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Fair Value Measurements (Schedule Of Fair Value Hierarchy Of Assets And Liabilities Measured At Fair Value On Recurring Basis) (Detail) (USD $)
In Millions, unless otherwise specified
Sep. 30, 2013
Dec. 31, 2012
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Nuclear Decommissioning Trust Fund $ 458 [1] $ 406 [2]
Assets fair value 499 435
Excluded receivables, payables, and accrued income, net 1 2
Union Electric Company
    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Nuclear Decommissioning Trust Fund 458 [1] 406 [2]
Assets fair value 499 434
Fuel Oils
    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Derivative assets 7 [3] 8 [3]
Natural Gas
    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Derivative assets 1 [3]  
Commodity Contract
    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Derivative assets 41 [3] 29 [3]
Derivative liabilities 182 [3] 230 [3]
Commodity Contract | Union Electric Company
    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Derivative assets 41 [3] 28 [3]
Derivative liabilities 26 [3] 25 [3]
Commodity Contract | Ameren Illinois Company
    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Derivative assets    1
Derivative liabilities 156 [3] 205 [3]
Commodity Contract | Fuel Oils
    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Derivative assets 8 [3] 12 [3]
Derivative liabilities 4 [3] 4 [3]
Commodity Contract | Fuel Oils | Union Electric Company
    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Derivative assets 8 [3] 12 [3]
Derivative liabilities 4 [3] 4 [3]
Commodity Contract | Natural Gas
    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Derivative assets 1 [3] 2 [3]
Derivative liabilities 75 [3] 109 [3]
Commodity Contract | Natural Gas | Union Electric Company
    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Derivative assets 1 [3] 1 [3]
Derivative liabilities 13 [3] 15 [3]
Commodity Contract | Natural Gas | Ameren Illinois Company
    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Derivative assets   1 [3]
Derivative liabilities 62 [3] 94 [3]
Commodity Contract | Power
    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Derivative assets 31 [3] 15 [3]
Derivative liabilities 97 [3] 115 [3]
Commodity Contract | Power | Union Electric Company
    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Derivative assets 31 [3] 15 [3]
Derivative liabilities 3 [3] 4 [3]
Commodity Contract | Power | Ameren Illinois Company
    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Derivative liabilities 94 [3] 111 [3]
Commodity Contract | Uranium
    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Derivative assets 1 [3]  
Derivative liabilities 6 [3] 2 [3]
Commodity Contract | Uranium | Union Electric Company
    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Derivative liabilities 6 [3] 2 [3]
Cash and cash equivalents
    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Nuclear Decommissioning Trust Fund 2 [1] 1 [2]
Cash and cash equivalents | Union Electric Company
    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Nuclear Decommissioning Trust Fund 2 [1] 1 [2]
Equity Securities | U.S. large capitalization
    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Nuclear Decommissioning Trust Fund 309 [1] 264 [2]
Equity Securities | U.S. large capitalization | Union Electric Company
    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Nuclear Decommissioning Trust Fund 309 [1] 264 [2]
Debt Securities | Corporate bonds
    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Nuclear Decommissioning Trust Fund 45 [1] 47 [2]
Debt Securities | Corporate bonds | Union Electric Company
    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Nuclear Decommissioning Trust Fund 45 [1] 47 [2]
Debt Securities | Municipal bonds
    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Nuclear Decommissioning Trust Fund 2 [1] 1 [2]
Debt Securities | Municipal bonds | Union Electric Company
    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Nuclear Decommissioning Trust Fund 2 [1] 1 [2]
Debt Securities | US treasury and government securities
    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Nuclear Decommissioning Trust Fund 90 [1] 81 [2]
Debt Securities | US treasury and government securities | Union Electric Company
    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Nuclear Decommissioning Trust Fund 90 [1] 81 [2]
Debt Securities | Asset-backed Securities
    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Nuclear Decommissioning Trust Fund 9 [1] 11 [2]
Debt Securities | Asset-backed Securities | Union Electric Company
    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Nuclear Decommissioning Trust Fund 9 [1] 11 [2]
Debt Securities | Other Debt Securities
    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Nuclear Decommissioning Trust Fund 1 [1] 1 [2]
Debt Securities | Other Debt Securities | Union Electric Company
    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Nuclear Decommissioning Trust Fund 1 [1] 1 [2]
Quoted Prices In Active Markets For Identical Assets or Liabilities (Level 1)
    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Nuclear Decommissioning Trust Fund 311 [1] 265 [2]
Assets fair value 312 269
Quoted Prices In Active Markets For Identical Assets or Liabilities (Level 1) | Union Electric Company
    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Nuclear Decommissioning Trust Fund 311 [1] 265 [2]
Assets fair value 312 269
Quoted Prices In Active Markets For Identical Assets or Liabilities (Level 1) | Ameren Illinois Company
    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Derivative liabilities     
Quoted Prices In Active Markets For Identical Assets or Liabilities (Level 1) | Commodity Contract
    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Derivative assets 1 [3] 4 [3]
Derivative liabilities 5 [3] 8 [3]
Quoted Prices In Active Markets For Identical Assets or Liabilities (Level 1) | Commodity Contract | Union Electric Company
    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Derivative assets 1 [3] 4 [3]
Derivative liabilities 5 [3] 8 [3]
Quoted Prices In Active Markets For Identical Assets or Liabilities (Level 1) | Commodity Contract | Fuel Oils
    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Derivative assets 1 [3] 4 [3]
Derivative liabilities    [3] 1 [3]
Quoted Prices In Active Markets For Identical Assets or Liabilities (Level 1) | Commodity Contract | Fuel Oils | Union Electric Company
    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Derivative assets 1 [3] 4 [3]
Derivative liabilities    [3] 1 [3]
Quoted Prices In Active Markets For Identical Assets or Liabilities (Level 1) | Commodity Contract | Natural Gas
    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Derivative assets    [3]    [3]
Derivative liabilities 5 [3] 7 [3]
Quoted Prices In Active Markets For Identical Assets or Liabilities (Level 1) | Commodity Contract | Natural Gas | Union Electric Company
    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Derivative assets    [3]    [3]
Derivative liabilities 5 [3] 7 [3]
Quoted Prices In Active Markets For Identical Assets or Liabilities (Level 1) | Commodity Contract | Natural Gas | Ameren Illinois Company
    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Derivative liabilities      [3]
Quoted Prices In Active Markets For Identical Assets or Liabilities (Level 1) | Cash and cash equivalents
    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Nuclear Decommissioning Trust Fund 2 [1] 1 [2]
Quoted Prices In Active Markets For Identical Assets or Liabilities (Level 1) | Cash and cash equivalents | Union Electric Company
    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Nuclear Decommissioning Trust Fund 2 [1] 1 [2]
Quoted Prices In Active Markets For Identical Assets or Liabilities (Level 1) | Equity Securities | U.S. large capitalization
    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Nuclear Decommissioning Trust Fund 309 [1] 264 [2]
Quoted Prices In Active Markets For Identical Assets or Liabilities (Level 1) | Equity Securities | U.S. large capitalization | Union Electric Company
    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Nuclear Decommissioning Trust Fund 309 [1] 264 [2]
Significant Other Observable Inputs (Level 2)
    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Nuclear Decommissioning Trust Fund 147 [1] 141 [2]
Assets fair value 149 144
Significant Other Observable Inputs (Level 2) | Union Electric Company
    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Nuclear Decommissioning Trust Fund 147 [1] 141 [2]
Assets fair value 149 143
Significant Other Observable Inputs (Level 2) | Commodity Contract
    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Derivative assets 2 [3] 3 [3]
Derivative liabilities 71 [3] 103 [3]
Significant Other Observable Inputs (Level 2) | Commodity Contract | Union Electric Company
    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Derivative assets 2 [3] 2 [3]
Derivative liabilities 9 [3] 9 [3]
Significant Other Observable Inputs (Level 2) | Commodity Contract | Ameren Illinois Company
    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Derivative liabilities 62 [3] 94 [3]
Significant Other Observable Inputs (Level 2) | Commodity Contract | Natural Gas
    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Derivative assets    [3] 2 [3]
Derivative liabilities 69 [3] 102 [3]
Significant Other Observable Inputs (Level 2) | Commodity Contract | Natural Gas | Union Electric Company
    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Derivative assets    [3] 1 [3]
Derivative liabilities 7 [3] 8 [3]
Significant Other Observable Inputs (Level 2) | Commodity Contract | Natural Gas | Ameren Illinois Company
    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Derivative assets   1 [3]
Derivative liabilities 62 [3] 94 [3]
Significant Other Observable Inputs (Level 2) | Commodity Contract | Power
    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Derivative assets 2 [3] 1 [3]
Derivative liabilities 2 [3] 1 [3]
Significant Other Observable Inputs (Level 2) | Commodity Contract | Power | Union Electric Company
    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Derivative assets 2 [3] 1 [3]
Derivative liabilities 2 [3] 1 [3]
Significant Other Observable Inputs (Level 2) | Commodity Contract | Power | Ameren Illinois Company
    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Derivative liabilities      [3]
Significant Other Observable Inputs (Level 2) | Debt Securities | Corporate bonds
    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Nuclear Decommissioning Trust Fund 45 [1] 47 [2]
Significant Other Observable Inputs (Level 2) | Debt Securities | Corporate bonds | Union Electric Company
    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Nuclear Decommissioning Trust Fund 45 [1] 47 [2]
Significant Other Observable Inputs (Level 2) | Debt Securities | Municipal bonds
    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Nuclear Decommissioning Trust Fund 2 [1] 1 [2]
Significant Other Observable Inputs (Level 2) | Debt Securities | Municipal bonds | Union Electric Company
    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Nuclear Decommissioning Trust Fund 2 [1] 1 [2]
Significant Other Observable Inputs (Level 2) | Debt Securities | US treasury and government securities
    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Nuclear Decommissioning Trust Fund 90 [1] 81 [2]
Significant Other Observable Inputs (Level 2) | Debt Securities | US treasury and government securities | Union Electric Company
    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Nuclear Decommissioning Trust Fund 90 [1] 81 [2]
Significant Other Observable Inputs (Level 2) | Debt Securities | Asset-backed Securities
    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Nuclear Decommissioning Trust Fund 9 [1] 11 [2]
Significant Other Observable Inputs (Level 2) | Debt Securities | Asset-backed Securities | Union Electric Company
    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Nuclear Decommissioning Trust Fund 9 [1] 11 [2]
Significant Other Observable Inputs (Level 2) | Debt Securities | Other Debt Securities
    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Nuclear Decommissioning Trust Fund 1 [1] 1 [2]
Significant Other Observable Inputs (Level 2) | Debt Securities | Other Debt Securities | Union Electric Company
    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Nuclear Decommissioning Trust Fund 1 [1] 1 [2]
Significant Other Unobservable Inputs (Level 3)
    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Assets fair value 38 [3] 22 [3]
Significant Other Unobservable Inputs (Level 3) | Union Electric Company
    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Assets fair value 38 [3] 22 [3]
Significant Other Unobservable Inputs (Level 3) | Commodity Contract
    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Derivative assets 38 [3] 22 [3]
Derivative liabilities 106 [3] 119 [3]
Significant Other Unobservable Inputs (Level 3) | Commodity Contract | Union Electric Company
    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Derivative assets 38 [3] 22 [3]
Derivative liabilities 12 [3] 8 [3]
Significant Other Unobservable Inputs (Level 3) | Commodity Contract | Ameren Illinois Company
    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Derivative liabilities 94 [3] 111 [3]
Significant Other Unobservable Inputs (Level 3) | Commodity Contract | Fuel Oils
    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Derivative assets 7 [3] 8 [3]
Derivative liabilities 4 [3] 3 [3]
Significant Other Unobservable Inputs (Level 3) | Commodity Contract | Fuel Oils | Union Electric Company
    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Derivative assets 7 [3] 8 [3]
Derivative liabilities 4 [3] 3 [3]
Significant Other Unobservable Inputs (Level 3) | Commodity Contract | Natural Gas
    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Derivative assets 1 [3]    [3]
Derivative liabilities 1 [3]    [3]
Significant Other Unobservable Inputs (Level 3) | Commodity Contract | Natural Gas | Union Electric Company
    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Derivative assets 1 [3]  
Derivative liabilities 1 [3]    [3]
Significant Other Unobservable Inputs (Level 3) | Commodity Contract | Natural Gas | Ameren Illinois Company
    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Derivative assets      [3]
Derivative liabilities      [3]
Significant Other Unobservable Inputs (Level 3) | Commodity Contract | Power
    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Derivative assets 29 [3] 14 [3]
Derivative liabilities 95 [3] 114 [3]
Significant Other Unobservable Inputs (Level 3) | Commodity Contract | Power | Union Electric Company
    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Derivative assets 29 [3] 14 [3]
Derivative liabilities 1 [3] 3 [3]
Significant Other Unobservable Inputs (Level 3) | Commodity Contract | Power | Ameren Illinois Company
    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Derivative liabilities 94 [3] 111 [3]
Significant Other Unobservable Inputs (Level 3) | Commodity Contract | Uranium
    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Derivative assets 1 [3]  
Derivative liabilities 6 [3] 2 [3]
Significant Other Unobservable Inputs (Level 3) | Commodity Contract | Uranium | Union Electric Company
    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Derivative assets 1 [3]  
Derivative liabilities $ 6 [3] $ 2 [3]
[1] Balance excludes $1 million of receivables, payables, and accrued income, net.
[2] Balance excludes $2 million of receivables, payables, and accrued income, net.
[3] The derivative asset and liability balances are presented net of counterparty credit considerations.