Estimated capital costs to comply with existing and known federal and state air emissions regulations

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- Definition

Number Of Remediation Sites

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- Definition

Number of Energy Center Scrubbers to achieve compliance with regulations

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- Definition

Percentage of Rate Base Related to Carbon Dioxide Energy Centers

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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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Reflects the estimated amount of loss from the specified contingency as of the balance sheet date.

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Commitments And Contingencies (Environmental Matters) (Detail) $ in Millions	Jun. 30, 2018 USD ($) scrubber site	Dec. 31, 2017 USD ($)
Loss Contingencies [Line Items]
Percentage of Rate Base Related to Carbon Dioxide Energy Centers		17.00%
Asset Retirement Obligation	$ 647.0	$ 644.0
Minimum
Loss Contingencies [Line Items]
Estimated capital costs to comply with existing and known federal and state air emissions regulations	325.0
Maximum
Loss Contingencies [Line Items]
Estimated capital costs to comply with existing and known federal and state air emissions regulations	425.0
Union Electric Company
Loss Contingencies [Line Items]
Percentage of Rate Base Related to Carbon Dioxide Energy Centers		33.00%
Asset Retirement Obligation	$ 643.0	$ 640.0
Number of Energy Center Scrubbers \| scrubber	2
Union Electric Company \| Minimum
Loss Contingencies [Line Items]
Estimated capital costs to comply with existing and known federal and state air emissions regulations	$ 325.0
Union Electric Company \| Maximum
Loss Contingencies [Line Items]
Estimated capital costs to comply with existing and known federal and state air emissions regulations	425.0
Ameren Illinois Company
Loss Contingencies [Line Items]
Asset Retirement Obligation	4.0	$ 4.0
Coal Combustion Residuals Estimate [Member] \| Union Electric Company \| Minimum
Loss Contingencies [Line Items]
Estimated capital costs to comply with existing and known federal and state air emissions regulations	300.0
Coal Combustion Residuals Estimate [Member] \| Union Electric Company \| Maximum
Loss Contingencies [Line Items]
Estimated capital costs to comply with existing and known federal and state air emissions regulations	350.0
Manufactured Gas Plant
Loss Contingencies [Line Items]
Accrual for environmental loss contingencies	$ 165.0
Manufactured Gas Plant \| Ameren Illinois Company
Loss Contingencies [Line Items]
Number of remediation sites \| site	44
Accrual for environmental loss contingencies	$ 165.0
Manufactured Gas Plant \| Ameren Illinois Company \| Minimum
Loss Contingencies [Line Items]
Estimate of possible loss	165.0
Manufactured Gas Plant \| Ameren Illinois Company \| Maximum
Loss Contingencies [Line Items]
Estimate of possible loss	236.0
Sauget Area Two \| Union Electric Company
Loss Contingencies [Line Items]
Accrual for environmental loss contingencies	1.0
Sauget Area Two \| Union Electric Company \| Minimum
Loss Contingencies [Line Items]
Estimate of possible loss	1.0
Sauget Area Two \| Union Electric Company \| Maximum
Loss Contingencies [Line Items]
Estimate of possible loss	2.5
New CCR Rules Estimate
Loss Contingencies [Line Items]
Asset Retirement Obligation	141.0
New CCR Rules Estimate \| Union Electric Company
Loss Contingencies [Line Items]
Asset Retirement Obligation	$ 141.0