Derivative Financial Instruments (Derivative Instruments Carrying Value) (Detail) - Not Designated As Hedging Instrument - USD ($) $ in Millions	Sep. 30, 2015	Dec. 31, 2014
Derivative [Line Items]
Derivative assets	$ 24	$ 19
Derivative liabilities	255	232
Fuel Oils | Other Current Assets
Derivative [Line Items]
Derivative assets		2
Fuel Oils | Other Current Liabilities
Derivative [Line Items]
Derivative liabilities	19	22
Fuel Oils | Other Deferred Credits And Liabilities
Derivative [Line Items]
Derivative liabilities	6	7
Natural Gas | Other Current Assets
Derivative [Line Items]
Derivative assets	1	2
Natural Gas | Other Current Liabilities
Derivative [Line Items]
Derivative liabilities	31	37
Natural Gas | Other Deferred Credits And Liabilities
Derivative [Line Items]
Derivative liabilities	27	19
Power | Other Current Assets
Derivative [Line Items]
Derivative assets	22	15
Power | Other Assets
Derivative [Line Items]
Derivative assets	1
Power | Other Current Liabilities
Derivative [Line Items]
Derivative liabilities	13	14
Power | Other Deferred Credits And Liabilities
Derivative [Line Items]
Derivative liabilities	158	131
Uranium | Other Current Liabilities
Derivative [Line Items]
Derivative liabilities	1	2
Union Electric Company
Derivative [Line Items]
Derivative assets	24	18
Derivative liabilities	41	46
Union Electric Company | Fuel Oils | Other Current Assets
Derivative [Line Items]
Derivative assets		2
Union Electric Company | Fuel Oils | Other Current Liabilities
Derivative [Line Items]
Derivative liabilities	19	22
Union Electric Company | Fuel Oils | Other Deferred Credits And Liabilities
Derivative [Line Items]
Derivative liabilities	6	7
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	5	6
Union Electric Company | Natural Gas | Other Deferred Credits And Liabilities
Derivative [Line Items]
Derivative liabilities	9	6
Union Electric Company | Power | Other Current Assets
Derivative [Line Items]
Derivative assets	22	15
Union Electric Company | Power | Other Assets
Derivative [Line Items]
Derivative assets	1
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 Current Liabilities
Derivative [Line Items]
Derivative liabilities	1	2
Ameren Illinois Company
Derivative [Line Items]
Derivative assets	0	1
Derivative liabilities	214	186
Ameren Illinois Company | Fuel Oils | Other Current Assets
Derivative [Line Items]
Derivative assets		0
Ameren Illinois Company | Fuel Oils | Other Current Liabilities
Derivative [Line Items]
Derivative liabilities	0	0
Ameren Illinois Company | Fuel Oils | Other Deferred Credits And Liabilities
Derivative [Line Items]
Derivative liabilities	0	0
Ameren Illinois Company | Natural Gas | Other Current Assets
Derivative [Line Items]
Derivative assets	0	1
Ameren Illinois Company | Natural Gas | Other Current Liabilities
Derivative [Line Items]
Derivative liabilities	0	0
Ameren Illinois Company | Natural Gas | Other Deferred Credits And Liabilities
Derivative [Line Items]
Derivative liabilities	18	13
Ameren Illinois Company | Natural Gas | Mark To Market Derivative Liabilities
Derivative [Line Items]
Derivative liabilities	26	31
Ameren Illinois Company | Power | Other Current Assets
Derivative [Line Items]
Derivative assets	0	0
Ameren Illinois Company | Power | Other Assets
Derivative [Line Items]
Derivative assets	0
Ameren Illinois Company | Power | Other Current Liabilities
Derivative [Line Items]
Derivative liabilities	0	0
Ameren Illinois Company | Power | Other Deferred Credits And Liabilities
Derivative [Line Items]
Derivative liabilities	158	131
Ameren Illinois Company | Power | Mark To Market Derivative Liabilities
Derivative [Line Items]
Derivative liabilities	12	11
Ameren Illinois Company | Uranium | Other Current Liabilities
Derivative [Line Items]
Derivative liabilities	$ 0	$ 0