Derivative Financial Instruments (Derivative Instruments Carrying Value) (Detail) (Not Designated As Hedging Instrument [Member], USD $) In Millions, unless otherwise specified	Mar. 31, 2014		Dec. 31, 2013
Derivative [Line Items]
Derivative assets	$ 28	[1]	$ 34	[1]
Derivative liabilities	172	[1]	178	[1]
Fuel Oils | Other Current Assets [Member]
Derivative [Line Items]
Derivative assets	5	[1]	6	[1]
Fuel Oils | Other Assets [Member]
Derivative [Line Items]
Derivative assets	1	[1]	3	[1]
Fuel Oils | Other Current Liabilities [Member]
Derivative [Line Items]
Derivative liabilities	2	[1]	2	[1]
Fuel Oils | Other Deferred Credits And Liabilities [Member]
Derivative [Line Items]
Derivative liabilities	2	[1]	1	[1]
Natural Gas | Other Current Assets [Member]
Derivative [Line Items]
Derivative assets	6	[1]	2	[1]
Natural Gas | Other Assets [Member]
Derivative [Line Items]
Derivative assets	1	[1]
Natural Gas | Other Current Liabilities [Member]
Derivative [Line Items]
Derivative liabilities	20	[1]	32	[1]
Natural Gas | Other Deferred Credits And Liabilities [Member]
Derivative [Line Items]
Derivative liabilities	17	[1]	25	[1]
Power | Other Current Assets [Member]
Derivative [Line Items]
Derivative assets	15	[1]	23	[1]
Power | Other Current Liabilities [Member]
Derivative [Line Items]
Derivative liabilities	14	[1]	13	[1]
Power | Other Deferred Credits And Liabilities [Member]
Derivative [Line Items]
Derivative liabilities	112	[1]	99	[1]
Uranium | Other Current Liabilities [Member]
Derivative [Line Items]
Derivative liabilities	4	[1]	5	[1]
Uranium | Other Deferred Credits And Liabilities [Member]
Derivative [Line Items]
Derivative liabilities	1	[1]	1	[1]
Union Electric Company
Derivative [Line Items]
Derivative assets	23	[1]	33	[1]
Derivative liabilities	23	[1]	24	[1]
Union Electric Company | Fuel Oils | Other Current Assets [Member]
Derivative [Line Items]
Derivative assets	5	[1]	6	[1]
Union Electric Company | Fuel Oils | Other Assets [Member]
Derivative [Line Items]
Derivative assets	1	[1]	3	[1]
Union Electric Company | Fuel Oils | Other Current Liabilities [Member]
Derivative [Line Items]
Derivative liabilities	2	[1]	2	[1]
Union Electric Company | Fuel Oils | Other Deferred Credits And Liabilities [Member]
Derivative [Line Items]
Derivative liabilities	2	[1]	1	[1]
Union Electric Company | Natural Gas | Other Current Assets [Member]
Derivative [Line Items]
Derivative assets	1	[1]	1	[1]
Union Electric Company | Natural Gas | Other Assets [Member]
Derivative [Line Items]
Derivative assets	1	[1]
Union Electric Company | Natural Gas | Other Current Liabilities [Member]
Derivative [Line Items]
Derivative liabilities	4	[1]	5	[1]
Union Electric Company | Natural Gas | Other Deferred Credits And Liabilities [Member]
Derivative [Line Items]
Derivative liabilities	4	[1]	6	[1]
Union Electric Company | Power | Other Current Assets [Member]
Derivative [Line Items]
Derivative assets	15	[1]	23	[1]
Union Electric Company | Power | Other Current Liabilities [Member]
Derivative [Line Items]
Derivative liabilities	6	[1]	4	[1]
Union Electric Company | Uranium | Other Current Liabilities [Member]
Derivative [Line Items]
Derivative liabilities	4	[1]	5	[1]
Union Electric Company | Uranium | Other Deferred Credits And Liabilities [Member]
Derivative [Line Items]
Derivative liabilities	1	[1]	1	[1]
Ameren Illinois Company
Derivative [Line Items]
Derivative assets	5	[1]	1	[1]
Derivative liabilities	149	[1]	154	[1]
Ameren Illinois Company | Natural Gas | Other Current Assets [Member]
Derivative [Line Items]
Derivative assets	5	[1]	1	[1]
Ameren Illinois Company | Natural Gas | Other Current Liabilities [Member]
Derivative [Line Items]
Derivative liabilities	16	[1]	27	[1]
Ameren Illinois Company | Natural Gas | Other Deferred Credits And Liabilities [Member]
Derivative [Line Items]
Derivative liabilities	13	[1]	19	[1]
Ameren Illinois Company | Power | Other Current Liabilities [Member]
Derivative [Line Items]
Derivative liabilities	8	[1]	9	[1]
Ameren Illinois Company | Power | Other Deferred Credits And Liabilities [Member]
Derivative [Line Items]
Derivative liabilities	$ 112	[1]	$ 99	[1]

[1] Includes derivatives subject to regulatory deferral.