Derivative Financial Instruments (Derivative Instruments Carrying Value) (Detail) - Not Designated As Hedging Instrument - USD ($) $ in Millions	Jun. 30, 2022	Dec. 31, 2021
Derivative [Line Items]
Derivative assets	$ 208	$ 90
Derivative Liability	236	202
Fuel oils | Mark-to-market derivative assets
Derivative [Line Items]
Derivative assets	22	8
Fuel oils | Other assets
Derivative [Line Items]
Derivative assets	8	5
Natural gas | Mark-to-market derivative assets
Derivative [Line Items]
Derivative assets	73	35
Natural gas | Other assets
Derivative [Line Items]
Derivative assets	41	18
Natural gas | Other current liabilities
Derivative [Line Items]
Derivative Liability	11	8
Natural gas | Other deferred credits and liabilities
Derivative [Line Items]
Derivative Liability	6	3
Power | Mark-to-market derivative assets
Derivative [Line Items]
Derivative assets	59	23
Power | Other assets
Derivative [Line Items]
Derivative assets	1	0
Power | Other current liabilities
Derivative [Line Items]
Derivative Liability	142	59
Power | Other deferred credits and liabilities
Derivative [Line Items]
Derivative Liability	77	131
Uranium | Mark-to-market derivative assets
Derivative [Line Items]
Derivative assets	3	0
Uranium | Other assets
Derivative [Line Items]
Derivative assets	1	1
Uranium | Other current liabilities
Derivative [Line Items]
Derivative Liability	0	1
Union Electric Company
Derivative [Line Items]
Derivative assets	112	49
Derivative Liability	171	77
Union Electric Company | Fuel oils | Mark-to-market derivative assets
Derivative [Line Items]
Derivative assets	22	8
Union Electric Company | Fuel oils | Other assets
Derivative [Line Items]
Derivative assets	8	5
Union Electric Company | Natural gas | Mark-to-market derivative assets
Derivative [Line Items]
Derivative assets	14	7
Union Electric Company | Natural gas | Other assets
Derivative [Line Items]
Derivative assets	14	5
Union Electric Company | Natural gas | Mark-to-market derivative liabilities
Derivative [Line Items]
Derivative Liability	4	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	2	1
Union Electric Company | Power | Mark-to-market derivative assets
Derivative [Line Items]
Derivative assets	50	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	141	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	24	23
Union Electric Company | Uranium | Mark-to-market derivative assets
Derivative [Line Items]
Derivative assets	3	0
Union Electric Company | Uranium | Other assets
Derivative [Line Items]
Derivative assets	1	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	96	41
Derivative Liability	65	125
Ameren Illinois Company | Fuel oils | Mark-to-market derivative 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	59	28
Ameren Illinois Company | Natural gas | Other assets
Derivative [Line Items]
Derivative assets	27	13
Ameren Illinois Company | Natural gas | Other current liabilities
Derivative [Line Items]
Derivative Liability	7	6
Ameren Illinois Company | Natural gas | Other deferred credits and liabilities
Derivative [Line Items]
Derivative Liability	4	2
Ameren Illinois Company | Power | Mark-to-market derivative assets
Derivative [Line Items]
Derivative assets	9	0
Ameren Illinois Company | Power | Other assets
Derivative [Line Items]
Derivative assets	1	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	53	108
Ameren Illinois Company | Uranium | Mark-to-market derivative 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