Derivative Financial Instruments (Derivative Instruments Carrying Value) (Detail) - Not Designated As Hedging Instrument - USD ($) $ in Millions	Sep. 30, 2023	Dec. 31, 2022
Derivative [Line Items]
Derivative Assets	$ 35	$ 89
Derivative Liability	125	136
Fuel oils | Other current assets
Derivative [Line Items]
Derivative Assets	6	13
Fuel oils | Other assets
Derivative [Line Items]
Derivative Assets	2	3
Natural gas | Other current assets
Derivative [Line Items]
Derivative Assets	7	30
Natural gas | Other assets
Derivative [Line Items]
Derivative Assets	8	20
Natural gas | Other current liabilities
Derivative [Line Items]
Derivative Liability	32	27
Natural gas | Other deferred credits and liabilities
Derivative [Line Items]
Derivative Liability	22	11
Power | Other current assets
Derivative [Line Items]
Derivative Assets	7	16
Power | Other assets
Derivative [Line Items]
Derivative Assets	0	4
Power | Other current liabilities
Derivative [Line Items]
Derivative Liability	15	61
Power | Other deferred credits and liabilities
Derivative [Line Items]
Derivative Liability	56	37
Uranium | Other current assets
Derivative [Line Items]
Derivative Assets	5	2
Uranium | Other assets
Derivative [Line Items]
Derivative Assets	0	1
Union Electric Company
Derivative [Line Items]
Derivative Assets	26	49
Derivative Liability	20	68
Union Electric Company | Fuel oils | Other current assets
Derivative [Line Items]
Derivative Assets	6	13
Union Electric Company | Fuel oils | Other assets
Derivative [Line Items]
Derivative Assets	2	3
Union Electric Company | Natural gas | Other current assets
Derivative [Line Items]
Derivative Assets	1	7
Union Electric Company | Natural gas | Other assets
Derivative [Line Items]
Derivative Assets	5	9
Union Electric Company | Natural gas | Other current liabilities
Derivative [Line Items]
Derivative Liability	7	7
Union Electric Company | Natural gas | Other deferred credits and liabilities
Derivative [Line Items]
Derivative Liability	6	2
Union Electric Company | Power | Other current assets
Derivative [Line Items]
Derivative Assets	7	14
Union Electric Company | Power | Other assets
Derivative [Line Items]
Derivative Assets	0	0
Union Electric Company | Power | Other current liabilities
Derivative [Line Items]
Derivative Liability	7	59
Union Electric Company | Power | Other deferred credits and liabilities
Derivative [Line Items]
Derivative Liability	0	0
Union Electric Company | Uranium | Other current assets
Derivative [Line Items]
Derivative Assets	5	2
Union Electric Company | Uranium | Other assets
Derivative [Line Items]
Derivative Assets	0	1
Ameren Illinois Company
Derivative [Line Items]
Derivative Assets	9	40
Derivative Liability	105	68
Ameren Illinois Company | Fuel oils | Other current assets
Derivative [Line Items]
Derivative Assets	0	0
Ameren Illinois Company | Fuel oils | Other assets
Derivative [Line Items]
Derivative Assets	0	0
Ameren Illinois Company | Natural gas | Other current assets
Derivative [Line Items]
Derivative Assets	6	23
Ameren Illinois Company | Natural gas | Other assets
Derivative [Line Items]
Derivative Assets	3	11
Ameren Illinois Company | Natural gas | Other current liabilities
Derivative [Line Items]
Derivative Liability	25	20
Ameren Illinois Company | Natural gas | Other deferred credits and liabilities
Derivative [Line Items]
Derivative Liability	16	9
Ameren Illinois Company | Power | Other current assets
Derivative [Line Items]
Derivative Assets	0	2
Ameren Illinois Company | Power | Other assets
Derivative [Line Items]
Derivative Assets	0	4
Ameren Illinois Company | Power | Other current liabilities
Derivative [Line Items]
Derivative Liability	8	2
Ameren Illinois Company | Power | Other deferred credits and liabilities
Derivative [Line Items]
Derivative Liability	56	37
Ameren Illinois Company | Uranium | Other current assets
Derivative [Line Items]
Derivative Assets	0	0
Ameren Illinois Company | Uranium | Other assets
Derivative [Line Items]
Derivative Assets	$ 0	$ 0