Derivative Financial Instruments (Derivative Instruments Carrying Value) (Detail) - Not Designated As Hedging Instrument - USD ($) $ in Millions	Sep. 30, 2026	Dec. 31, 2025
Derivative [Line Items]
Derivative Assets	$ 107	$ 16
Derivative Liability	100	126
Fuel oils \| Balance Sheet Location [Axis]: us-gaap:OtherAssetsCurrent
Derivative [Line Items]
Derivative Assets	15	0
Fuel oils \| Balance Sheet Location [Axis]: us-gaap:OtherAssetsNoncurrent
Derivative [Line Items]
Derivative Assets	6	0
Fuel oils \| Balance Sheet Location [Axis]: us-gaap:OtherLiabilitiesCurrent
Derivative [Line Items]
Derivative Liability	0	3
Fuel oils \| Balance Sheet Location [Axis]: us-gaap:OtherLiabilitiesNoncurrent
Derivative [Line Items]
Derivative Liability	0	2
Natural gas \| Balance Sheet Location [Axis]: us-gaap:OtherAssetsCurrent
Derivative [Line Items]
Derivative Assets	2	2
Natural gas \| Balance Sheet Location [Axis]: us-gaap:OtherAssetsNoncurrent
Derivative [Line Items]
Derivative Assets	4	3
Natural gas \| Balance Sheet Location [Axis]: us-gaap:OtherLiabilitiesCurrent
Derivative [Line Items]
Derivative Liability	37	26
Natural gas \| Balance Sheet Location [Axis]: us-gaap:OtherLiabilitiesNoncurrent
Derivative [Line Items]
Derivative Liability	13	16
Power \| Balance Sheet Location [Axis]: us-gaap:OtherAssetsCurrent
Derivative [Line Items]
Derivative Assets	15	11
Power \| Balance Sheet Location [Axis]: us-gaap:OtherAssetsNoncurrent
Derivative [Line Items]
Derivative Assets	65	0
Power \| Balance Sheet Location [Axis]: us-gaap:OtherLiabilitiesCurrent
Derivative [Line Items]
Derivative Liability	32	24
Power \| Balance Sheet Location [Axis]: us-gaap:OtherLiabilitiesNoncurrent
Derivative [Line Items]
Derivative Liability	18	55
Union Electric Company
Derivative [Line Items]
Derivative Assets	97	12
Derivative Liability	14	15
Union Electric Company \| Fuel oils \| Balance Sheet Location [Axis]: us-gaap:OtherAssetsCurrent
Derivative [Line Items]
Derivative Assets	15	0
Union Electric Company \| Fuel oils \| Balance Sheet Location [Axis]: us-gaap:OtherAssetsNoncurrent
Derivative [Line Items]
Derivative Assets	6	0
Union Electric Company \| Fuel oils \| Balance Sheet Location [Axis]: us-gaap:OtherLiabilitiesCurrent
Derivative [Line Items]
Derivative Liability	0	3
Union Electric Company \| Fuel oils \| Balance Sheet Location [Axis]: us-gaap:OtherLiabilitiesNoncurrent
Derivative [Line Items]
Derivative Liability	0	2
Union Electric Company \| Natural gas \| Balance Sheet Location [Axis]: us-gaap:OtherAssetsCurrent
Derivative [Line Items]
Derivative Assets	0	1
Union Electric Company \| Natural gas \| Balance Sheet Location [Axis]: us-gaap:OtherAssetsNoncurrent
Derivative [Line Items]
Derivative Assets	1	0
Union Electric Company \| Natural gas \| Balance Sheet Location [Axis]: us-gaap:OtherLiabilitiesCurrent
Derivative [Line Items]
Derivative Liability	7	5
Union Electric Company \| Natural gas \| Balance Sheet Location [Axis]: us-gaap:OtherLiabilitiesNoncurrent
Derivative [Line Items]
Derivative Liability	5	5
Union Electric Company \| Power \| Balance Sheet Location [Axis]: us-gaap:OtherAssetsCurrent
Derivative [Line Items]
Derivative Assets	14	11
Union Electric Company \| Power \| Balance Sheet Location [Axis]: us-gaap:OtherAssetsNoncurrent
Derivative [Line Items]
Derivative Assets	61	0
Union Electric Company \| Power \| Balance Sheet Location [Axis]: us-gaap:OtherLiabilitiesCurrent
Derivative [Line Items]
Derivative Liability	2	0
Union Electric Company \| Power \| Balance Sheet Location [Axis]: us-gaap:OtherLiabilitiesNoncurrent
Derivative [Line Items]
Derivative Liability	0	0
Ameren Illinois Company
Derivative [Line Items]
Derivative Assets	10	4
Derivative Liability	86	111
Ameren Illinois Company \| Fuel oils \| Balance Sheet Location [Axis]: us-gaap:OtherAssetsCurrent
Derivative [Line Items]
Derivative Assets	0	0
Ameren Illinois Company \| Fuel oils \| Balance Sheet Location [Axis]: us-gaap:OtherAssetsNoncurrent
Derivative [Line Items]
Derivative Assets	0	0
Ameren Illinois Company \| Fuel oils \| Balance Sheet Location [Axis]: us-gaap:OtherLiabilitiesCurrent
Derivative [Line Items]
Derivative Liability	0	0
Ameren Illinois Company \| Fuel oils \| Balance Sheet Location [Axis]: us-gaap:OtherLiabilitiesNoncurrent
Derivative [Line Items]
Derivative Liability	0	0
Ameren Illinois Company \| Natural gas \| Balance Sheet Location [Axis]: us-gaap:OtherAssetsCurrent
Derivative [Line Items]
Derivative Assets	2	1
Ameren Illinois Company \| Natural gas \| Balance Sheet Location [Axis]: us-gaap:OtherAssetsNoncurrent
Derivative [Line Items]
Derivative Assets	3	3
Ameren Illinois Company \| Natural gas \| Balance Sheet Location [Axis]: us-gaap:OtherLiabilitiesCurrent
Derivative [Line Items]
Derivative Liability	0	0
Ameren Illinois Company \| Natural gas \| Balance Sheet Location [Axis]: us-gaap:OtherLiabilitiesNoncurrent
Derivative [Line Items]
Derivative Liability	8	11
Ameren Illinois Company \| Natural gas \| Balance Sheet Location [Axis]: aee:MarkToMarketDerivativeLiabilities
Derivative [Line Items]
Derivative Liability	30	21
Ameren Illinois Company \| Power \| Balance Sheet Location [Axis]: us-gaap:OtherAssetsCurrent
Derivative [Line Items]
Derivative Assets	1	0
Ameren Illinois Company \| Power \| Balance Sheet Location [Axis]: us-gaap:OtherAssetsNoncurrent
Derivative [Line Items]
Derivative Assets	4	0
Ameren Illinois Company \| Power \| Balance Sheet Location [Axis]: us-gaap:OtherLiabilitiesCurrent
Derivative [Line Items]
Derivative Liability	0	0
Ameren Illinois Company \| Power \| Balance Sheet Location [Axis]: us-gaap:OtherLiabilitiesNoncurrent
Derivative [Line Items]
Derivative Liability	18	55
Ameren Illinois Company \| Power \| Balance Sheet Location [Axis]: aee:MarkToMarketDerivativeLiabilities
Derivative [Line Items]
Derivative Liability	$ 30	$ 24