Derivative Financial Instruments (Open Gross Derivative Volumes By Commodity Type) (Detail) gal in Millions, MWh in Millions, MMBTU in Millions	6 Months Ended	12 Months Ended
Jun. 30, 2026 MWh MMBTU gal	Dec. 31, 2025 MWh MMBTU gal
Fuel oils
Derivative [Line Items]
Derivative, Nonmonetary Notional Amount, Volume \| gal	21	25
Natural gas
Derivative [Line Items]
Derivative, Nonmonetary Notional Amount, Energy Measure \| MMBTU	284	263
Power
Derivative [Line Items]
Derivative, Nonmonetary Notional Amount, Energy Measure \| MWh	14	7
Union Electric Company \| Fuel oils
Derivative [Line Items]
Derivative, Nonmonetary Notional Amount, Volume \| gal	21	25
Union Electric Company \| Natural gas
Derivative [Line Items]
Derivative, Nonmonetary Notional Amount, Energy Measure \| MMBTU	58	46
Union Electric Company \| Power
Derivative [Line Items]
Derivative, Nonmonetary Notional Amount, Energy Measure \| MWh	7	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	226	217
Ameren Illinois Company \| Power
Derivative [Line Items]
Derivative, Nonmonetary Notional Amount, Energy Measure \| MWh	7	7

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.

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Topic 815
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 5A
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147480434/815-10-50-5A

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Topic 505
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 18
-Subparagraph (c)
-Publisher FASB
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Reference 3: http://www.xbrl.org/2009/role/commonPracticeRef
-Topic 815
-SubTopic 20
-Name Accounting Standards Codification
-Section 25
-Paragraph 6A
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147480682/815-20-25-6A

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Nominal energy measure used to calculate payments on a derivative instrument.

+ References

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Topic 815
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 5
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147480434/815-10-50-5

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Nominal volume used to calculate payments on a derivative instrument.

+ References

Reference 1: http://www.xbrl.org/2003/role/exampleRef
-Topic 815
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-Name Accounting Standards Codification
-Section 55
-Paragraph 181
-Publisher FASB
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