Derivative Financial Instruments (Derivative Instruments Carrying Value) (Details) (Not Designated As Hedging Instrument, USD $) In Millions, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013
Derivative [Line Items]
Derivative assets	$ 19	$ 34
Derivative liabilities	232	178
Fuel Oils | Other current assets
Derivative [Line Items]
Derivative assets	2	6
Fuel Oils | Other assets
Derivative [Line Items]
Derivative assets		3
Fuel Oils | Other current liabilities
Derivative [Line Items]
Derivative liabilities	22	2
Fuel Oils | Other deferred credits and liabilities
Derivative [Line Items]
Derivative liabilities	7	1
Natural Gas | Other current assets
Derivative [Line Items]
Derivative assets	2	2
Natural Gas | Other current liabilities
Derivative [Line Items]
Derivative liabilities	37	32
Natural Gas | Other deferred credits and liabilities
Derivative [Line Items]
Derivative liabilities	19	25
Power | Other current assets
Derivative [Line Items]
Derivative assets	15	23
Power | Other current liabilities
Derivative [Line Items]
Derivative liabilities	14	13
Power | Other deferred credits and liabilities
Derivative [Line Items]
Derivative liabilities	131	99
Uranium | Other current liabilities
Derivative [Line Items]
Derivative liabilities	2	5
Uranium | Other deferred credits and liabilities
Derivative [Line Items]
Derivative liabilities		1
Union Electric Company
Derivative [Line Items]
Derivative assets	18	33
Derivative liabilities	46	24
Union Electric Company | Fuel Oils | Other current assets
Derivative [Line Items]
Derivative assets	2	6
Union Electric Company | Fuel Oils | Other assets
Derivative [Line Items]
Derivative assets		3
Union Electric Company | Fuel Oils | Other current liabilities
Derivative [Line Items]
Derivative liabilities	22	2
Union Electric Company | Fuel Oils | Other deferred credits and liabilities
Derivative [Line Items]
Derivative liabilities	7	1
Union Electric Company | Natural Gas | Other current assets
Derivative [Line Items]
Derivative assets	1	1
Union Electric Company | Natural Gas | Other current liabilities
Derivative [Line Items]
Derivative liabilities	6	5
Union Electric Company | Natural Gas | Other deferred credits and liabilities
Derivative [Line Items]
Derivative liabilities	6	6
Union Electric Company | Power | Other current assets
Derivative [Line Items]
Derivative assets	15	23
Union Electric Company | Power | Other current liabilities
Derivative [Line Items]
Derivative liabilities	3	4
Union Electric Company | Uranium | Other current liabilities
Derivative [Line Items]
Derivative liabilities	2	5
Union Electric Company | Uranium | Other deferred credits and liabilities
Derivative [Line Items]
Derivative liabilities		1
Ameren Illinois Company
Derivative [Line Items]
Derivative assets	1	1
Derivative liabilities	186	154
Ameren Illinois Company | Natural Gas | Other current assets
Derivative [Line Items]
Derivative assets	1	1
Ameren Illinois Company | Natural Gas | MTM derivative liabilities
Derivative [Line Items]
Derivative liabilities	31	27
Ameren Illinois Company | Natural Gas | Other deferred credits and liabilities
Derivative [Line Items]
Derivative liabilities	13	19
Ameren Illinois Company | Power | MTM derivative liabilities
Derivative [Line Items]
Derivative liabilities	11	9
Ameren Illinois Company | Power | Other deferred credits and liabilities
Derivative [Line Items]
Derivative liabilities	$ 131	$ 99