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 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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Percentage of Rate Base Related to Carbon Dioxide Energy Centers

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Number of years Clean Water Act postponed for two specific waste streams

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Waste Streams identified in Clean Water Act with Postponed Compliance Dates

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

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Commitments And Contingencies (Environmental Matters) (Detail) $ in Millions	9 Months Ended
	Sep. 30, 2019 USD ($) waste_stream scrubber center site	Dec. 31, 2018 USD ($)
Loss Contingencies [Line Items]
Percentage of Rate Base Related to Carbon Dioxide Energy Centers		16.00%
Asset Retirement Obligation	$ 694	$ 650
Minimum
Loss Contingencies [Line Items]
Estimated capital costs to comply with existing and known federal and state air emissions regulations	300
Maximum
Loss Contingencies [Line Items]
Estimated capital costs to comply with existing and known federal and state air emissions regulations	$ 400
Union Electric Company
Loss Contingencies [Line Items]
Percentage of Rate Base Related to Carbon Dioxide Energy Centers		32.00%
Number of Energy Center Scrubbers \| scrubber	2
Postponement of EPA's 2015 Rule	two years
Waste Streams with Postponed Compliance Dates \| waste_stream	2
Number of Energy Centers Constructing Wastewater Treatment Facilities \| center	3
Asset Retirement Obligation	$ 690	$ 646
Union Electric Company \| Minimum
Loss Contingencies [Line Items]
Estimated capital costs to comply with existing and known federal and state air emissions regulations	300
Union Electric Company \| Maximum
Loss Contingencies [Line Items]
Estimated capital costs to comply with existing and known federal and state air emissions regulations	400
Ameren Illinois Company
Loss Contingencies [Line Items]
Asset Retirement Obligation	4	$ 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	150
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	200
Manufactured Gas Plant
Loss Contingencies [Line Items]
Accrual for environmental loss contingencies	$ 145
Manufactured Gas Plant \| Ameren Illinois Company
Loss Contingencies [Line Items]
Number of remediation sites \| site	44
Accrual for environmental loss contingencies	$ 139
Manufactured Gas Plant \| Ameren Illinois Company \| Minimum
Loss Contingencies [Line Items]
Estimate of possible loss	139
Manufactured Gas Plant \| Ameren Illinois Company \| Maximum
Loss Contingencies [Line Items]
Estimate of possible loss	209
Rush Island Energy Center [Member] \| Union Electric Company
Loss Contingencies [Line Items]
Estimated capital costs to comply with preliminary court order	1,000
Rush Island Energy Center [Member] \| Union Electric Company \| Minimum
Loss Contingencies [Line Items]
Estimated operations and maintenance costs to comply with preliminary court order	30
Rush Island Energy Center [Member] \| 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	162
New CCR Rules Estimate \| Union Electric Company
Loss Contingencies [Line Items]
Asset Retirement Obligation	$ 160