Derivative Financial Instruments (Open Gross Derivative Volumes By Commodity Type) (Detail) lb in Thousands, gal in Millions, MWh in Millions, MMBTU in Millions	6 Months Ended	12 Months Ended
Jun. 30, 2023 MWh MMBTU lb gal	Dec. 31, 2022 MMBTU MWh lb gal
Fuel oils
Derivative [Line Items]
Derivative, Nonmonetary Notional Amount, Volume \| gal	18	18
Natural gas
Derivative [Line Items]
Derivative, Nonmonetary Notional Amount, Energy Measure \| MMBTU	275	205
Power
Derivative [Line Items]
Derivative, Nonmonetary Notional Amount, Energy Measure \| MWh	6	7
Uranium
Derivative [Line Items]
Derivative, Nonmonetary Notional Amount, Mass \| lb	186	514
Union Electric Company \| Fuel oils
Derivative [Line Items]
Derivative, Nonmonetary Notional Amount, Volume \| gal	18	18
Union Electric Company \| Natural gas
Derivative [Line Items]
Derivative, Nonmonetary Notional Amount, Energy Measure \| MMBTU	57	48
Union Electric Company \| Power
Derivative [Line Items]
Derivative, Nonmonetary Notional Amount, Energy Measure \| MWh	1	1
Union Electric Company \| Uranium
Derivative [Line Items]
Derivative, Nonmonetary Notional Amount, Mass \| lb	186	514
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	218	157
Ameren Illinois Company \| Power
Derivative [Line Items]
Derivative, Nonmonetary Notional Amount, Energy Measure \| MWh	5	6
Ameren Illinois Company \| Uranium
Derivative [Line Items]
Derivative, Nonmonetary Notional Amount, Mass \| lb	0	0

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/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

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- Definition

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

+ References

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- Definition

Nominal mass used to calculate payments on a derivative instrument.

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- Definition

Nominal volume used to calculate payments on a derivative instrument.

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