Commitments And Contingencies (Environmental Matters) (Detail) $ in Millions	Mar. 31, 2025 USD ($) scrubber site	Dec. 31, 2024 USD ($)
Loss Contingencies [Line Items]
Estimated capital costs to comply with existing and known federal CO2 emissions regulations	$ 580
Estimated capital costs to comply with existing and known federal MATS regulations	320
Asset Retirement Obligation	835	$ 827
Minimum
Loss Contingencies [Line Items]
Estimated capital costs to comply with existing and known federal and state air emissions regulations	900
Maximum
Loss Contingencies [Line Items]
Estimated capital costs to comply with existing and known federal and state air emissions regulations	$ 1,000
Union Electric Company
Loss Contingencies [Line Items]
Number of Energy Center Scrubbers | scrubber	2
Estimated capital costs to comply with existing and known federal CO2 emissions regulations	$ 580
Estimated capital costs to comply with existing and known federal MATS regulations	320
Asset Retirement Obligation	831	823
Union Electric Company | Minimum
Loss Contingencies [Line Items]
Estimated capital costs to comply with existing and known federal and state air emissions regulations	900
Union Electric Company | Maximum
Loss Contingencies [Line Items]
Estimated capital costs to comply with existing and known federal and state air emissions regulations	1,000
Ameren Illinois Company
Loss Contingencies [Line Items]
Asset Retirement Obligation	$ 4	$ 4
Number of remediation sites | site	3
Manufactured Gas Plant
Loss Contingencies [Line Items]
Accrual for environmental loss contingencies	$ 47
Manufactured Gas Plant | Ameren Illinois Company
Loss Contingencies [Line Items]
Number of remediation sites | site	44
Accrual for environmental loss contingencies	$ 47
Manufactured Gas Plant | Ameren Illinois Company | Minimum
Loss Contingencies [Line Items]
Estimate of possible loss	47
Manufactured Gas Plant | Ameren Illinois Company | Maximum
Loss Contingencies [Line Items]
Estimate of possible loss	91
New CCR Rules Estimate
Loss Contingencies [Line Items]
Asset Retirement Obligation	46
New CCR Rules Estimate | Union Electric Company
Loss Contingencies [Line Items]
Asset Retirement Obligation	$ 46