Derivative Financial Instruments (Derivative Instruments Carrying Value) (Details) - Not Designated As Hedging Instrument - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
Derivative [Line Items]
Derivative assets	$ 17	$ 27
Derivative liabilities	226	211
Fuel Oils | Other current assets
Derivative [Line Items]
Derivative assets	5	2
Fuel Oils | Other assets
Derivative [Line Items]
Derivative assets	2	1
Fuel Oils | Other current liabilities
Derivative [Line Items]
Derivative liabilities		5
Natural Gas | Other current assets
Derivative [Line Items]
Derivative assets		12
Natural Gas | Other assets
Derivative [Line Items]
Derivative assets	1	3
Natural Gas | Other current liabilities
Derivative [Line Items]
Derivative liabilities	17	4
Natural Gas | Other deferred credits and liabilities
Derivative [Line Items]
Derivative liabilities	13	10
Power | Other current assets
Derivative [Line Items]
Derivative assets	9	9
Power | Other current liabilities
Derivative [Line Items]
Derivative liabilities	14	15
Power | Other deferred credits and liabilities
Derivative [Line Items]
Derivative liabilities	182	173
Uranium | Other deferred credits and liabilities
Derivative [Line Items]
Derivative liabilities	0	4
Union Electric Company
Derivative [Line Items]
Derivative assets	17	14
Derivative liabilities	9	18
Union Electric Company | Fuel Oils | Other current assets
Derivative [Line Items]
Derivative assets	5	2
Union Electric Company | Fuel Oils | Other assets
Derivative [Line Items]
Derivative assets	2	1
Union Electric Company | Fuel Oils | Other current liabilities
Derivative [Line Items]
Derivative liabilities		5
Union Electric Company | Natural Gas | Other current assets
Derivative [Line Items]
Derivative assets		1
Union Electric Company | Natural Gas | Other assets
Derivative [Line Items]
Derivative assets	1	1
Union Electric Company | Natural Gas | Other current liabilities
Derivative [Line Items]
Derivative liabilities	5	1
Union Electric Company | Natural Gas | Other deferred credits and liabilities
Derivative [Line Items]
Derivative liabilities	3	5
Union Electric Company | Power | Other current assets
Derivative [Line Items]
Derivative assets	9	9
Union Electric Company | Power | Other current liabilities
Derivative [Line Items]
Derivative liabilities	1	3
Union Electric Company | Power | Other deferred credits and liabilities
Derivative [Line Items]
Derivative liabilities	0	0
Union Electric Company | Uranium | Other deferred credits and liabilities
Derivative [Line Items]
Derivative liabilities	0	4
Ameren Illinois Company
Derivative [Line Items]
Derivative assets	0	13
Derivative liabilities	217	193
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
Ameren Illinois Company | Natural Gas | Other current assets
Derivative [Line Items]
Derivative assets		11
Ameren Illinois Company | Natural Gas | Other assets
Derivative [Line Items]
Derivative assets	0	2
Ameren Illinois Company | Natural Gas | Other current liabilities
Derivative [Line Items]
Derivative liabilities	12	3
Ameren Illinois Company | Natural Gas | Other deferred credits and liabilities
Derivative [Line Items]
Derivative liabilities	10	5
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	13	12
Ameren Illinois Company | Power | Other deferred credits and liabilities
Derivative [Line Items]
Derivative liabilities	182	173
Ameren Illinois Company | Uranium | Other deferred credits and liabilities
Derivative [Line Items]
Derivative liabilities	$ 0	$ 0