Derivative Financial Instruments (Derivative Instruments Carrying Value) (Detail) - Not Designated As Hedging Instrument - USD ($) $ in Millions	Jun. 30, 2023	Dec. 31, 2022
Derivative [Line Items]
Derivative assets	$ 43	$ 89
Derivative Liability	145	136
Fuel oils | Other current assets
Derivative [Line Items]
Derivative assets	5	13
Fuel oils | Other assets
Derivative [Line Items]
Derivative assets	1	3
Fuel oils | Other current liabilities
Derivative [Line Items]
Derivative Liability	1	0
Fuel oils | Other deferred credits and liabilities
Derivative [Line Items]
Derivative Liability	1	0
Natural gas | Other current assets
Derivative [Line Items]
Derivative assets	10	30
Natural gas | Other assets
Derivative [Line Items]
Derivative assets	10	20
Natural gas | Other current liabilities
Derivative [Line Items]
Derivative Liability	35	27
Natural gas | Other deferred credits and liabilities
Derivative [Line Items]
Derivative Liability	27	11
Power | Other current assets
Derivative [Line Items]
Derivative assets	15	16
Power | Other assets
Derivative [Line Items]
Derivative assets	0	4
Power | Other current liabilities
Derivative [Line Items]
Derivative Liability	23	61
Power | Other deferred credits and liabilities
Derivative [Line Items]
Derivative Liability	58	37
Uranium | Other current assets
Derivative [Line Items]
Derivative assets	2	2
Uranium | Other assets
Derivative [Line Items]
Derivative assets	0	1
Union Electric Company
Derivative [Line Items]
Derivative assets	30	49
Derivative Liability	31	68
Union Electric Company | Fuel oils | Other current assets
Derivative [Line Items]
Derivative assets	5	13
Union Electric Company | Fuel oils | Other assets
Derivative [Line Items]
Derivative assets	1	3
Union Electric Company | Fuel oils | Other current liabilities
Derivative [Line Items]
Derivative Liability	1	0
Union Electric Company | Fuel oils | Other deferred credits and liabilities
Derivative [Line Items]
Derivative Liability	1	0
Union Electric Company | Natural gas | Other current assets
Derivative [Line Items]
Derivative assets	2	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	8	7
Union Electric Company | Natural gas | Other deferred credits and liabilities
Derivative [Line Items]
Derivative Liability	8	2
Union Electric Company | Power | Other current assets
Derivative [Line Items]
Derivative assets	15	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	13	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	2	2
Union Electric Company | Uranium | Other assets
Derivative [Line Items]
Derivative assets	0	1
Ameren Illinois Company
Derivative [Line Items]
Derivative assets	13	40
Derivative Liability	114	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 | 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	8	23
Ameren Illinois Company | Natural gas | Other assets
Derivative [Line Items]
Derivative assets	5	11
Ameren Illinois Company | Natural gas | Other current liabilities
Derivative [Line Items]
Derivative Liability	27	20
Ameren Illinois Company | Natural gas | Other deferred credits and liabilities
Derivative [Line Items]
Derivative Liability	19	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	10	2
Ameren Illinois Company | Power | Other deferred credits and liabilities
Derivative [Line Items]
Derivative Liability	58	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