Derivative Financial Instruments (Derivative Instruments Carrying Value) (Detail) - Not Designated As Hedging Instrument - USD ($) $ in Millions	Mar. 31, 2022	Dec. 31, 2021
Derivative [Line Items]
Derivative assets	$ 180	$ 90
Derivative Liability	218	202
Fuel oils | Mark-to-market derivative assets
Derivative [Line Items]
Derivative assets	18	8
Fuel oils | Other current assets
Derivative [Line Items]
Derivative assets	0	0
Fuel oils | Other assets
Derivative [Line Items]
Derivative assets	8	5
Natural gas | Mark-to-market derivative assets
Derivative [Line Items]
Derivative assets	90	35
Natural gas | Other current assets
Derivative [Line Items]
Derivative assets	0	0
Natural gas | Other assets
Derivative [Line Items]
Derivative assets	33	18
Natural gas | Other current liabilities
Derivative [Line Items]
Derivative Liability	2	8
Natural gas | Other deferred credits and liabilities
Derivative [Line Items]
Derivative Liability	2	3
Power | Mark-to-market derivative assets
Derivative [Line Items]
Derivative assets	24	23
Power | Other current assets
Derivative [Line Items]
Derivative assets	0	0
Power | Other assets
Derivative [Line Items]
Derivative assets	0	0
Power | Other current liabilities
Derivative [Line Items]
Derivative Liability	110	59
Power | Other deferred credits and liabilities
Derivative [Line Items]
Derivative Liability	104	131
Uranium | Mark-to-market derivative assets
Derivative [Line Items]
Derivative assets	5	0
Uranium | Other current assets
Derivative [Line Items]
Derivative assets	0	0
Uranium | Other assets
Derivative [Line Items]
Derivative assets	2	1
Uranium | Other current liabilities
Derivative [Line Items]
Derivative Liability	0	1
Union Electric Company
Derivative [Line Items]
Derivative assets	78	49
Derivative Liability	136	77
Union Electric Company | Fuel oils | Other current assets
Derivative [Line Items]
Derivative assets	18	8
Union Electric Company | Fuel oils | Other assets
Derivative [Line Items]
Derivative assets	8	5
Union Electric Company | Natural gas | Other current assets
Derivative [Line Items]
Derivative assets	16	7
Union Electric Company | Natural gas | Other assets
Derivative [Line Items]
Derivative assets	10	5
Union Electric Company | Natural gas | Mark-to-market derivative liabilities
Derivative [Line Items]
Derivative Liability	0	2
Union Electric Company | Natural gas | Other current liabilities
Derivative [Line Items]
Derivative Liability	0	0
Union Electric Company | Natural gas | Other deferred credits and liabilities
Derivative [Line Items]
Derivative Liability	1	1
Union Electric Company | Power | Other current assets
Derivative [Line Items]
Derivative assets	19	23
Union Electric Company | Power | Other assets
Derivative [Line Items]
Derivative assets	0	0
Union Electric Company | Power | Mark-to-market derivative liabilities
Derivative [Line Items]
Derivative Liability	109	50
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	26	23
Union Electric Company | Uranium | Other current assets
Derivative [Line Items]
Derivative assets	5	0
Union Electric Company | Uranium | Other assets
Derivative [Line Items]
Derivative assets	2	1
Union Electric Company | Uranium | Mark-to-market derivative liabilities
Derivative [Line Items]
Derivative Liability	0	1
Union Electric Company | Uranium | Other current liabilities
Derivative [Line Items]
Derivative Liability	0	0
Ameren Illinois Company
Derivative [Line Items]
Derivative assets	102	41
Derivative Liability	82	125
Ameren Illinois Company | Fuel oils | Mark-to-market derivative assets
Derivative [Line Items]
Derivative assets	0	0
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 | Mark-to-market derivative assets
Derivative [Line Items]
Derivative assets	74	28
Ameren Illinois Company | Natural gas | Other current assets
Derivative [Line Items]
Derivative assets	0	0
Ameren Illinois Company | Natural gas | Other assets
Derivative [Line Items]
Derivative assets	23	13
Ameren Illinois Company | Natural gas | Other current liabilities
Derivative [Line Items]
Derivative Liability	2	6
Ameren Illinois Company | Natural gas | Other deferred credits and liabilities
Derivative [Line Items]
Derivative Liability	1	2
Ameren Illinois Company | Power | Mark-to-market derivative assets
Derivative [Line Items]
Derivative assets	5	0
Ameren Illinois Company | Power | Other current assets
Derivative [Line Items]
Derivative assets	0	0
Ameren Illinois Company | Power | Other assets
Derivative [Line Items]
Derivative assets	0	0
Ameren Illinois Company | Power | Other current liabilities
Derivative [Line Items]
Derivative Liability	1	9
Ameren Illinois Company | Power | Other deferred credits and liabilities
Derivative [Line Items]
Derivative Liability	78	108
Ameren Illinois Company | Uranium | Mark-to-market derivative assets
Derivative [Line Items]
Derivative assets	0	0
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
Ameren Illinois Company | Uranium | Other current liabilities
Derivative [Line Items]
Derivative Liability	$ 0	$ 0