Derivative Financial Instruments (Derivative Instruments Carrying Value) (Detail) - Not Designated As Hedging Instrument - USD ($) $ in Millions	Jun. 30, 2026	Dec. 31, 2025
Derivative [Line Items]
Derivative Assets	$ 45	$ 16
Derivative Liability	126	126
Fuel oils \| Balance Sheet Location [Axis]: us-gaap:OtherAssetsCurrent
Derivative [Line Items]
Derivative Assets	7	0
Fuel oils \| Balance Sheet Location [Axis]: us-gaap:OtherAssetsNoncurrent
Derivative [Line Items]
Derivative Assets	4	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	1	2
Natural gas \| Balance Sheet Location [Axis]: us-gaap:OtherAssetsNoncurrent
Derivative [Line Items]
Derivative Assets	1	3
Natural gas \| Balance Sheet Location [Axis]: us-gaap:OtherLiabilitiesCurrent
Derivative [Line Items]
Derivative Liability	35	26
Natural gas \| Balance Sheet Location [Axis]: us-gaap:OtherLiabilitiesNoncurrent
Derivative [Line Items]
Derivative Liability	27	16
Power \| Balance Sheet Location [Axis]: us-gaap:OtherAssetsCurrent
Derivative [Line Items]
Derivative Assets	17	11
Power \| Balance Sheet Location [Axis]: us-gaap:OtherAssetsNoncurrent
Derivative [Line Items]
Derivative Assets	15	0
Power \| Balance Sheet Location [Axis]: us-gaap:OtherLiabilitiesCurrent
Derivative [Line Items]
Derivative Liability	33	24
Power \| Balance Sheet Location [Axis]: us-gaap:OtherLiabilitiesNoncurrent
Derivative [Line Items]
Derivative Liability	31	55
Union Electric Company
Derivative [Line Items]
Derivative Assets	43	12
Derivative Liability	16	15
Union Electric Company \| Fuel oils \| Balance Sheet Location [Axis]: us-gaap:OtherAssetsCurrent
Derivative [Line Items]
Derivative Assets	7	0
Union Electric Company \| Fuel oils \| Balance Sheet Location [Axis]: us-gaap:OtherAssetsNoncurrent
Derivative [Line Items]
Derivative Assets	4	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	0	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	8	5
Union Electric Company \| Power \| Balance Sheet Location [Axis]: us-gaap:OtherAssetsCurrent
Derivative [Line Items]
Derivative Assets	17	11
Union Electric Company \| Power \| Balance Sheet Location [Axis]: us-gaap:OtherAssetsNoncurrent
Derivative [Line Items]
Derivative Assets	15	0
Union Electric Company \| Power \| Balance Sheet Location [Axis]: us-gaap:OtherLiabilitiesCurrent
Derivative [Line Items]
Derivative Liability	0	0
Union Electric Company \| Power \| Balance Sheet Location [Axis]: us-gaap:OtherLiabilitiesNoncurrent
Derivative [Line Items]
Derivative Liability	1	0
Ameren Illinois Company
Derivative [Line Items]
Derivative Assets	2	4
Derivative Liability	110	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	1	1
Ameren Illinois Company \| Natural gas \| Balance Sheet Location [Axis]: us-gaap:OtherAssetsNoncurrent
Derivative [Line Items]
Derivative Assets	1	3
Ameren Illinois Company \| Natural gas \| Balance Sheet Location [Axis]: us-gaap:OtherLiabilitiesCurrent
Derivative [Line Items]
Derivative Liability	28	21
Ameren Illinois Company \| Natural gas \| Balance Sheet Location [Axis]: us-gaap:OtherLiabilitiesNoncurrent
Derivative [Line Items]
Derivative Liability	19	11
Ameren Illinois Company \| Power \| Balance Sheet Location [Axis]: us-gaap:OtherAssetsCurrent
Derivative [Line Items]
Derivative Assets	0	0
Ameren Illinois Company \| Power \| Balance Sheet Location [Axis]: us-gaap:OtherAssetsNoncurrent
Derivative [Line Items]
Derivative Assets	0	0
Ameren Illinois Company \| Power \| Balance Sheet Location [Axis]: us-gaap:OtherLiabilitiesCurrent
Derivative [Line Items]
Derivative Liability	33	24
Ameren Illinois Company \| Power \| Balance Sheet Location [Axis]: us-gaap:OtherLiabilitiesNoncurrent
Derivative [Line Items]
Derivative Liability	$ 30	$ 55