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Long-Term Debt And Equity Financings (Schedule Of Long-Term Debt Outstanding) (Details) (USD $)
12 Months Ended
Dec. 31, 2011
Dec. 31, 2010
Sep. 30, 2010
Debt Instrument [Line Items]      
Less: Unamortized discount and premium $ (1,000,000)    
Less: Maturities due within one year (179,000,000) (155,000,000)  
Long-term debt, net 6,677,000,000 6,853,000,000  
Series 7.69% Due 2036 [Member]
     
Debt Instrument [Line Items]      
Debt instrument face amount     66,000,000
Parent Company [Member]
     
Debt Instrument [Line Items]      
Less: Unamortized discount and premium (1,000,000) (2,000,000)  
Long-term debt, net 424,000,000 423,000,000  
Parent Company [Member] | Senior Unsecured Notes 8.875% Due 2014 [Member]
     
Debt Instrument [Line Items]      
Debt instrument face amount 425,000,000 425,000,000  
Union Electric Company [Member]
     
Debt Instrument [Line Items]      
Long-term debt, gross 3,955,000,000 3,960,000,000  
Less: Unamortized discount and premium (5,000,000) (6,000,000)  
Less: Maturities due within one year (178,000,000) (5,000,000)  
Long-term debt, net 3,772,000,000 3,949,000,000  
Union Electric Company [Member] | City Of Bowling Green Capital Lease Peno Creek Ct [Member]
     
Debt Instrument [Line Items]      
Capital lease obligations 69,000,000 74,000,000  
Union Electric Company [Member] | Audrain County Capital Lease Audrain County Ct [Member]
     
Debt Instrument [Line Items]      
Capital lease obligations 240,000,000 240,000,000  
Union Electric Company [Member] | First Mortgage [Member] | Senior Secured Notes 5.25% Due 2012 [Member]
     
Debt Instrument [Line Items]      
Debt instrument face amount 173,000,000 [1] 173,000,000 [1]  
Union Electric Company [Member] | First Mortgage [Member] | Senior Secured Notes 4.65% Due 2013 [Member]
     
Debt Instrument [Line Items]      
Debt instrument face amount 200,000,000 [1] 200,000,000 [1]  
Union Electric Company [Member] | First Mortgage [Member] | Senior Secured Notes 5.50% Due 2014 [Member]
     
Debt Instrument [Line Items]      
Debt instrument face amount 104,000,000 [1] 104,000,000 [1]  
Union Electric Company [Member] | First Mortgage [Member] | Senior Secured Notes 4.75% Due 2015 [Member]
     
Debt Instrument [Line Items]      
Debt instrument face amount 114,000,000 [1] 114,000,000 [1]  
Union Electric Company [Member] | First Mortgage [Member] | Senior Secured Notes 5.40% Due 2016 [Member]
     
Debt Instrument [Line Items]      
Debt instrument face amount 260,000,000 [1] 260,000,000 [1]  
Union Electric Company [Member] | First Mortgage [Member] | Senior Secured Notes 6.40% Due 2017 [Member]
     
Debt Instrument [Line Items]      
Debt instrument face amount 425,000,000 [1] 425,000,000 [1]  
Union Electric Company [Member] | First Mortgage [Member] | Senior Secured Notes 6.00% Due 2018 [Member]
     
Debt Instrument [Line Items]      
Debt instrument face amount 250,000,000 [1],[2] 250,000,000 [1],[2]  
Union Electric Company [Member] | First Mortgage [Member] | Senior Secured Notes 5.10% Due 2018 [Member]
     
Debt Instrument [Line Items]      
Debt instrument face amount 200,000,000 [1] 200,000,000 [1]  
Union Electric Company [Member] | First Mortgage [Member] | Senior Secured Notes 6.70% Due 2019 [Member]
     
Debt Instrument [Line Items]      
Debt instrument face amount 450,000,000 [1],[2] 450,000,000 [1],[2]  
Union Electric Company [Member] | First Mortgage [Member] | Senior Secured Notes 5.10% Due 2019 [Member]
     
Debt Instrument [Line Items]      
Debt instrument face amount 300,000,000 [1] 300,000,000 [1]  
Union Electric Company [Member] | First Mortgage [Member] | Senior Secured Notes 5.00% Due 2020 [Member]
     
Debt Instrument [Line Items]      
Debt instrument face amount 85,000,000 [1] 85,000,000 [1]  
Union Electric Company [Member] | First Mortgage [Member] | Senior Secured Notes 5.50% Due 2034 [Member]
     
Debt Instrument [Line Items]      
Debt instrument face amount 184,000,000 [1] 184,000,000 [1]  
Union Electric Company [Member] | First Mortgage [Member] | Senior Secured Notes 5.30% Due 2037 [Member]
     
Debt Instrument [Line Items]      
Debt instrument face amount 300,000,000 [1] 300,000,000 [1]  
Union Electric Company [Member] | First Mortgage [Member] | Senior Secured Notes 8.45% Due 2039 [Member]
     
Debt Instrument [Line Items]      
Debt instrument face amount 350,000,000 [1],[2] 350,000,000 [1],[2]  
Union Electric Company [Member] | Environmental Improvement And Pollution Control Revenue Bonds [Member] | Series 1992 Due 2022 [Member]
     
Debt Instrument [Line Items]      
Debt instrument face amount 47,000,000 [3],[4] 47,000,000 [3],[4]  
Union Electric Company [Member] | Environmental Improvement And Pollution Control Revenue Bonds [Member] | Series 1993 5.45% Due 2028 [Member]
     
Debt Instrument [Line Items]      
Debt instrument face amount 44,000,000 [4] 44,000,000 [4]  
Union Electric Company [Member] | Environmental Improvement And Pollution Control Revenue Bonds [Member] | Series A 1998 Due 2033 [Member]
     
Debt Instrument [Line Items]      
Debt instrument face amount 60,000,000 [3],[5] 60,000,000 [3],[5]  
Union Electric Company [Member] | Environmental Improvement And Pollution Control Revenue Bonds [Member] | Series B 1998 Due 2033 [Member]
     
Debt Instrument [Line Items]      
Debt instrument face amount 50,000,000 [3],[5] 50,000,000 [3],[5]  
Union Electric Company [Member] | Environmental Improvement And Pollution Control Revenue Bonds [Member] | Series C 1998 Due 2033 [Member]
     
Debt Instrument [Line Items]      
Debt instrument face amount 50,000,000 [3],[5] 50,000,000 [3],[5]  
Ameren Illinois Company [Member]
     
Debt Instrument [Line Items]      
Debt instrument face amount     40,000,000
Fair-market value adjustments 5,000,000 5,000,000  
Long-term debt, gross 1,666,000,000 1,816,000,000  
Less: Unamortized discount and premium (8,000,000) (9,000,000)  
Less: Maturities due within one year (1,000,000) (150,000,000)  
Long-term debt, net 1,657,000,000 1,657,000,000  
Ameren Illinois Company [Member] | First Mortgage [Member] | Senior Secured Notes 6.625% Due 2011 [Member]
     
Debt Instrument [Line Items]      
Debt instrument face amount   150,000,000  
Ameren Illinois Company [Member] | First Mortgage [Member] | Senior Secured Notes 8.875% Due 2013 [Member]
     
Debt Instrument [Line Items]      
Debt instrument face amount 150,000,000 [6],[7] 150,000,000 [6],[7]  
Ameren Illinois Company [Member] | First Mortgage [Member] | Senior Secured Notes 6.20% Due 2016 [Member]
     
Debt Instrument [Line Items]      
Debt instrument face amount 54,000,000 [6] 54,000,000 [6]  
Ameren Illinois Company [Member] | First Mortgage [Member] | Senior Secured Notes 6.25% Due 2016 [Member]
     
Debt Instrument [Line Items]      
Debt instrument face amount 75,000,000 [8] 75,000,000 [8]  
Ameren Illinois Company [Member] | First Mortgage [Member] | Senior Secured Notes 6.125% Due 2017 [Member]
     
Debt Instrument [Line Items]      
Debt instrument face amount 250,000,000 [7],[9] 250,000,000 [7],[9]  
Ameren Illinois Company [Member] | First Mortgage [Member] | Senior Secured Notes 6.25% Due 2018 [Member]
     
Debt Instrument [Line Items]      
Debt instrument face amount 337,000,000 [7],[9] 337,000,000 [7],[9]  
Ameren Illinois Company [Member] | First Mortgage [Member] | Senior Secured Notes 9.75% Due 2018 [Member]
     
Debt Instrument [Line Items]      
Debt instrument face amount 400,000,000 [7],[9] 400,000,000 [7],[9]  
Ameren Illinois Company [Member] | First Mortgage [Member] | Senior Secured Notes 6.125% Due 2028 [Member]
     
Debt Instrument [Line Items]      
Debt instrument face amount 60,000,000 [9] 60,000,000 [9]  
Ameren Illinois Company [Member] | First Mortgage [Member] | Senior Secured Notes 6.70% Due 2036 [Member]
     
Debt Instrument [Line Items]      
Debt instrument face amount 61,000,000 [9] 61,000,000 [9]  
Ameren Illinois Company [Member] | First Mortgage [Member] | Senior Secured Notes 6.70% Due 2036 [Member]
     
Debt Instrument [Line Items]      
Debt instrument face amount 42,000,000 [6] 42,000,000 [6]  
Ameren Illinois Company [Member] | Environmental Improvement And Pollution Control Revenue Bonds [Member] | Series1992 B 6.20% Due 2012 [Member]
     
Debt Instrument [Line Items]      
Debt instrument face amount 1,000,000 [6] 1,000,000 [6]  
Ameren Illinois Company [Member] | Environmental Improvement And Pollution Control Revenue Bonds [Member] | Series A 2000 5.50% Due 2014 [Member]
     
Debt Instrument [Line Items]      
Debt instrument face amount 51,000,000 51,000,000  
Ameren Illinois Company [Member] | Environmental Improvement And Pollution Control Revenue Bonds [Member] | Series1993 5.90% Due 2023 [Member]
     
Debt Instrument [Line Items]      
Debt instrument face amount 32,000,000 [6] 32,000,000 [6]  
Ameren Illinois Company [Member] | Environmental Improvement And Pollution Control Revenue Bonds [Member] | Series1994A 5.70% Due 2024 [Member]
     
Debt Instrument [Line Items]      
Debt instrument face amount 36,000,000 [10] 36,000,000 [10]  
Ameren Illinois Company [Member] | Environmental Improvement And Pollution Control Revenue Bonds [Member] | Series C-1 1993 5.95% Due 2026 [Member]
     
Debt Instrument [Line Items]      
Debt instrument face amount 35,000,000 35,000,000  
Ameren Illinois Company [Member] | Environmental Improvement And Pollution Control Revenue Bonds [Member] | Series C-2 1993 5.70% Due 2026 [Member]
     
Debt Instrument [Line Items]      
Debt instrument face amount 8,000,000 8,000,000  
Ameren Illinois Company [Member] | Environmental Improvement And Pollution Control Revenue Bonds [Member] | Series B-1 1993 Due 2028 [Member]
     
Debt Instrument [Line Items]      
Debt instrument face amount 17,000,000 [3] 17,000,000 [3]  
Ameren Illinois Company [Member] | Environmental Improvement And Pollution Control Revenue Bonds [Member] | Series1998A 5.40% Due 2028 [Member]
     
Debt Instrument [Line Items]      
Debt instrument face amount 19,000,000 [10] 19,000,000 [10]  
Ameren Illinois Company [Member] | Environmental Improvement And Pollution Control Revenue Bonds [Member] | Series1998B 5.40% Due 2028 [Member]
     
Debt Instrument [Line Items]      
Debt instrument face amount 33,000,000 [10] 33,000,000 [10]  
Ameren Energy Generating Company [Member]
     
Debt Instrument [Line Items]      
Long-term debt, gross 825,000,000 825,000,000  
Less: Unamortized discount and premium (1,000,000) (1,000,000)  
Long-term debt, net 824,000,000 824,000,000  
Ameren Energy Generating Company [Member] | Senior Notes Series F 7.95% Due 2032 [Member]
     
Debt Instrument [Line Items]      
Debt instrument face amount 275,000,000 275,000,000  
Ameren Energy Generating Company [Member] | Senior Notes Series H 7.00% Due 2018 [Member]
     
Debt Instrument [Line Items]      
Debt instrument face amount 300,000,000 300,000,000  
Ameren Energy Generating Company [Member] | Senior Notes Series I 6.30% Due 2020 [Member]
     
Debt Instrument [Line Items]      
Debt instrument face amount 250,000,000 250,000,000  
Ameren Missouri [Member]
     
Debt Instrument [Line Items]      
Less: Unamortized discount and premium $ (5,000,000)    
[1] These notes are collaterally secured by first mortgage bonds issued by Ameren Missouri under the UE mortgage indenture. The notes have a fall-away lien provision and will remain secured only as long as any first mortgage bonds issued under the UE mortgage indenture remain outstanding. Redemption, purchase, or maturity of all first mortgage bonds, including first mortgage bonds currently outstanding and any that may be issued in the future, would result in a release of the first mortgage bonds currently securing these notes, at which time these notes would become unsecured obligations. Based on the Ameren Missouri first mortgage bonds and senior secured notes currently outstanding, and assuming no early retirement of any series of such securities in full, we do not expect the first mortgage bond lien protection associated with these notes to fall away until 2039.
[2] Ameren Missouri has agreed, during the life of these notes, not to optionally redeem, purchase or otherwise retire in full its first mortgage bonds. Ameren Missouri has also agreed to prevent a first mortgage bond release date from occurring as long as any of the 8.45% Senior secured notes due 2039 remain outstanding.
[3] (d) Interest rates, and periods during which such rates apply, vary depending on our selection of defined rate modes. Maximum interest rates could range up to 18% depending on the series of bonds. The average interest rates for 2011 and 2010 were as follows: 2011 2010 Ameren Missouri 1992 Series 0.34% 0.47% Ameren Missouri 1998 Series A 0.69% 0.71% Ameren Missouri 1998 Series B 0.68% 0.73% Ameren Missouri 1998 Series C 0.69% 0.74% Ameren Illinois 1993 Series B-1 0.28% 0.59%
[4] These bonds are first mortgage bonds issued by Ameren Missouri under the UE mortgage bond indenture and are secured by substantially all Ameren Missouri property and franchises. The bonds are callable at 100% of par value.
[5] These bonds are secured by first mortgage bonds issued by Ameren Missouri under the UE mortgage indenture and have a fall-away lien provision similar to that of the company's senior secured notes. The bonds are also backed by an insurance guarantee policy.
[6] These notes are collaterally secured by first mortgage bonds issued by Ameren Illinois under the CILCO mortgage indenture. The notes have a fall-away lien provision and will remain secured only as long as any series of first mortgage bonds issued under the CILCO mortgage indenture remain outstanding. Redemption, purchase, or maturity of all first mortgage bonds, including first mortgage bonds currently outstanding and any that may be issued in the future, would result in a release of the first mortgage bonds currently securing these notes, at which time these notes would become unsecured obligations. Based on the CILCO first mortgage bonds and senior secured notes currently outstanding, and assuming no early retirement of any series of such securities in full, we do not expect the first mortgage bond lien protection associated with these notes to fall away until 2023.
[7] Ameren Illinois has agreed, during the life of these notes, not to optionally redeem, purchase or otherwise retire in full its Ameren Illinois mortgage bonds.
[8] These notes are collaterally secured by mortgage bonds issued by Ameren Illinois under the Ameren Illinois mortgage indenture. The notes have a fall-away lien provision and will remain secured only as long as any series of first mortgage bonds issued under the Ameren Illinois mortgage indenture remain outstanding. Redemption, purchase, or maturity of all mortgage bonds, including first mortgage bonds currently outstanding and any that may be issued in the future, would result in a release of the mortgage bonds currently securing these notes, at which time these notes would become unsecured obligations. Based on the Ameren Illinois mortgage bonds and senior secured notes currently outstanding, and assuming no early retirement of any series of such securities in full, we do not expect the mortgage bond lien protection associated with these notes to fall away until 2028.
[9] These bonds are mortgage bonds issued by Ameren Illinois under the Ameren Illinois mortgage indenture and are secured by substantially all property of the former IP and CIPS. The bonds are callable at 100% of par value. The bonds are also backed by an insurance guarantee policy.
[10] These bonds are first mortgage bonds issued by Ameren Illinois under the CILCO mortgage indenture and are secured by substantially all property of the former CILCO. The bonds are callable at 100% of par value.