Derivative Financial Instruments (Derivative Instruments Carrying Value) (Detail) - Not Designated As Hedging Instrument - USD ($) $ in Millions	Mar. 31, 2025	Dec. 31, 2024
Derivative [Line Items]
Derivative Assets	$ 27	$ 16
Derivative Liability	99	103
Fuel oils | Other current liabilities
Derivative [Line Items]
Derivative Liability	2	2
Fuel oils | Other deferred credits and liabilities
Derivative [Line Items]
Derivative Liability	1	2
Natural gas | Other current assets
Derivative [Line Items]
Derivative Assets	15	4
Natural gas | Other assets
Derivative [Line Items]
Derivative Assets	7	6
Natural gas | Other current liabilities
Derivative [Line Items]
Derivative Liability	8	27
Natural gas | Other deferred credits and liabilities
Derivative [Line Items]
Derivative Liability	21	19
Power | Other current assets
Derivative [Line Items]
Derivative Assets	5	6
Power | Other current liabilities
Derivative [Line Items]
Derivative Liability	7	10
Power | Other deferred credits and liabilities
Derivative [Line Items]
Derivative Liability	60	43
Union Electric Company
Derivative [Line Items]
Derivative Assets	11	10
Derivative Liability	12	15
Union Electric Company | Fuel oils | Other current liabilities
Derivative [Line Items]
Derivative Liability	2	2
Union Electric Company | Fuel oils | Other deferred credits and liabilities
Derivative [Line Items]
Derivative Liability	1	2
Union Electric Company | Natural gas | Other current assets
Derivative [Line Items]
Derivative Assets	4	2
Union Electric Company | Natural gas | Other assets
Derivative [Line Items]
Derivative Assets	2	2
Union Electric Company | Natural gas | Other current liabilities
Derivative [Line Items]
Derivative Liability	2	5
Union Electric Company | Natural gas | Other deferred credits and liabilities
Derivative [Line Items]
Derivative Liability	7	6
Union Electric Company | Power | Other current assets
Derivative [Line Items]
Derivative Assets	5	6
Union Electric Company | Power | Other current liabilities
Derivative [Line Items]
Derivative Liability	0	0
Union Electric Company | Power | Other deferred credits and liabilities
Derivative [Line Items]
Derivative Liability	0	0
Ameren Illinois Company
Derivative [Line Items]
Derivative Assets	16	6
Derivative Liability	87	88
Ameren Illinois Company | Fuel oils | Other current liabilities
Derivative [Line Items]
Derivative Liability	0	0
Ameren Illinois Company | Fuel oils | Other deferred credits and liabilities
Derivative [Line Items]
Derivative Liability	0	0
Ameren Illinois Company | Natural gas | Other current assets
Derivative [Line Items]
Derivative Assets	11	2
Ameren Illinois Company | Natural gas | Other assets
Derivative [Line Items]
Derivative Assets	5	4
Ameren Illinois Company | Natural gas | Other current liabilities
Derivative [Line Items]
Derivative Liability	6	22
Ameren Illinois Company | Natural gas | Other deferred credits and liabilities
Derivative [Line Items]
Derivative Liability	14	13
Ameren Illinois Company | Power | Other current assets
Derivative [Line Items]
Derivative Assets	0	0
Ameren Illinois Company | Power | Other current liabilities
Derivative [Line Items]
Derivative Liability	7	10
Ameren Illinois Company | Power | Other deferred credits and liabilities
Derivative [Line Items]
Derivative Liability	$ 60	$ 43