Commitments And Contingencies (Environmental Matters) (Detail) $ in Millions	3 Months Ended
	Sep. 30, 2024 USD ($) scrubber site	Mar. 31, 2024 USD ($)	Dec. 31, 2023 USD ($)
Loss Contingencies [Line Items]
Asset Retirement Obligation	$ 817		$ 791
Minimum
Loss Contingencies [Line Items]
Estimated capital costs to comply with existing and known federal and state air emissions regulations	90
Maximum
Loss Contingencies [Line Items]
Estimated capital costs to comply with existing and known federal and state air emissions regulations	$ 120
Union Electric Company
Loss Contingencies [Line Items]
Number of Energy Center Scrubbers | scrubber	2
Asset Retirement Obligation	$ 814		787
Union Electric Company | Minimum
Loss Contingencies [Line Items]
Estimated capital costs to comply with existing and known federal and state air emissions regulations	90
Union Electric Company | Maximum
Loss Contingencies [Line Items]
Estimated capital costs to comply with existing and known federal and state air emissions regulations	120
Ameren Illinois Company
Loss Contingencies [Line Items]
Asset Retirement Obligation	3		$ 4
Rush Island Energy Center Additional Mitigation Relief Proposed by Ameren Missouri | Union Electric Company
Loss Contingencies [Line Items]
Estimate of possible loss	64
Loss Contingency, Loss in Period	44	$ 15
Rush Island Energy Center Additional Mitigation Relief Proposed by Ameren Missouri | Union Electric Company | Other current liabilities
Loss Contingencies [Line Items]
Accrual for environmental loss contingencies	40
Rush Island Energy Center Additional Mitigation Relief Proposed by Ameren Missouri | Union Electric Company | Other Noncurrent Liabilities
Loss Contingencies [Line Items]
Accrual for environmental loss contingencies	24
Manufactured Gas Plant
Loss Contingencies [Line Items]
Accrual for environmental loss contingencies	45
Manufactured Gas Plant | Ameren Illinois Company
Loss Contingencies [Line Items]
Accrual for environmental loss contingencies	$ 45
Number of remediation sites | site	44
Manufactured Gas Plant | Ameren Illinois Company | Minimum
Loss Contingencies [Line Items]
Estimate of possible loss	$ 45
Manufactured Gas Plant | Ameren Illinois Company | Maximum
Loss Contingencies [Line Items]
Estimate of possible loss	93
New CCR Rules Estimate
Loss Contingencies [Line Items]
Asset Retirement Obligation	47
New CCR Rules Estimate | Union Electric Company
Loss Contingencies [Line Items]
Asset Retirement Obligation	$ 47