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Derivative Financial Instruments (Derivative Instruments Carrying Value) (Detail) (USD $)
In Millions, unless otherwise specified
Sep. 30, 2012
Dec. 31, 2011
Derivative [Line Items]    
Derivative liabilities $ 328 [1] $ 281 [1]
Designated As Hedging Instrument [Member]
    
Derivative [Line Items]    
Derivative assets 46 [1] 24 [1]
Derivative liabilities 2 [1] 1 [1]
Designated As Hedging Instrument [Member] | Power [Member] | Mark To Market Derivative Assets [Member]
    
Derivative [Line Items]    
Derivative assets 23 [1] 8 [1]
Designated As Hedging Instrument [Member] | Power [Member] | Other Assets [Member]
    
Derivative [Line Items]    
Derivative assets 23 [1] 16 [1]
Designated As Hedging Instrument [Member] | Power [Member] | Mark To Market Derivative Liabilities [Member]
    
Derivative [Line Items]    
Derivative liabilities 2 [1]  
Designated As Hedging Instrument [Member] | Power [Member] | Other Deferred Credits And Liabilities [Member]
    
Derivative [Line Items]    
Derivative liabilities   1 [1]
Not Designated As Hedging Instrument [Member]
    
Derivative [Line Items]    
Derivative assets 151 [1],[2] 214 [1],[2]
Derivative liabilities 326 [1],[2] 280 [1],[2]
Not Designated As Hedging Instrument [Member] | Power [Member] | Mark To Market Derivative Assets [Member]
    
Derivative [Line Items]    
Derivative assets 85 [1],[2] 72 [1],[2]
Not Designated As Hedging Instrument [Member] | Power [Member] | Other Assets [Member]
    
Derivative [Line Items]    
Derivative assets 32 [1],[2] 99 [1],[2]
Not Designated As Hedging Instrument [Member] | Power [Member] | Mark To Market Derivative Liabilities [Member]
    
Derivative [Line Items]    
Derivative liabilities 74 [1],[2] 53 [1],[2]
Not Designated As Hedging Instrument [Member] | Power [Member] | Other Deferred Credits And Liabilities [Member]
    
Derivative [Line Items]    
Derivative liabilities 111 [1],[2] 26 [1],[2]
Not Designated As Hedging Instrument [Member] | Fuel Oils [Member] | Mark To Market Derivative Assets [Member]
    
Derivative [Line Items]    
Derivative assets 17 [1],[2] 29 [1],[2]
Not Designated As Hedging Instrument [Member] | Fuel Oils [Member] | Other Assets [Member]
    
Derivative [Line Items]    
Derivative assets 6 [1],[2] 8 [1],[2]
Not Designated As Hedging Instrument [Member] | Fuel Oils [Member] | Mark To Market Derivative Liabilities [Member]
    
Derivative [Line Items]    
Derivative liabilities 1 [1],[2] 2 [1],[2]
Not Designated As Hedging Instrument [Member] | Fuel Oils [Member] | Other Deferred Credits And Liabilities [Member]
    
Derivative [Line Items]    
Derivative liabilities 3 [1],[2]  
Not Designated As Hedging Instrument [Member] | Natural Gas [Member] | Mark To Market Derivative Assets [Member]
    
Derivative [Line Items]    
Derivative assets 9 [1],[2] 6 [1],[2]
Not Designated As Hedging Instrument [Member] | Natural Gas [Member] | Other Assets [Member]
    
Derivative [Line Items]    
Derivative assets 2 [1],[2]  
Not Designated As Hedging Instrument [Member] | Natural Gas [Member] | Mark To Market Derivative Liabilities [Member]
    
Derivative [Line Items]    
Derivative liabilities 71 [1],[2] 106 [1],[2]
Not Designated As Hedging Instrument [Member] | Natural Gas [Member] | Other Deferred Credits And Liabilities [Member]
    
Derivative [Line Items]    
Derivative liabilities 54 [1],[2] 92 [1],[2]
Not Designated As Hedging Instrument [Member] | Coal [Member] | Mark To Market Derivative Liabilities [Member]
    
Derivative [Line Items]    
Derivative liabilities 6 [1],[2]  
Not Designated As Hedging Instrument [Member] | Coal [Member] | Other Deferred Credits And Liabilities [Member]
    
Derivative [Line Items]    
Derivative liabilities 4 [1],[2]  
Not Designated As Hedging Instrument [Member] | Uranium [Member] | Mark To Market Derivative Liabilities [Member]
    
Derivative [Line Items]    
Derivative liabilities 1 [1],[2]  
Not Designated As Hedging Instrument [Member] | Uranium [Member] | Other Deferred Credits And Liabilities [Member]
    
Derivative [Line Items]    
Derivative liabilities 1 [1],[2] 1 [1],[2]
Ameren Missouri [Member]
    
Derivative [Line Items]    
Derivative liabilities 32 37
Ameren Missouri [Member] | Not Designated As Hedging Instrument [Member]
    
Derivative [Line Items]    
Derivative assets 45 [2] 55 [2]
Derivative liabilities 32 [2] 37 [2]
Ameren Missouri [Member] | Not Designated As Hedging Instrument [Member] | Power [Member] | Other Assets [Member]
    
Derivative [Line Items]    
Derivative assets 3 [2]  
Ameren Missouri [Member] | Not Designated As Hedging Instrument [Member] | Power [Member] | Other Current Assets [Member]
    
Derivative [Line Items]    
Derivative assets 25 [2] 30 [2]
Ameren Missouri [Member] | Not Designated As Hedging Instrument [Member] | Power [Member] | Other Current Liabilities [Member]
    
Derivative [Line Items]    
Derivative liabilities 9 [2] 9 [2]
Ameren Missouri [Member] | Not Designated As Hedging Instrument [Member] | Power [Member] | Other Deferred Credits And Liabilities [Member]
    
Derivative [Line Items]    
Derivative liabilities 2 [2]  
Ameren Missouri [Member] | Not Designated As Hedging Instrument [Member] | Fuel Oils [Member] | Other Assets [Member]
    
Derivative [Line Items]    
Derivative assets 4 [2] 6 [2]
Ameren Missouri [Member] | Not Designated As Hedging Instrument [Member] | Fuel Oils [Member] | Other Current Assets [Member]
    
Derivative [Line Items]    
Derivative assets 11 [2] 17 [2]
Ameren Missouri [Member] | Not Designated As Hedging Instrument [Member] | Fuel Oils [Member] | Other Current Liabilities [Member]
    
Derivative [Line Items]    
Derivative liabilities 1 [2] 1 [2]
Ameren Missouri [Member] | Not Designated As Hedging Instrument [Member] | Fuel Oils [Member] | Other Deferred Credits And Liabilities [Member]
    
Derivative [Line Items]    
Derivative liabilities 1 [2]  
Ameren Missouri [Member] | Not Designated As Hedging Instrument [Member] | Natural Gas [Member] | Other Assets [Member]
    
Derivative [Line Items]    
Derivative assets 1 [2]  
Ameren Missouri [Member] | Not Designated As Hedging Instrument [Member] | Natural Gas [Member] | Other Current Assets [Member]
    
Derivative [Line Items]    
Derivative assets 1 [2] 2 [2]
Ameren Missouri [Member] | Not Designated As Hedging Instrument [Member] | Natural Gas [Member] | Other Current Liabilities [Member]
    
Derivative [Line Items]    
Derivative liabilities 8 [2] 13 [2]
Ameren Missouri [Member] | Not Designated As Hedging Instrument [Member] | Natural Gas [Member] | Other Deferred Credits And Liabilities [Member]
    
Derivative [Line Items]    
Derivative liabilities 9 [2] 13 [2]
Ameren Missouri [Member] | Not Designated As Hedging Instrument [Member] | Uranium [Member] | Other Current Liabilities [Member]
    
Derivative [Line Items]    
Derivative liabilities 1 [2]  
Ameren Missouri [Member] | Not Designated As Hedging Instrument [Member] | Uranium [Member] | Other Deferred Credits And Liabilities [Member]
    
Derivative [Line Items]    
Derivative liabilities 1 [2] 1 [2]
Ameren Illinois Company [Member]
    
Derivative [Line Items]    
Derivative assets   79
Derivative liabilities 269 386
Ameren Illinois Company [Member] | Not Designated As Hedging Instrument [Member]
    
Derivative [Line Items]    
Derivative assets 2 [2] 79 [2]
Derivative liabilities 269 [2] 386 [2]
Ameren Illinois Company [Member] | Not Designated As Hedging Instrument [Member] | Power [Member] | Other Assets [Member]
    
Derivative [Line Items]    
Derivative assets   77 [2]
Ameren Illinois Company [Member] | Not Designated As Hedging Instrument [Member] | Power [Member] | Mark To Market Derivative Liabilities [Member]
    
Derivative [Line Items]    
Derivative liabilities 19 [2] 9 [2]
Ameren Illinois Company [Member] | Not Designated As Hedging Instrument [Member] | Power [Member] | Other Deferred Credits And Liabilities [Member]
    
Derivative [Line Items]    
Derivative liabilities 87 [2] 8 [2]
Ameren Illinois Company [Member] | Not Designated As Hedging Instrument [Member] | Power [Member] | Mark To Market Derivative Liabilities - Affiliates [Member]
    
Derivative [Line Items]    
Derivative liabilities 59 [2] 200 [2]
Ameren Illinois Company [Member] | Not Designated As Hedging Instrument [Member] | Natural Gas [Member] | Other Assets [Member]
    
Derivative [Line Items]    
Derivative assets   1 [2]
Ameren Illinois Company [Member] | Not Designated As Hedging Instrument [Member] | Natural Gas [Member] | Mark To Market Derivative Liabilities [Member]
    
Derivative [Line Items]    
Derivative liabilities 59 [2] 90 [2]
Ameren Illinois Company [Member] | Not Designated As Hedging Instrument [Member] | Natural Gas [Member] | Other Current Assets [Member]
    
Derivative [Line Items]    
Derivative assets 2 [2] 1 [2]
Ameren Illinois Company [Member] | Not Designated As Hedging Instrument [Member] | Natural Gas [Member] | Other Deferred Credits And Liabilities [Member]
    
Derivative [Line Items]    
Derivative liabilities 45 [2] 79 [2]
Genco [Member]
    
Derivative [Line Items]    
Derivative assets 14 13
Derivative liabilities 14 3
Genco [Member] | Not Designated As Hedging Instrument [Member]
    
Derivative [Line Items]    
Derivative assets 14 [2] 13 [2]
Derivative liabilities 14 [2] 3 [2]
Genco [Member] | Not Designated As Hedging Instrument [Member] | Fuel Oils [Member] | Other Assets [Member]
    
Derivative [Line Items]    
Derivative assets 1 [2] 1 [2]
Genco [Member] | Not Designated As Hedging Instrument [Member] | Fuel Oils [Member] | Other Current Assets [Member]
    
Derivative [Line Items]    
Derivative assets 6 [2] 10 [2]
Genco [Member] | Not Designated As Hedging Instrument [Member] | Fuel Oils [Member] | Other Current Liabilities [Member]
    
Derivative [Line Items]    
Derivative liabilities   1 [2]
Genco [Member] | Not Designated As Hedging Instrument [Member] | Fuel Oils [Member] | Other Deferred Credits And Liabilities [Member]
    
Derivative [Line Items]    
Derivative liabilities 2 [2]  
Genco [Member] | Not Designated As Hedging Instrument [Member] | Natural Gas [Member] | Other Assets [Member]
    
Derivative [Line Items]    
Derivative assets 1 [2]  
Genco [Member] | Not Designated As Hedging Instrument [Member] | Natural Gas [Member] | Other Current Assets [Member]
    
Derivative [Line Items]    
Derivative assets 6 [2] 2 [2]
Genco [Member] | Not Designated As Hedging Instrument [Member] | Natural Gas [Member] | Other Current Liabilities [Member]
    
Derivative [Line Items]    
Derivative liabilities 4 [2] 2 [2]
Genco [Member] | Not Designated As Hedging Instrument [Member] | Coal [Member] | Other Current Liabilities [Member]
    
Derivative [Line Items]    
Derivative liabilities 5 [2]  
Genco [Member] | Not Designated As Hedging Instrument [Member] | Coal [Member] | Other Deferred Credits And Liabilities [Member]
    
Derivative [Line Items]    
Derivative liabilities $ 3 [2]  
[1] Includes amounts for Ameren registrant and nonregistrant subsidiaries and intercompany eliminations.
[2] Includes derivatives subject to regulatory deferral.