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Derivative Financial Instruments (Open Gross Derivative Volumes By Commodity Type) (Detail) gal in Millions, MWh in Millions, MMBTU in Millions | 3 Months Ended | 9 Months Ended
--- | --- | ---
Mar. 31, 2025 MWh MMBTU gal | Sep. 30, 2024 MWh MMBTU gal
Fuel oils |  |
Derivative [Line Items] |  |
Derivative, Nonmonetary Notional Amount, Volume | gal | 24 | 23
Natural gas |  |
Derivative [Line Items] |  |
Derivative, Nonmonetary Notional Amount, Energy Measure | MMBTU | 268 | 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 | 24 | 23
Union Electric Company | Natural gas |  |
Derivative [Line Items] |  |
Derivative, Nonmonetary Notional Amount, Energy Measure | MMBTU | 47 | 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 | 221 | 213
Ameren Illinois Company | Power |  |
Derivative [Line Items] |  |
Derivative, Nonmonetary Notional Amount, Energy Measure | MWh | 4 | 4

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- 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. + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Topic 815 -SubTopic 40 -Name Accounting Standards Codification -Section 50 -Paragraph 5 -Subparagraph (f) -Publisher FASB -URI https://asc.fasb.org/1943274/2147480237/815-40-50-5 + Details
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- Definition Nominal volume used to calculate payments on a derivative instrument. + References No definition available. + Details
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