Derivative Financial Instruments (Derivative Instruments Carrying Value) (Details) - Not Designated As Hedging Instrument - USD ($) $ in Millions	Dec. 31, 2018	Dec. 31, 2017
Derivative [Line Items]
Derivative assets	$ 15	$ 17
Derivative liabilities	219	226
Fuel Oils | Other current assets
Derivative [Line Items]
Derivative assets	3	5
Fuel Oils | Other assets
Derivative [Line Items]
Derivative assets	5	2
Fuel Oils | Other current liabilities
Derivative [Line Items]
Derivative liabilities	4	0
Fuel Oils | Other deferred credits and liabilities
Derivative [Line Items]
Derivative liabilities	9	0
Natural gas | Other current assets
Derivative [Line Items]
Derivative assets	1	0
Natural gas | Other assets
Derivative [Line Items]
Derivative assets	2	1
Natural gas | Other current liabilities
Derivative [Line Items]
Derivative liabilities	12	17
Natural gas | Other deferred credits and liabilities
Derivative [Line Items]
Derivative liabilities	7	13
Power | Other current assets
Derivative [Line Items]
Derivative assets	4	9
Power | Other current liabilities
Derivative [Line Items]
Derivative liabilities	18	14
Power | Other deferred credits and liabilities
Derivative [Line Items]
Derivative liabilities	169	182
Union Electric Company
Derivative [Line Items]
Derivative assets	12	17
Derivative liabilities	22	9
Union Electric Company | Fuel Oils | Other current assets
Derivative [Line Items]
Derivative assets	3	5
Union Electric Company | Fuel Oils | Other assets
Derivative [Line Items]
Derivative assets	5	2
Union Electric Company | Fuel Oils | Other current liabilities
Derivative [Line Items]
Derivative liabilities	4	0
Union Electric Company | Fuel Oils | Other deferred credits and liabilities
Derivative [Line Items]
Derivative liabilities	9	0
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	1
Union Electric Company | Natural gas | Other current liabilities
Derivative [Line Items]
Derivative liabilities	4	5
Union Electric Company | Natural gas | Other deferred credits and liabilities
Derivative [Line Items]
Derivative liabilities	1	3
Union Electric Company | Power | Other current assets
Derivative [Line Items]
Derivative assets	4	9
Union Electric Company | Power | Other current liabilities
Derivative [Line Items]
Derivative liabilities	4	1
Union Electric Company | Power | Other deferred credits and liabilities
Derivative [Line Items]
Derivative liabilities	0	0
Ameren Illinois Company
Derivative [Line Items]
Derivative assets	3	0
Derivative liabilities	197	217
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	1	0
Ameren Illinois Company | Natural gas | Other assets
Derivative [Line Items]
Derivative assets	2	0
Ameren Illinois Company | Natural gas | Other current liabilities
Derivative [Line Items]
Derivative liabilities	8	12
Ameren Illinois Company | Natural gas | Other deferred credits and liabilities
Derivative [Line Items]
Derivative liabilities	6	10
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	14	13
Ameren Illinois Company | Power | Other deferred credits and liabilities
Derivative [Line Items]
Derivative liabilities	$ 169	$ 182