Derivative Financial Instruments (Derivative Instruments Carrying Value) (Details) - Not Designated As Hedging Instrument - USD ($) $ in Millions	Dec. 31, 2021	Dec. 31, 2020
Derivative [Line Items]
Derivative assets	$ 90	$ 22
Derivative liabilities	202	221
Fuel Oils | Other current assets
Derivative [Line Items]
Derivative assets	8	2
Fuel Oils | Other assets
Derivative [Line Items]
Derivative assets	5	0
Fuel Oils | Other current liabilities
Derivative [Line Items]
Derivative liabilities	0	7
Fuel Oils | Other deferred credits and liabilities
Derivative [Line Items]
Derivative liabilities	0	2
Natural gas | Other current assets
Derivative [Line Items]
Derivative assets	35	9
Natural gas | Other assets
Derivative [Line Items]
Derivative assets	18	4
Natural gas | Other current liabilities
Derivative [Line Items]
Derivative liabilities	8	2
Natural gas | Other deferred credits and liabilities
Derivative [Line Items]
Derivative liabilities	3	1
Power | Other current assets
Derivative [Line Items]
Derivative assets	23	7
Power | Other current liabilities
Derivative [Line Items]
Derivative liabilities	59	20
Power | Other deferred credits and liabilities
Derivative [Line Items]
Derivative liabilities	131	189
Uranium | Other assets
Derivative [Line Items]
Derivative assets	1	0
Uranium | Other current liabilities
Derivative [Line Items]
Derivative liabilities	1	0
Union Electric Company
Derivative [Line Items]
Derivative assets	49	12
Derivative liabilities	77	21
Union Electric Company | Fuel Oils | Other current assets
Derivative [Line Items]
Derivative assets	8	2
Union Electric Company | Fuel Oils | Other assets
Derivative [Line Items]
Derivative assets	5	0
Union Electric Company | Fuel Oils | Other current liabilities
Derivative [Line Items]
Derivative liabilities	0	7
Union Electric Company | Fuel Oils | Other deferred credits and liabilities
Derivative [Line Items]
Derivative liabilities	0	2
Union Electric Company | Natural gas | Other current assets
Derivative [Line Items]
Derivative assets	7	1
Union Electric Company | Natural gas | Other assets
Derivative [Line Items]
Derivative assets	5	2
Union Electric Company | Natural gas | Other current liabilities
Derivative [Line Items]
Derivative liabilities	2	1
Union Electric Company | Natural gas | Other deferred credits and liabilities
Derivative [Line Items]
Derivative liabilities	1	0
Union Electric Company | Power | Other current assets
Derivative [Line Items]
Derivative assets	23	7
Union Electric Company | Power | Other current liabilities
Derivative [Line Items]
Derivative liabilities	50	3
Union Electric Company | Power | Other deferred credits and liabilities
Derivative [Line Items]
Derivative liabilities	23	8
Union Electric Company | Uranium | Other assets
Derivative [Line Items]
Derivative assets	1	0
Union Electric Company | Uranium | Other current liabilities
Derivative [Line Items]
Derivative liabilities	1	0
Ameren Illinois Company
Derivative [Line Items]
Derivative assets	41	10
Derivative liabilities	125	200
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 liabilities	0	0
Ameren Illinois Company | Fuel Oils | Other deferred credits and liabilities
Derivative [Line Items]
Derivative liabilities	0	0
Ameren Illinois Company | Natural gas | Other current assets
Derivative [Line Items]
Derivative assets	28	8
Ameren Illinois Company | Natural gas | Other assets
Derivative [Line Items]
Derivative assets	13	2
Ameren Illinois Company | Natural gas | Other current liabilities
Derivative [Line Items]
Derivative liabilities	6	1
Ameren Illinois Company | Natural gas | Other deferred credits and liabilities
Derivative [Line Items]
Derivative liabilities	2	1
Ameren Illinois Company | Power | Other current assets
Derivative [Line Items]
Derivative assets	0	0
Ameren Illinois Company | Power | Other current liabilities
Derivative [Line Items]
Derivative liabilities	9	17
Ameren Illinois Company | Power | Other deferred credits and liabilities
Derivative [Line Items]
Derivative liabilities	108	181
Ameren Illinois Company | Uranium | Other assets
Derivative [Line Items]
Derivative assets	0	0
Ameren Illinois Company | Uranium | Other current liabilities
Derivative [Line Items]
Derivative liabilities	$ 0	$ 0