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Derivative Financial Instruments (Derivative Instruments Carrying Value) (Detail) (USD $)
In Millions, unless otherwise specified
Jun. 30, 2012
Dec. 31, 2011
Derivative [Line Items]    
Derivative liabilities used as hedging instruments $ 404 [1],[2] $ 281 [1],[2]
Designated As Hedging Instrument [Member]
    
Derivative [Line Items]    
Derivative assets used hedging instruments 54 [2] 24 [2]
Derivative liabilities used as hedging instruments 1 [2] 1 [2]
Designated As Hedging Instrument [Member] | Power [Member] | Mark To Market Derivative Assets [Member]
    
Derivative [Line Items]    
Derivative assets used hedging instruments 23 [2] 8 [2]
Designated As Hedging Instrument [Member] | Power [Member] | Other Assets [Member]
    
Derivative [Line Items]    
Derivative assets used hedging instruments 31 [2] 16 [2]
Designated As Hedging Instrument [Member] | Power [Member] | Mark To Market Derivative Liabilities [Member]
    
Derivative [Line Items]    
Derivative liabilities used as hedging instruments 1 [2]  
Designated As Hedging Instrument [Member] | Power [Member] | Other Deferred Credits And Liabilities [Member]
    
Derivative [Line Items]    
Derivative liabilities used as hedging instruments   1 [2]
Not Designated As Hedging Instrument [Member]
    
Derivative [Line Items]    
Derivative assets used hedging instruments 173 [2],[3] 214 [2],[3]
Derivative liabilities used as hedging instruments 403 [2],[3] 280 [2],[3]
Not Designated As Hedging Instrument [Member] | Power [Member] | Mark To Market Derivative Assets [Member]
    
Derivative [Line Items]    
Derivative assets used hedging instruments 110 [2],[3] 72 [2],[3]
Not Designated As Hedging Instrument [Member] | Power [Member] | Other Assets [Member]
    
Derivative [Line Items]    
Derivative assets used hedging instruments 35 [2],[3] 99 [2],[3]
Not Designated As Hedging Instrument [Member] | Power [Member] | Mark To Market Derivative Liabilities [Member]
    
Derivative [Line Items]    
Derivative liabilities used as hedging instruments 96 [2],[3] 53 [2],[3]
Not Designated As Hedging Instrument [Member] | Power [Member] | Other Deferred Credits And Liabilities [Member]
    
Derivative [Line Items]    
Derivative liabilities used as hedging instruments 117 [2],[3] 26 [2],[3]
Not Designated As Hedging Instrument [Member] | Fuel Oils [Member] | Mark To Market Derivative Assets [Member]
    
Derivative [Line Items]    
Derivative assets used hedging instruments 13 [2],[3] 29 [2],[3]
Not Designated As Hedging Instrument [Member] | Fuel Oils [Member] | Other Assets [Member]
    
Derivative [Line Items]    
Derivative assets used hedging instruments 5 [2],[3] 8 [2],[3]
Not Designated As Hedging Instrument [Member] | Fuel Oils [Member] | Mark To Market Derivative Liabilities [Member]
    
Derivative [Line Items]    
Derivative liabilities used as hedging instruments 5 [2],[3] 2 [2],[3]
Not Designated As Hedging Instrument [Member] | Fuel Oils [Member] | Other Deferred Credits And Liabilities [Member]
    
Derivative [Line Items]    
Derivative liabilities used as hedging instruments 6 [2],[3]  
Not Designated As Hedging Instrument [Member] | Natural Gas [Member] | Mark To Market Derivative Assets [Member]
    
Derivative [Line Items]    
Derivative assets used hedging instruments 10 [2],[3] 6 [2],[3]
Not Designated As Hedging Instrument [Member] | Natural Gas [Member] | Mark To Market Derivative Liabilities [Member]
    
Derivative [Line Items]    
Derivative liabilities used as hedging instruments 91 [2],[3] 106 [2],[3]
Not Designated As Hedging Instrument [Member] | Natural Gas [Member] | Other Deferred Credits And Liabilities [Member]
    
Derivative [Line Items]    
Derivative liabilities used as hedging instruments 77 [2],[3] 92 [2],[3]
Not Designated As Hedging Instrument [Member] | Coal [Member] | Mark To Market Derivative Liabilities [Member]
    
Derivative [Line Items]    
Derivative liabilities used as hedging instruments 4 [2],[3]  
Not Designated As Hedging Instrument [Member] | Coal [Member] | Other Deferred Credits And Liabilities [Member]
    
Derivative [Line Items]    
Derivative liabilities used as hedging instruments 6 [2],[3]  
Not Designated As Hedging Instrument [Member] | Uranium [Member] | Mark To Market Derivative Liabilities [Member]
    
Derivative [Line Items]    
Derivative liabilities used as hedging instruments 1 [2],[3]  
Not Designated As Hedging Instrument [Member] | Uranium [Member] | Other Deferred Credits And Liabilities [Member]
    
Derivative [Line Items]    
Derivative liabilities used as hedging instruments   1 [2],[3]
Ameren Missouri [Member]
    
Derivative [Line Items]    
Derivative liabilities used as hedging instruments 45 [1] 37 [1]
Ameren Missouri [Member] | Not Designated As Hedging Instrument [Member]
    
Derivative [Line Items]    
Derivative assets used hedging instruments 55 [3] 55 [3]
Derivative liabilities used as hedging instruments 45 [3] 37 [3]
Ameren Missouri [Member] | Not Designated As Hedging Instrument [Member] | Power [Member] | Other Assets [Member]
    
Derivative [Line Items]    
Derivative assets used hedging instruments 2 [3]  
Ameren Missouri [Member] | Not Designated As Hedging Instrument [Member] | Power [Member] | Other Current Assets [Member]
    
Derivative [Line Items]    
Derivative assets used hedging instruments 39 [3] 30 [3]
Ameren Missouri [Member] | Not Designated As Hedging Instrument [Member] | Power [Member] | Other Current Liabilities [Member]
    
Derivative [Line Items]    
Derivative liabilities used as hedging instruments 15 [3] 9 [3]
Ameren Missouri [Member] | Not Designated As Hedging Instrument [Member] | Power [Member] | Other Deferred Credits And Liabilities [Member]
    
Derivative [Line Items]    
Derivative liabilities used as hedging instruments 2 [3]  
Ameren Missouri [Member] | Not Designated As Hedging Instrument [Member] | Fuel Oils [Member] | Other Assets [Member]
    
Derivative [Line Items]    
Derivative assets used hedging instruments 4 [3] 6 [3]
Ameren Missouri [Member] | Not Designated As Hedging Instrument [Member] | Fuel Oils [Member] | Other Current Assets [Member]
    
Derivative [Line Items]    
Derivative assets used hedging instruments 8 [3] 17 [3]
Ameren Missouri [Member] | Not Designated As Hedging Instrument [Member] | Fuel Oils [Member] | Other Current Liabilities [Member]
    
Derivative [Line Items]    
Derivative liabilities used as hedging instruments 2 [3] 1 [3]
Ameren Missouri [Member] | Not Designated As Hedging Instrument [Member] | Fuel Oils [Member] | Other Deferred Credits And Liabilities [Member]
    
Derivative [Line Items]    
Derivative liabilities used as hedging instruments 2 [3]  
Ameren Missouri [Member] | Not Designated As Hedging Instrument [Member] | Natural Gas [Member] | Other Current Assets [Member]
    
Derivative [Line Items]    
Derivative assets used hedging instruments 2 [3] 2 [3]
Ameren Missouri [Member] | Not Designated As Hedging Instrument [Member] | Natural Gas [Member] | Other Current Liabilities [Member]
    
Derivative [Line Items]    
Derivative liabilities used as hedging instruments 12 [3] 13 [3]
Ameren Missouri [Member] | Not Designated As Hedging Instrument [Member] | Natural Gas [Member] | Other Deferred Credits And Liabilities [Member]
    
Derivative [Line Items]    
Derivative liabilities used as hedging instruments 11 [3] 13 [3]
Ameren Missouri [Member] | Not Designated As Hedging Instrument [Member] | Uranium [Member] | Other Current Liabilities [Member]
    
Derivative [Line Items]    
Derivative liabilities used as hedging instruments 1 [3]  
Ameren Missouri [Member] | Not Designated As Hedging Instrument [Member] | Uranium [Member] | Other Deferred Credits And Liabilities [Member]
    
Derivative [Line Items]    
Derivative liabilities used as hedging instruments   1 [3]
Ameren Illinois Company [Member]
    
Derivative [Line Items]    
Derivative assets used hedging instruments   79 [1]
Derivative liabilities used as hedging instruments 365 [1] 386 [1]
Ameren Illinois Company [Member] | Not Designated As Hedging Instrument [Member]
    
Derivative [Line Items]    
Derivative assets used hedging instruments 2 [3] 79 [3]
Derivative liabilities used as hedging instruments 365 [3] 386 [3]
Ameren Illinois Company [Member] | Not Designated As Hedging Instrument [Member] | Power [Member] | Other Assets [Member]
    
Derivative [Line Items]    
Derivative assets used hedging instruments   77 [3]
Ameren Illinois Company [Member] | Not Designated As Hedging Instrument [Member] | Power [Member] | Mark To Market Derivative Liabilities [Member]
    
Derivative [Line Items]    
Derivative liabilities used as hedging instruments 19 [3] 9 [3]
Ameren Illinois Company [Member] | Not Designated As Hedging Instrument [Member] | Power [Member] | Other Deferred Credits And Liabilities [Member]
    
Derivative [Line Items]    
Derivative liabilities used as hedging instruments 88 [3] 8 [3]
Ameren Illinois Company [Member] | Not Designated As Hedging Instrument [Member] | Power [Member] | Mark To Market Derivative Liabilities - Affiliates [Member]
    
Derivative [Line Items]    
Derivative liabilities used as hedging instruments 114 [3] 200 [3]
Ameren Illinois Company [Member] | Not Designated As Hedging Instrument [Member] | Natural Gas [Member] | Other Assets [Member]
    
Derivative [Line Items]    
Derivative assets used hedging instruments   1 [3]
Ameren Illinois Company [Member] | Not Designated As Hedging Instrument [Member] | Natural Gas [Member] | Mark To Market Derivative Liabilities [Member]
    
Derivative [Line Items]    
Derivative liabilities used as hedging instruments 78 [3] 90 [3]
Ameren Illinois Company [Member] | Not Designated As Hedging Instrument [Member] | Natural Gas [Member] | Other Current Assets [Member]
    
Derivative [Line Items]    
Derivative assets used hedging instruments 2 [3] 1 [3]
Ameren Illinois Company [Member] | Not Designated As Hedging Instrument [Member] | Natural Gas [Member] | Other Deferred Credits And Liabilities [Member]
    
Derivative [Line Items]    
Derivative liabilities used as hedging instruments 66 [3] 79 [3]
Genco [Member]
    
Derivative [Line Items]    
Derivative assets used hedging instruments 10 [1] 13 [1]
Derivative liabilities used as hedging instruments 14 [1] 3 [1]
Genco [Member] | Not Designated As Hedging Instrument [Member]
    
Derivative [Line Items]    
Derivative assets used hedging instruments 10 [3] 13 [3]
Derivative liabilities used as hedging instruments 14 [3] 3 [3]
Genco [Member] | Not Designated As Hedging Instrument [Member] | Fuel Oils [Member] | Other Assets [Member]
    
Derivative [Line Items]    
Derivative assets used hedging instruments 1 [3] 1 [3]
Genco [Member] | Not Designated As Hedging Instrument [Member] | Fuel Oils [Member] | Other Current Assets [Member]
    
Derivative [Line Items]    
Derivative assets used hedging instruments 4 [3] 10 [3]
Genco [Member] | Not Designated As Hedging Instrument [Member] | Fuel Oils [Member] | Other Current Liabilities [Member]
    
Derivative [Line Items]    
Derivative liabilities used as hedging instruments 2 [3] 1 [3]
Genco [Member] | Not Designated As Hedging Instrument [Member] | Fuel Oils [Member] | Other Deferred Credits And Liabilities [Member]
    
Derivative [Line Items]    
Derivative liabilities used as hedging instruments 3 [3]  
Genco [Member] | Not Designated As Hedging Instrument [Member] | Natural Gas [Member] | Other Current Assets [Member]
    
Derivative [Line Items]    
Derivative assets used hedging instruments 5 [3] 2 [3]
Genco [Member] | Not Designated As Hedging Instrument [Member] | Natural Gas [Member] | Other Current Liabilities [Member]
    
Derivative [Line Items]    
Derivative liabilities used as hedging instruments 1 [3] 2 [3]
Genco [Member] | Not Designated As Hedging Instrument [Member] | Coal [Member] | Other Current Liabilities [Member]
    
Derivative [Line Items]    
Derivative liabilities used as hedging instruments 4 [3]  
Genco [Member] | Not Designated As Hedging Instrument [Member] | Coal [Member] | Other Deferred Credits And Liabilities [Member]
    
Derivative [Line Items]    
Derivative liabilities used as hedging instruments $ 4 [3]  
[1] The derivative asset and liability balances are presented net of counterparty credit considerations.
[2] Includes amounts for Ameren registrant and nonregistrant subsidiaries and intercompany eliminations.
[3] Includes derivatives subject to regulatory deferral.