Derivative Financial Instruments (Derivative Instruments Carrying Value) (Details) - Not Designated As Hedging Instrument - USD ($) $ in Millions	Dec. 31, 2019	Dec. 31, 2018
Derivative [Line Items]
Derivative assets	$ 26	$ 15
Derivative liabilities	255	219
Fuel Oils | Other current assets
Derivative [Line Items]
Derivative assets	4	3
Fuel Oils | Other assets
Derivative [Line Items]
Derivative assets	2	5
Fuel Oils | Other current liabilities
Derivative [Line Items]
Derivative liabilities	4	4
Fuel Oils | Other deferred credits and liabilities
Derivative [Line Items]
Derivative liabilities	3	9
Natural gas | Other current assets
Derivative [Line Items]
Derivative assets	3	1
Natural gas | Other assets
Derivative [Line Items]
Derivative assets	1	2
Natural gas | Other current liabilities
Derivative [Line Items]
Derivative liabilities	13	12
Natural gas | Other deferred credits and liabilities
Derivative [Line Items]
Derivative liabilities	7	7
Power | Other current assets
Derivative [Line Items]
Derivative assets	14	4
Power | Other assets
Derivative [Line Items]
Derivative assets	2	0
Power | Other current liabilities
Derivative [Line Items]
Derivative liabilities	19	18
Power | Other deferred credits and liabilities
Derivative [Line Items]
Derivative liabilities	208	169
Uranium | Other deferred credits and liabilities
Derivative [Line Items]
Derivative liabilities	1	0
Union Electric Company
Derivative [Line Items]
Derivative assets	22	12
Derivative liabilities	13	22
Union Electric Company | Fuel Oils | Other current assets
Derivative [Line Items]
Derivative assets	4	3
Union Electric Company | Fuel Oils | Other assets
Derivative [Line Items]
Derivative assets	2	5
Union Electric Company | Fuel Oils | Other current liabilities
Derivative [Line Items]
Derivative liabilities	4	4
Union Electric Company | Fuel Oils | Other deferred credits and liabilities
Derivative [Line Items]
Derivative liabilities	3	9
Union Electric Company | Natural gas | Other current assets
Derivative [Line Items]
Derivative assets	0	0
Union Electric Company | Natural gas | Other assets
Derivative [Line Items]
Derivative assets	0	0
Union Electric Company | Natural gas | Other current liabilities
Derivative [Line Items]
Derivative liabilities	1	4
Union Electric Company | Natural gas | Other deferred credits and liabilities
Derivative [Line Items]
Derivative liabilities	1	1
Union Electric Company | Power | Other current assets
Derivative [Line Items]
Derivative assets	14	4
Union Electric Company | Power | Other assets
Derivative [Line Items]
Derivative assets	2
Union Electric Company | Power | Other current liabilities
Derivative [Line Items]
Derivative liabilities	2	4
Union Electric Company | Power | Other deferred credits and liabilities
Derivative [Line Items]
Derivative liabilities	1	0
Union Electric Company | Uranium | Other deferred credits and liabilities
Derivative [Line Items]
Derivative liabilities	1	0
Ameren Illinois Company
Derivative [Line Items]
Derivative assets	4	3
Derivative liabilities	242	197
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	3	1
Ameren Illinois Company | Natural gas | Other assets
Derivative [Line Items]
Derivative assets	1	2
Ameren Illinois Company | Natural gas | Other current liabilities
Derivative [Line Items]
Derivative liabilities	12	8
Ameren Illinois Company | Natural gas | Other deferred credits and liabilities
Derivative [Line Items]
Derivative liabilities	6	6
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	17	14
Ameren Illinois Company | Power | Other deferred credits and liabilities
Derivative [Line Items]
Derivative liabilities	207	169
Ameren Illinois Company | Uranium | Other deferred credits and liabilities
Derivative [Line Items]
Derivative liabilities	$ 0	$ 0