Derivative Financial Instruments (Derivative Instruments Carrying Value) (Detail) - Not Designated As Hedging Instrument - USD ($) $ in Millions	Mar. 31, 2023	Dec. 31, 2022
Derivative [Line Items]
Derivative assets	$ 39	$ 89
Derivative Liability	130	136
Fuel oils | Other current assets
Derivative [Line Items]
Derivative assets	8	13
Fuel oils | Other assets
Derivative [Line Items]
Derivative assets	2	3
Fuel oils | Other deferred credits and liabilities
Derivative [Line Items]
Derivative Liability	1	0
Natural gas | Other current assets
Derivative [Line Items]
Derivative assets	6	30
Natural gas | Other assets
Derivative [Line Items]
Derivative assets	16	20
Natural gas | Other deferred credits and liabilities
Derivative [Line Items]
Derivative Liability	17	11
Natural gas | Other current liabilities
Derivative [Line Items]
Derivative Liability	42	27
Power | Other current assets
Derivative [Line Items]
Derivative assets	5	16
Power | Other assets
Derivative [Line Items]
Derivative assets	1	4
Power | Other deferred credits and liabilities
Derivative [Line Items]
Derivative Liability	43	37
Power | Other current liabilities
Derivative [Line Items]
Derivative Liability	27	61
Uranium | Other current assets
Derivative [Line Items]
Derivative assets	1	2
Uranium | Other assets
Derivative [Line Items]
Derivative assets	0	1
Union Electric Company
Derivative [Line Items]
Derivative assets	25	49
Derivative Liability	32	68
Union Electric Company | Fuel oils | Other current assets
Derivative [Line Items]
Derivative assets	8	13
Union Electric Company | Fuel oils | Other assets
Derivative [Line Items]
Derivative assets	2	3
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	1	7
Union Electric Company | Natural gas | Other assets
Derivative [Line Items]
Derivative assets	8	9
Union Electric Company | Natural gas | Other deferred credits and liabilities
Derivative [Line Items]
Derivative Liability	5	2
Union Electric Company | Natural gas | Other current liabilities
Derivative [Line Items]
Derivative Liability	9	7
Union Electric Company | Power | Other current assets
Derivative [Line Items]
Derivative assets	5	14
Union Electric Company | Power | Other assets
Derivative [Line Items]
Derivative assets	0	0
Union Electric Company | Power | Other deferred credits and liabilities
Derivative [Line Items]
Derivative Liability	0	0
Union Electric Company | Power | Other current liabilities
Derivative [Line Items]
Derivative Liability	17	59
Union Electric Company | Uranium | Other current assets
Derivative [Line Items]
Derivative assets	1	2
Union Electric Company | Uranium | Other assets
Derivative [Line Items]
Derivative assets	0	1
Ameren Illinois Company
Derivative [Line Items]
Derivative assets	14	40
Derivative Liability	98	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 deferred credits and liabilities
Derivative [Line Items]
Derivative Liability	0	0
Ameren Illinois Company | Natural gas | Other current assets
Derivative [Line Items]
Derivative assets	5	23
Ameren Illinois Company | Natural gas | Other assets
Derivative [Line Items]
Derivative assets	8	11
Ameren Illinois Company | Natural gas | Other deferred credits and liabilities
Derivative [Line Items]
Derivative Liability	12	9
Ameren Illinois Company | Natural gas | Other current liabilities
Derivative [Line Items]
Derivative Liability	33	20
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	1	4
Ameren Illinois Company | Power | Other deferred credits and liabilities
Derivative [Line Items]
Derivative Liability	43	37
Ameren Illinois Company | Power | Other current liabilities
Derivative [Line Items]
Derivative Liability	10	2
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