Commitments And Contingencies (Environmental Matters) (Detail) $ in Millions	6 Months Ended
	Jun. 30, 2022 USD ($) energyCenter scrubber site	Dec. 31, 2021 USD ($)
Loss Contingencies [Line Items]
Asset Retirement Obligation	$ 781	$ 764
Minimum
Loss Contingencies [Line Items]
Estimated capital costs to comply with existing and known federal and state air emissions regulations	125
Maximum
Loss Contingencies [Line Items]
Estimated capital costs to comply with existing and known federal and state air emissions regulations	$ 175
Union Electric Company
Loss Contingencies [Line Items]
Number of Energy Center Scrubbers | scrubber	2
Number of Months of System Support Agreement	12 months
Number of Energy Centers Constructing Wastewater Treatment Facilities | energyCenter	3
Number of energy centers | energyCenter	4
Number of Days to Comply after Final EPA Determination	135 days
Asset Retirement Obligation	$ 777	760
Number of Ameren Missouri Natural Gas Energy Centers Subject To IETL | energyCenter	4
Union Electric Company | Coal Fired Electric Generation Equipment
Loss Contingencies [Line Items]
Plant To Be Abandoned, Net	$ 600
Union Electric Company | Minimum
Loss Contingencies [Line Items]
Estimated capital costs to comply with existing and known federal and state air emissions regulations	125
Union Electric Company | Maximum
Loss Contingencies [Line Items]
Estimated capital costs to comply with existing and known federal and state air emissions regulations	175
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	60
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	80
Manufactured Gas Plant
Loss Contingencies [Line Items]
Accrual for environmental loss contingencies	$ 78
Manufactured Gas Plant | Ameren Illinois Company
Loss Contingencies [Line Items]
Number of remediation sites | site	44
Accrual for environmental loss contingencies	$ 78
Manufactured Gas Plant | Ameren Illinois Company | Minimum
Loss Contingencies [Line Items]
Estimate of possible loss	78
Manufactured Gas Plant | Ameren Illinois Company | Maximum
Loss Contingencies [Line Items]
Estimate of possible loss	147
Rush Island Energy Center | Union Electric Company
Loss Contingencies [Line Items]
Rate Base	500
New CCR Rules Estimate
Loss Contingencies [Line Items]
Asset Retirement Obligation	77
New CCR Rules Estimate | Union Electric Company
Loss Contingencies [Line Items]
Asset Retirement Obligation	$ 77