energy Measure | MWh | 0 | 0
Ameren Illinois Company | Fuel oils |  |
Derivative [Line Items] |  |
Derivative, Nonmonetary Notional Amount, Volume | gal | 0 | 0
Ameren Illinois Company | Natural gas |  |
Derivative [Line Items] |  |
Derivative, Nonmonetary Notional Amount, Energy Measure | MMBTU | 211 | 213
Ameren Illinois Company | Power |  |
Derivative [Line Items] |  |
Derivative, Nonmonetary Notional Amount, Energy Measure | MWh | 4 | 4

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