Estimated capital costs to comply with existing and known federal and state air emission standards.

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Number of Energy Center Scrubbers to achieve compliance with regulations

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Number Of Remediation Sites

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Plant To Be Abandoned, Net

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The carrying amount of a liability for an asset retirement obligation. An asset retirement obligation is a legal obligation associated with the disposal or retirement of a tangible long-lived asset that results from the acquisition, construction or development, or the normal operations of a long-lived asset, except for certain obligations of lessees.

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Commitments And Contingencies (Environmental Matters) (Detail) $ in Millions	May 30, 2024 USD ($)	Mar. 31, 2024 USD ($) scrubber site	Dec. 31, 2023 USD ($)
Loss Contingencies [Line Items]
Asset Retirement Obligation		$ 798	$ 791
Minimum
Loss Contingencies [Line Items]
Estimated capital costs to comply with existing and known federal and state air emissions regulations		90
Maximum
Loss Contingencies [Line Items]
Estimated capital costs to comply with existing and known federal and state air emissions regulations		$ 120
Union Electric Company
Loss Contingencies [Line Items]
Number of Energy Center Scrubbers \| scrubber		2
Asset Retirement Obligation		$ 795	787
Union Electric Company \| Coal Fired Electric Generation Equipment
Loss Contingencies [Line Items]
Plant To Be Abandoned, Net		520
Union Electric Company \| Minimum
Loss Contingencies [Line Items]
Estimated capital costs to comply with existing and known federal and state air emissions regulations		90
Union Electric Company \| Maximum
Loss Contingencies [Line Items]
Estimated capital costs to comply with existing and known federal and state air emissions regulations		120
Ameren Illinois Company
Loss Contingencies [Line Items]
Asset Retirement Obligation		3	$ 4
Rush Island Energy Center Additional Mitigation Relief Proposed by Ameren Missouri \| Union Electric Company
Loss Contingencies [Line Items]
Estimate of possible loss		20
Accrual for environmental loss contingencies		20
Rush Island Energy Center Additional Mitigation Relief Proposed by Department of Justice \| Union Electric Company \| Subsequent Event
Loss Contingencies [Line Items]
Estimate of possible loss	$ 120
Manufactured Gas Plant
Loss Contingencies [Line Items]
Accrual for environmental loss contingencies		46
Manufactured Gas Plant \| Ameren Illinois Company
Loss Contingencies [Line Items]
Accrual for environmental loss contingencies		$ 46
Number of remediation sites \| site		44
Manufactured Gas Plant \| Ameren Illinois Company \| Minimum
Loss Contingencies [Line Items]
Estimate of possible loss		$ 46
Manufactured Gas Plant \| Ameren Illinois Company \| Maximum
Loss Contingencies [Line Items]
Estimate of possible loss		96
New CCR Rules Estimate
Loss Contingencies [Line Items]
Asset Retirement Obligation		40
New CCR Rules Estimate \| Union Electric Company
Loss Contingencies [Line Items]
Asset Retirement Obligation		$ 40