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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
Jun. 30, 2013
Dec. 31, 2012
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Nuclear Decommissioning Trust Fund $ 440 [1] $ 406 [1]
Assets fair value 497 435
Excluded receivables, payables, and accrued income, net 2 2
Ameren Missouri [Member]
    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Nuclear Decommissioning Trust Fund 440 [1] 406 [1]
Assets fair value 493 434
Fuel Oils [Member]
    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Derivative assets 7 [2] 8 [2]
Natural Gas [Member]
    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Derivative assets 2 [2]  
Commodity Contracts [Member]
    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Derivative assets 57 [2] 29 [2]
Derivative liabilities 183 [2] 230 [2]
Commodity Contracts [Member] | Ameren Missouri [Member]
    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Derivative assets 53 [2] 28 [2]
Derivative liabilities 28 [2] 25 [2]
Commodity Contracts [Member] | Ameren Illinois Company [Member]
    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Derivative assets 4 1
Derivative liabilities 155 [2] 205 [2]
Commodity Contracts [Member] | Fuel Oils [Member]
    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Derivative assets 7 [2] 12 [2]
Derivative liabilities 4 [2] 4 [2]
Commodity Contracts [Member] | Fuel Oils [Member] | Ameren Missouri [Member]
    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Derivative assets 7 [2] 12 [2]
Derivative liabilities 4 [2] 4 [2]
Commodity Contracts [Member] | Natural Gas [Member]
    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Derivative assets 3 [2] 2 [2]
Derivative liabilities 85 [2] 109 [2]
Commodity Contracts [Member] | Natural Gas [Member] | Ameren Missouri [Member]
    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Derivative assets 1 [2] 1 [2]
Derivative liabilities 12 [2] 15 [2]
Commodity Contracts [Member] | Natural Gas [Member] | Ameren Illinois Company [Member]
    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Derivative assets 2 [2] 1 [2]
Derivative liabilities 73 [2] 94 [2]
Commodity Contracts [Member] | Power [Member]
    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Derivative assets 47 [2] 15 [2]
Derivative liabilities 91 [2] 115 [2]
Commodity Contracts [Member] | Power [Member] | Ameren Missouri [Member]
    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Derivative assets 45 [2] 15 [2]
Derivative liabilities 9 [2] 4 [2]
Commodity Contracts [Member] | Power [Member] | Ameren Illinois Company [Member]
    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Derivative assets 2 [2]  
Derivative liabilities 82 [2] 111 [2]
Commodity Contracts [Member] | Uranium [Member]
    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Derivative liabilities 3 [2] 2 [2]
Commodity Contracts [Member] | Uranium [Member] | Ameren Missouri [Member]
    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Derivative liabilities 3 [2] 2 [2]
Cash and cash equivalents [Member]
    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Nuclear Decommissioning Trust Fund 3 [1] 1 [1]
Cash and cash equivalents [Member] | Ameren Missouri [Member]
    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Nuclear Decommissioning Trust Fund 3 [1] 1 [1]
Equity Securities [Member] | U.S. large capitalization [Member]
    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Nuclear Decommissioning Trust Fund 294 [1] 264 [1]
Equity Securities [Member] | U.S. large capitalization [Member] | Ameren Missouri [Member]
    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Nuclear Decommissioning Trust Fund 294 [1] 264 [1]
Debt Securities [Member] | Corporate bonds [Member]
    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Nuclear Decommissioning Trust Fund 40 [1] 47 [1]
Debt Securities [Member] | Corporate bonds [Member] | Ameren Missouri [Member]
    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Nuclear Decommissioning Trust Fund 40 [1] 47 [1]
Debt Securities [Member] | Municipal bonds [Member]
    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Nuclear Decommissioning Trust Fund 1 [1] 1 [1]
Debt Securities [Member] | Municipal bonds [Member] | Ameren Missouri [Member]
    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Nuclear Decommissioning Trust Fund 1 [1] 1 [1]
Debt Securities [Member] | US treasury and government securities [Member]
    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Nuclear Decommissioning Trust Fund 91 [1] 81 [1]
Debt Securities [Member] | US treasury and government securities [Member] | Ameren Missouri [Member]
    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Nuclear Decommissioning Trust Fund 91 [1] 81 [1]
Debt Securities [Member] | Asset-backed Securities [Member]
    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Nuclear Decommissioning Trust Fund 10 [1] 11 [1]
Debt Securities [Member] | Asset-backed Securities [Member] | Ameren Missouri [Member]
    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Nuclear Decommissioning Trust Fund 10 [1] 11 [1]
Debt Securities [Member] | Other Debt Securities [Member]
    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Nuclear Decommissioning Trust Fund 1 [1] 1 [1]
Debt Securities [Member] | Other Debt Securities [Member] | Ameren Missouri [Member]
    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Nuclear Decommissioning Trust Fund 1 [1] 1 [1]
Quoted Prices In Active Markets For Identical Assets or Liabilities (Level 1) [Member]
    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Nuclear Decommissioning Trust Fund 297 [1] 265 [1]
Assets fair value 297 269
Quoted Prices In Active Markets For Identical Assets or Liabilities (Level 1) [Member] | Ameren Missouri [Member]
    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Nuclear Decommissioning Trust Fund 297 [1] 265 [1]
Assets fair value 297 269
Quoted Prices In Active Markets For Identical Assets or Liabilities (Level 1) [Member] | Ameren Illinois Company [Member]
    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Derivative liabilities     
Quoted Prices In Active Markets For Identical Assets or Liabilities (Level 1) [Member] | Commodity Contracts [Member]
    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Derivative assets    [2] 4 [2]
Derivative liabilities 5 [2] 8 [2]
Quoted Prices In Active Markets For Identical Assets or Liabilities (Level 1) [Member] | Commodity Contracts [Member] | Ameren Missouri [Member]
    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Derivative assets    [2] 4 [2]
Derivative liabilities 5 [2] 8 [2]
Quoted Prices In Active Markets For Identical Assets or Liabilities (Level 1) [Member] | Commodity Contracts [Member] | Ameren Illinois Company [Member]
    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Derivative assets     
Quoted Prices In Active Markets For Identical Assets or Liabilities (Level 1) [Member] | Commodity Contracts [Member] | Fuel Oils [Member]
    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Derivative assets    [2] 4 [2]
Derivative liabilities    [2] 1 [2]
Quoted Prices In Active Markets For Identical Assets or Liabilities (Level 1) [Member] | Commodity Contracts [Member] | Fuel Oils [Member] | Ameren Missouri [Member]
    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Derivative assets    [2] 4 [2]
Derivative liabilities    [2] 1 [2]
Quoted Prices In Active Markets For Identical Assets or Liabilities (Level 1) [Member] | Commodity Contracts [Member] | Natural Gas [Member]
    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Derivative assets    [2]    [2]
Derivative liabilities 5 [2] 7 [2]
Quoted Prices In Active Markets For Identical Assets or Liabilities (Level 1) [Member] | Commodity Contracts [Member] | Natural Gas [Member] | Ameren Missouri [Member]
    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Derivative assets    [2]    [2]
Derivative liabilities 5 [2] 7 [2]
Quoted Prices In Active Markets For Identical Assets or Liabilities (Level 1) [Member] | Commodity Contracts [Member] | Natural Gas [Member] | Ameren Illinois Company [Member]
    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Derivative assets    [2]  
Derivative liabilities      [2]
Quoted Prices In Active Markets For Identical Assets or Liabilities (Level 1) [Member] | Cash and cash equivalents [Member]
    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Nuclear Decommissioning Trust Fund 3 [1] 1 [1]
Quoted Prices In Active Markets For Identical Assets or Liabilities (Level 1) [Member] | Cash and cash equivalents [Member] | Ameren Missouri [Member]
    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Nuclear Decommissioning Trust Fund 3 [1] 1 [1]
Quoted Prices In Active Markets For Identical Assets or Liabilities (Level 1) [Member] | Equity Securities [Member] | U.S. large capitalization [Member]
    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Nuclear Decommissioning Trust Fund 294 [1] 264 [1]
Quoted Prices In Active Markets For Identical Assets or Liabilities (Level 1) [Member] | Equity Securities [Member] | U.S. large capitalization [Member] | Ameren Missouri [Member]
    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Nuclear Decommissioning Trust Fund 294 [1] 264 [1]
Significant Other Observable Inputs (Level 2) [Member]
    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Nuclear Decommissioning Trust Fund 143 [1] 141 [1]
Assets fair value 147 144
Significant Other Observable Inputs (Level 2) [Member] | Ameren Missouri [Member]
    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Nuclear Decommissioning Trust Fund 143 [1] 141 [1]
Assets fair value 147 143
Significant Other Observable Inputs (Level 2) [Member] | Commodity Contracts [Member]
    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Derivative assets 4 [2] 3 [2]
Derivative liabilities 83 [2] 103 [2]
Significant Other Observable Inputs (Level 2) [Member] | Commodity Contracts [Member] | Ameren Missouri [Member]
    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Derivative assets 4 [2] 2 [2]
Derivative liabilities 10 [2] 9 [2]
Significant Other Observable Inputs (Level 2) [Member] | Commodity Contracts [Member] | Ameren Illinois Company [Member]
    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Derivative assets     
Derivative liabilities 73 [2] 94 [2]
Significant Other Observable Inputs (Level 2) [Member] | Commodity Contracts [Member] | Natural Gas [Member]
    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Derivative assets 1 [2] 2 [2]
Derivative liabilities 79 [2] 102 [2]
Significant Other Observable Inputs (Level 2) [Member] | Commodity Contracts [Member] | Natural Gas [Member] | Ameren Missouri [Member]
    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Derivative assets 1 [2] 1 [2]
Derivative liabilities 6 [2] 8 [2]
Significant Other Observable Inputs (Level 2) [Member] | Commodity Contracts [Member] | Natural Gas [Member] | Ameren Illinois Company [Member]
    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Derivative assets    [2] 1 [2]
Derivative liabilities 73 [2] 94 [2]
Significant Other Observable Inputs (Level 2) [Member] | Commodity Contracts [Member] | Power [Member]
    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Derivative assets 3 [2] 1 [2]
Derivative liabilities 4 [2] 1 [2]
Significant Other Observable Inputs (Level 2) [Member] | Commodity Contracts [Member] | Power [Member] | Ameren Missouri [Member]
    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Derivative assets 3 [2] 1 [2]
Derivative liabilities 4 [2] 1 [2]
Significant Other Observable Inputs (Level 2) [Member] | Commodity Contracts [Member] | Power [Member] | Ameren Illinois Company [Member]
    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Derivative liabilities      [2]
Significant Other Observable Inputs (Level 2) [Member] | Debt Securities [Member] | Corporate bonds [Member]
    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Nuclear Decommissioning Trust Fund 40 [1] 47 [1]
Significant Other Observable Inputs (Level 2) [Member] | Debt Securities [Member] | Corporate bonds [Member] | Ameren Missouri [Member]
    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Nuclear Decommissioning Trust Fund 40 [1] 47 [1]
Significant Other Observable Inputs (Level 2) [Member] | Debt Securities [Member] | Municipal bonds [Member]
    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Nuclear Decommissioning Trust Fund 1 [1] 1 [1]
Significant Other Observable Inputs (Level 2) [Member] | Debt Securities [Member] | Municipal bonds [Member] | Ameren Missouri [Member]
    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Nuclear Decommissioning Trust Fund 1 [1] 1 [1]
Significant Other Observable Inputs (Level 2) [Member] | Debt Securities [Member] | US treasury and government securities [Member]
    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Nuclear Decommissioning Trust Fund 91 [1] 81 [1]
Significant Other Observable Inputs (Level 2) [Member] | Debt Securities [Member] | US treasury and government securities [Member] | Ameren Missouri [Member]
    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Nuclear Decommissioning Trust Fund 91 [1] 81 [1]
Significant Other Observable Inputs (Level 2) [Member] | Debt Securities [Member] | Asset-backed Securities [Member]
    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Nuclear Decommissioning Trust Fund 10 [1] 11 [1]
Significant Other Observable Inputs (Level 2) [Member] | Debt Securities [Member] | Asset-backed Securities [Member] | Ameren Missouri [Member]
    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Nuclear Decommissioning Trust Fund 10 [1] 11 [1]
Significant Other Observable Inputs (Level 2) [Member] | Debt Securities [Member] | Other Debt Securities [Member]
    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Nuclear Decommissioning Trust Fund 1 [1] 1 [1]
Significant Other Observable Inputs (Level 2) [Member] | Debt Securities [Member] | Other Debt Securities [Member] | Ameren Missouri [Member]
    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Nuclear Decommissioning Trust Fund 1 [1] 1 [1]
Significant Other Unobservable Inputs (Level 3) [Member]
    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Assets fair value 53 22 [2]
Significant Other Unobservable Inputs (Level 3) [Member] | Ameren Missouri [Member]
    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Assets fair value 49 22
Significant Other Unobservable Inputs (Level 3) [Member] | Commodity Contracts [Member]
    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Derivative assets 53 [2] 22 [2]
Derivative liabilities 95 [2] 119 [2]
Significant Other Unobservable Inputs (Level 3) [Member] | Commodity Contracts [Member] | Ameren Missouri [Member]
    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Derivative assets 49 [2] 22 [2]
Derivative liabilities 13 [2] 8 [2]
Significant Other Unobservable Inputs (Level 3) [Member] | Commodity Contracts [Member] | Ameren Illinois Company [Member]
    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Derivative assets 4  
Derivative liabilities 82 [2] 111 [2]
Significant Other Unobservable Inputs (Level 3) [Member] | Commodity Contracts [Member] | Fuel Oils [Member]
    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Derivative assets 7 [2] 8 [2]
Derivative liabilities 4 [2] 3 [2]
Significant Other Unobservable Inputs (Level 3) [Member] | Commodity Contracts [Member] | Fuel Oils [Member] | Ameren Missouri [Member]
    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Derivative assets 7 [2] 8 [2]
Derivative liabilities 4 [2] 3 [2]
Significant Other Unobservable Inputs (Level 3) [Member] | Commodity Contracts [Member] | Natural Gas [Member]
    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Derivative assets 2 [2]    [2]
Derivative liabilities 1 [2]    [2]
Significant Other Unobservable Inputs (Level 3) [Member] | Commodity Contracts [Member] | Natural Gas [Member] | Ameren Missouri [Member]
    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Derivative assets    [2]  
Derivative liabilities 1 [2]    [2]
Significant Other Unobservable Inputs (Level 3) [Member] | Commodity Contracts [Member] | Natural Gas [Member] | Ameren Illinois Company [Member]
    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Derivative assets 2 [2]    [2]
Derivative liabilities      [2]
Significant Other Unobservable Inputs (Level 3) [Member] | Commodity Contracts [Member] | Power [Member]
    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Derivative assets 44 [2] 14 [2]
Derivative liabilities 87 [2] 114 [2]
Significant Other Unobservable Inputs (Level 3) [Member] | Commodity Contracts [Member] | Power [Member] | Ameren Missouri [Member]
    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Derivative assets 42 [2] 14 [2]
Derivative liabilities 5 [2] 3 [2]
Significant Other Unobservable Inputs (Level 3) [Member] | Commodity Contracts [Member] | Power [Member] | Ameren Illinois Company [Member]
    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Derivative assets 2 [2]  
Derivative liabilities 82 [2] 111 [2]
Significant Other Unobservable Inputs (Level 3) [Member] | Commodity Contracts [Member] | Uranium [Member]
    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Derivative liabilities 3 [2] 2 [2]
Significant Other Unobservable Inputs (Level 3) [Member] | Commodity Contracts [Member] | Uranium [Member] | Ameren Missouri [Member]
    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Derivative liabilities $ 3 [2] $ 2 [2]
[1] Balance excludes $2 million of receivables, payables, and accrued income, net.
[2] The derivative asset and liability balances are presented net of counterparty credit considerations.