Derivative Financial Instruments (Derivative Instruments Carrying Value) (Detail) (Not Designated As Hedging Instrument [Member], USD $) In Millions, unless otherwise specified	Mar. 31, 2015	Dec. 31, 2014
Derivative [Line Items]
Derivative assets	$ 9	$ 19
Derivative liabilities	258	232
Fuel Oils | Other Current Assets [Member]
Derivative [Line Items]
Derivative assets	1	2
Fuel Oils | Other Current Liabilities [Member]
Derivative [Line Items]
Derivative liabilities	21	22
Fuel Oils | Other Deferred Credits And Liabilities [Member]
Derivative [Line Items]
Derivative liabilities	7	7
Natural Gas | Other Current Assets [Member]
Derivative [Line Items]
Derivative assets	1	2
Natural Gas | Other Current Liabilities [Member]
Derivative [Line Items]
Derivative liabilities	38	37
Natural Gas | Other Deferred Credits And Liabilities [Member]
Derivative [Line Items]
Derivative liabilities	26	19
Power | Other Current Assets [Member]
Derivative [Line Items]
Derivative assets	7	15
Power | Other Current Liabilities [Member]
Derivative [Line Items]
Derivative liabilities	12	14
Power | Other Deferred Credits And Liabilities [Member]
Derivative [Line Items]
Derivative liabilities	153	131
Uranium | Other Current Liabilities [Member]
Derivative [Line Items]
Derivative liabilities	1	2
Union Electric Company
Derivative [Line Items]
Derivative assets	8	18
Derivative liabilities	44	46
Union Electric Company | Fuel Oils | Other Current Assets [Member]
Derivative [Line Items]
Derivative assets	1	2
Union Electric Company | Fuel Oils | Other Current Liabilities [Member]
Derivative [Line Items]
Derivative liabilities	21	22
Union Electric Company | Fuel Oils | Other Deferred Credits And Liabilities [Member]
Derivative [Line Items]
Derivative liabilities	7	7
Union Electric Company | Natural Gas | Other Current Assets [Member]
Derivative [Line Items]
Derivative assets		1
Union Electric Company | Natural Gas | Other Current Liabilities [Member]
Derivative [Line Items]
Derivative liabilities	6	6
Union Electric Company | Natural Gas | Other Deferred Credits And Liabilities [Member]
Derivative [Line Items]
Derivative liabilities	8	6
Union Electric Company | Power | Other Current Assets [Member]
Derivative [Line Items]
Derivative assets	7	15
Union Electric Company | Power | Other Current Liabilities [Member]
Derivative [Line Items]
Derivative liabilities	1	3
Union Electric Company | Uranium | Other Current Liabilities [Member]
Derivative [Line Items]
Derivative liabilities	1	2
Ameren Illinois Company
Derivative [Line Items]
Derivative assets	1	1
Derivative liabilities	214	186
Ameren Illinois Company | Natural Gas | Other Current Assets [Member]
Derivative [Line Items]
Derivative assets	1	1
Ameren Illinois Company | Natural Gas | Other Deferred Credits And Liabilities [Member]
Derivative [Line Items]
Derivative liabilities	18	13
Ameren Illinois Company | Natural Gas | Mark To Market Derivative Liabilities [Member]
Derivative [Line Items]
Derivative liabilities	32	31
Ameren Illinois Company | Power | Other Deferred Credits And Liabilities [Member]
Derivative [Line Items]
Derivative liabilities	153	131
Ameren Illinois Company | Power | Mark To Market Derivative Liabilities [Member]
Derivative [Line Items]
Derivative liabilities	$ 11	$ 11