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

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Estimated capital costs to comply with preliminary court order

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Estimated operations and maintenance costs to comply with preliminary court order. [Monetary Items]

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Number of Energy Centers

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Number of Energy Centers Closing Surface Impoundments

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Number Of Remediation Sites

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Number of Energy Center Scrubbers to achieve compliance with regulations

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Number of Energy Centers Constructing Wastewater Treatment Facilities

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Total costs accrued as of the balance sheet date for environmental loss contingencies.

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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	Sep. 30, 2020 USD ($) center scrubber site	Dec. 31, 2019 USD ($)
Loss Contingencies [Line Items]
Asset Retirement Obligation	$ 728	$ 691
Minimum
Loss Contingencies [Line Items]
Estimated capital costs to comply with existing and known federal and state air emissions regulations	200
Maximum
Loss Contingencies [Line Items]
Estimated capital costs to comply with existing and known federal and state air emissions regulations	$ 250
Union Electric Company
Loss Contingencies [Line Items]
Number of Energy Center Scrubbers \| scrubber	2
Number of Energy Centers Constructing Wastewater Treatment Facilities \| center	3
Number of Energy Centers Closing Surface Impoundments \| center	3
Asset Retirement Obligation	$ 723	687
Number of energy centers \| center	4
Union Electric Company \| Minimum
Loss Contingencies [Line Items]
Estimated capital costs to comply with existing and known federal and state air emissions regulations	$ 200
Union Electric Company \| Maximum
Loss Contingencies [Line Items]
Estimated capital costs to comply with existing and known federal and state air emissions regulations	250
Ameren Illinois Company
Loss Contingencies [Line Items]
Asset Retirement Obligation	5	$ 4
Coal Combustion Residuals Estimate \| Union Electric Company \| Minimum
Loss Contingencies [Line Items]
Estimated capital costs to comply with existing and known federal and state air emissions regulations	75
Coal Combustion Residuals Estimate \| Union Electric Company \| Maximum
Loss Contingencies [Line Items]
Estimated capital costs to comply with existing and known federal and state air emissions regulations	125
Manufactured Gas Plant
Loss Contingencies [Line Items]
Accrual for environmental loss contingencies	$ 106
Manufactured Gas Plant \| Ameren Illinois Company
Loss Contingencies [Line Items]
Number of remediation sites \| site	44
Accrual for environmental loss contingencies	$ 106
Manufactured Gas Plant \| Ameren Illinois Company \| Minimum
Loss Contingencies [Line Items]
Estimate of possible loss	106
Manufactured Gas Plant \| Ameren Illinois Company \| Maximum
Loss Contingencies [Line Items]
Estimate of possible loss	178
Rush Island Energy Center \| Union Electric Company
Loss Contingencies [Line Items]
Estimated capital costs to comply with preliminary court order	1,000
Rush Island Energy Center \| Union Electric Company \| Minimum
Loss Contingencies [Line Items]
Estimated operations and maintenance costs to comply with preliminary court order	30
Rush Island Energy Center \| Union Electric Company \| Maximum
Loss Contingencies [Line Items]
Estimated operations and maintenance costs to comply with preliminary court order	50
New CCR Rules Estimate
Loss Contingencies [Line Items]
Asset Retirement Obligation	125
New CCR Rules Estimate \| Union Electric Company
Loss Contingencies [Line Items]
Asset Retirement Obligation	$ 125