Derivative Financial Instruments (Derivative Instruments Carrying Value) (Detail) (Not Designated As Hedging Instrument [Member], USD $) In Millions, unless otherwise specified	Jun. 30, 2014	Dec. 31, 2013
Derivative [Line Items]
Derivative assets	$ 34	$ 34
Derivative liabilities	149	178
Fuel Oils | Other Current Assets [Member]
Derivative [Line Items]
Derivative assets	5	6
Fuel Oils | Other Assets [Member]
Derivative [Line Items]
Derivative assets	2	3
Fuel Oils | Other Current Liabilities [Member]
Derivative [Line Items]
Derivative liabilities	2	2
Fuel Oils | Other Deferred Credits And Liabilities [Member]
Derivative [Line Items]
Derivative liabilities	1	1
Natural Gas | Other Current Assets [Member]
Derivative [Line Items]
Derivative assets	4	2
Natural Gas | Other Assets [Member]
Derivative [Line Items]
Derivative assets	1
Natural Gas | Other Current Liabilities [Member]
Derivative [Line Items]
Derivative liabilities	20	32
Natural Gas | Other Deferred Credits And Liabilities [Member]
Derivative [Line Items]
Derivative liabilities	10	25
Power | Other Current Assets [Member]
Derivative [Line Items]
Derivative assets	22	23
Power | Other Current Liabilities [Member]
Derivative [Line Items]
Derivative liabilities	13	13
Power | Other Deferred Credits And Liabilities [Member]
Derivative [Line Items]
Derivative liabilities	96	99
Uranium | Other Current Liabilities [Member]
Derivative [Line Items]
Derivative liabilities	5	5
Uranium | Other Deferred Credits And Liabilities [Member]
Derivative [Line Items]
Derivative liabilities	2	1
Union Electric Company
Derivative [Line Items]
Derivative assets	30	33
Derivative liabilities	22	24
Union Electric Company | Fuel Oils | Other Current Assets [Member]
Derivative [Line Items]
Derivative assets	5	6
Union Electric Company | Fuel Oils | Other Assets [Member]
Derivative [Line Items]
Derivative assets	2	3
Union Electric Company | Fuel Oils | Other Current Liabilities [Member]
Derivative [Line Items]
Derivative liabilities	2	2
Union Electric Company | Fuel Oils | Other Deferred Credits And Liabilities [Member]
Derivative [Line Items]
Derivative liabilities	1	1
Union Electric Company | Natural Gas | Other Current Assets [Member]
Derivative [Line Items]
Derivative assets	1	1
Union Electric Company | Natural Gas | Other Current Liabilities [Member]
Derivative [Line Items]
Derivative liabilities	4	5
Union Electric Company | Natural Gas | Other Deferred Credits And Liabilities [Member]
Derivative [Line Items]
Derivative liabilities	2	6
Union Electric Company | Power | Other Current Assets [Member]
Derivative [Line Items]
Derivative assets	22	23
Union Electric Company | Power | Other Current Liabilities [Member]
Derivative [Line Items]
Derivative liabilities	6	4
Union Electric Company | Uranium | Other Current Liabilities [Member]
Derivative [Line Items]
Derivative liabilities	5	5
Union Electric Company | Uranium | Other Deferred Credits And Liabilities [Member]
Derivative [Line Items]
Derivative liabilities	2	1
Ameren Illinois Company
Derivative [Line Items]
Derivative assets	4	1
Derivative liabilities	127	154
Ameren Illinois Company | Natural Gas | Other Current Assets [Member]
Derivative [Line Items]
Derivative assets	3	1
Ameren Illinois Company | Natural Gas | Other Assets [Member]
Derivative [Line Items]
Derivative assets	1
Ameren Illinois Company | Natural Gas | Other Current Liabilities [Member]
Derivative [Line Items]
Derivative liabilities	16	27
Ameren Illinois Company | Natural Gas | Other Deferred Credits And Liabilities [Member]
Derivative [Line Items]
Derivative liabilities	8	19
Ameren Illinois Company | Power | Other Current Liabilities [Member]
Derivative [Line Items]
Derivative liabilities	7	9
Ameren Illinois Company | Power | Other Deferred Credits And Liabilities [Member]
Derivative [Line Items]
Derivative liabilities	$ 96	$ 99