Derivative Financial Instruments (Derivative Instruments Carrying Value) (Details) - Not Designated As Hedging Instrument - USD ($) $ in Millions	Dec. 31, 2020	Dec. 31, 2019
Derivative [Line Items]
Derivative assets	$ 22	$ 26
Derivative liabilities	221	255
Fuel Oils | Other current assets
Derivative [Line Items]
Derivative assets	2	4
Fuel Oils | Other assets
Derivative [Line Items]
Derivative assets	0	2
Fuel Oils | Other current liabilities
Derivative [Line Items]
Derivative liabilities	7	4
Fuel Oils | Other deferred credits and liabilities
Derivative [Line Items]
Derivative liabilities	2	3
Natural gas | Other current assets
Derivative [Line Items]
Derivative assets	9	3
Natural gas | Other assets
Derivative [Line Items]
Derivative assets	4	1
Natural gas | Other current liabilities
Derivative [Line Items]
Derivative liabilities	2	13
Natural gas | Other deferred credits and liabilities
Derivative [Line Items]
Derivative liabilities	1	7
Power | Other current assets
Derivative [Line Items]
Derivative assets	7	14
Power | Other assets
Derivative [Line Items]
Derivative assets	0	2
Power | Other current liabilities
Derivative [Line Items]
Derivative liabilities	20	19
Power | Other deferred credits and liabilities
Derivative [Line Items]
Derivative liabilities	189	208
Uranium | Other deferred credits and liabilities
Derivative [Line Items]
Derivative liabilities	0	1
Union Electric Company
Derivative [Line Items]
Derivative assets	12	22
Derivative liabilities	21	13
Union Electric Company | Fuel Oils | Other current assets
Derivative [Line Items]
Derivative assets	2	4
Union Electric Company | Fuel Oils | Other assets
Derivative [Line Items]
Derivative assets	0	2
Union Electric Company | Fuel Oils | Other current liabilities
Derivative [Line Items]
Derivative liabilities	7	4
Union Electric Company | Fuel Oils | Other deferred credits and liabilities
Derivative [Line Items]
Derivative liabilities	2	3
Union Electric Company | Natural gas | Other current assets
Derivative [Line Items]
Derivative assets	1	0
Union Electric Company | Natural gas | Other assets
Derivative [Line Items]
Derivative assets	2	0
Union Electric Company | Natural gas | Other current liabilities
Derivative [Line Items]
Derivative liabilities	1	1
Union Electric Company | Natural gas | Other deferred credits and liabilities
Derivative [Line Items]
Derivative liabilities	0	1
Union Electric Company | Power | Other current assets
Derivative [Line Items]
Derivative assets	7	14
Union Electric Company | Power | Other assets
Derivative [Line Items]
Derivative assets	0	2
Union Electric Company | Power | Other current liabilities
Derivative [Line Items]
Derivative liabilities	3	2
Union Electric Company | Power | Other deferred credits and liabilities
Derivative [Line Items]
Derivative liabilities	8	1
Union Electric Company | Uranium | Other deferred credits and liabilities
Derivative [Line Items]
Derivative liabilities	0	1
Ameren Illinois Company
Derivative [Line Items]
Derivative assets	10	4
Derivative liabilities	200	242
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	8	3
Ameren Illinois Company | Natural gas | Other assets
Derivative [Line Items]
Derivative assets	2	1
Ameren Illinois Company | Natural gas | Other current liabilities
Derivative [Line Items]
Derivative liabilities	1	12
Ameren Illinois Company | Natural gas | Other deferred credits and liabilities
Derivative [Line Items]
Derivative liabilities	1	6
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 liabilities	17	17
Ameren Illinois Company | Power | Other deferred credits and liabilities
Derivative [Line Items]
Derivative liabilities	181	207
Ameren Illinois Company | Uranium | Other deferred credits and liabilities
Derivative [Line Items]
Derivative liabilities	$ 0	$ 0