XML 60 R59.htm IDEA: XBRL DOCUMENT v2.4.0.6
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
Mar. 31, 2013
Dec. 31, 2012
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Nuclear Decommissioning Trust Fund $ 434 [1] $ 406 [2]
Assets fair value 471 435
Excluded receivables, payables, and accrued income, net 3 2
Union Electric Company [Member]
    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Nuclear Decommissioning Trust Fund 434 [1] 406 [2]
Assets fair value 462 434
Fuel Oils [Member]
    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Derivative assets 7 [3] 8 [3]
Natural Gas [Member]
    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Derivative assets 3 [3]  
Commodity Contracts [Member]
    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Derivative assets 37 [3] 29 [3]
Derivative liabilities 183 [3] 230 [3]
Commodity Contracts [Member] | Union Electric Company [Member]
    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Derivative assets 28 [3] 28 [3]
Derivative liabilities 32 [3] 25 [3]
Commodity Contracts [Member] | Ameren Illinois Company [Member]
    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Derivative assets 9 1
Derivative liabilities 151 [3] 205 [3]
Commodity Contracts [Member] | Fuel Oils [Member]
    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Derivative assets 10 [3] 12 [3]
Derivative liabilities 3 [3] 4 [3]
Commodity Contracts [Member] | Fuel Oils [Member] | Union Electric Company [Member]
    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Derivative assets 10 [3] 12 [3]
Derivative liabilities 3 [3] 4 [3]
Commodity Contracts [Member] | Natural Gas [Member]
    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Derivative assets 8 [3] 2 [3]
Derivative liabilities 79 [3] 109 [3]
Commodity Contracts [Member] | Natural Gas [Member] | Union Electric Company [Member]
    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Derivative assets 2 [3] 1 [3]
Derivative liabilities 12 [3] 15 [3]
Commodity Contracts [Member] | Natural Gas [Member] | Ameren Illinois Company [Member]
    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Derivative assets 6 [3] 1 [3]
Derivative liabilities 67 [3] 94 [3]
Commodity Contracts [Member] | Power [Member]
    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Derivative assets 19 [3] 15 [3]
Derivative liabilities 99 [3] 115 [3]
Commodity Contracts [Member] | Power [Member] | Union Electric Company [Member]
    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Derivative assets 16 [3] 15 [3]
Derivative liabilities 15 [3] 4 [3]
Commodity Contracts [Member] | Power [Member] | Ameren Illinois Company [Member]
    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Derivative assets 3 [3]    [3]
Derivative liabilities 84 [3] 111 [3]
Commodity Contracts [Member] | Uranium [Member]
    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Derivative liabilities 2 [3] 2 [3]
Commodity Contracts [Member] | Uranium [Member] | Union Electric Company [Member]
    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Derivative liabilities 2 [3] 2 [3]
Cash and cash equivalents [Member]
    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Nuclear Decommissioning Trust Fund 2 [1] 1 [2]
Cash and cash equivalents [Member] | Union Electric Company [Member]
    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Nuclear Decommissioning Trust Fund 2 [1] 1 [2]
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 [2]
Equity Securities [Member] | U.S. large capitalization [Member] | Union Electric Company [Member]
    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Nuclear Decommissioning Trust Fund 294 [1] 264 [2]
Debt Securities [Member] | Corporate bonds [Member]
    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Nuclear Decommissioning Trust Fund 48 [1] 47 [2]
Debt Securities [Member] | Corporate bonds [Member] | Union Electric Company [Member]
    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Nuclear Decommissioning Trust Fund 48 [1] 47 [2]
Debt Securities [Member] | Municipal bonds [Member]
    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Nuclear Decommissioning Trust Fund 1 [1] 1 [2]
Debt Securities [Member] | Municipal bonds [Member] | Union Electric Company [Member]
    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Nuclear Decommissioning Trust Fund 1 [1] 1 [2]
Debt Securities [Member] | US treasury and government securities [Member]
    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Nuclear Decommissioning Trust Fund 77 [1] 81 [2]
Debt Securities [Member] | US treasury and government securities [Member] | Union Electric Company [Member]
    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Nuclear Decommissioning Trust Fund 77 [1] 81 [2]
Debt Securities [Member] | Asset-backed Securities [Member]
    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Nuclear Decommissioning Trust Fund 11 [1] 11 [2]
Debt Securities [Member] | Asset-backed Securities [Member] | Union Electric Company [Member]
    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Nuclear Decommissioning Trust Fund 11 [1] 11 [2]
Debt Securities [Member] | Other Debt Securities [Member]
    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Nuclear Decommissioning Trust Fund 1 [1] 1 [2]
Debt Securities [Member] | Other Debt Securities [Member] | Union Electric Company [Member]
    
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) [Member]
    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Nuclear Decommissioning Trust Fund 296 [1] 265 [2]
Assets fair value 300 269
Quoted Prices In Active Markets For Identical Assets or Liabilities (Level 1) [Member] | Union Electric Company [Member]
    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Nuclear Decommissioning Trust Fund 296 [1] 265 [2]
Assets fair value 299 269
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 4 [3] 4 [3]
Derivative liabilities 6 [3] 8 [3]
Quoted Prices In Active Markets For Identical Assets or Liabilities (Level 1) [Member] | Commodity Contracts [Member] | Union Electric Company [Member]
    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Derivative assets 3 [3] 4 [3]
Derivative liabilities 6 [3] 8 [3]
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 1  
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 3 [3] 4 [3]
Derivative liabilities 1 [3] 1 [3]
Quoted Prices In Active Markets For Identical Assets or Liabilities (Level 1) [Member] | Commodity Contracts [Member] | Fuel Oils [Member] | Union Electric Company [Member]
    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Derivative assets 3 [3] 4 [3]
Derivative liabilities 1 [3] 1 [3]
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 1 [3]    [3]
Derivative liabilities 5 [3] 7 [3]
Quoted Prices In Active Markets For Identical Assets or Liabilities (Level 1) [Member] | Commodity Contracts [Member] | Natural Gas [Member] | Union Electric Company [Member]
    
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) [Member] | Commodity Contracts [Member] | Natural Gas [Member] | Ameren Illinois Company [Member]
    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Derivative assets 1 [3]  
Derivative liabilities      [3]
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 2 [1] 1 [2]
Quoted Prices In Active Markets For Identical Assets or Liabilities (Level 1) [Member] | Cash and cash equivalents [Member] | Union Electric Company [Member]
    
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) [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 [2]
Quoted Prices In Active Markets For Identical Assets or Liabilities (Level 1) [Member] | Equity Securities [Member] | U.S. large capitalization [Member] | Union Electric Company [Member]
    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Nuclear Decommissioning Trust Fund 294 [1] 264 [2]
Significant Other Observable Inputs (Level 2) [Member]
    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Nuclear Decommissioning Trust Fund 138 [1] 141 [2]
Assets fair value 149 144
Significant Other Observable Inputs (Level 2) [Member] | Union Electric Company [Member]
    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Nuclear Decommissioning Trust Fund 138 [1] 141 [2]
Assets fair value 146 143
Significant Other Observable Inputs (Level 2) [Member] | Commodity Contracts [Member]
    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Derivative assets 11 [3] 3 [3]
Derivative liabilities 81 [3] 103 [3]
Significant Other Observable Inputs (Level 2) [Member] | Commodity Contracts [Member] | Union Electric Company [Member]
    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Derivative assets 8 [3] 2 [3]
Derivative liabilities 14 [3] 9 [3]
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 3 1
Derivative liabilities 67 [3] 94 [3]
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 4 [3] 2 [3]
Derivative liabilities 73 [3] 102 [3]
Significant Other Observable Inputs (Level 2) [Member] | Commodity Contracts [Member] | Natural Gas [Member] | Union Electric Company [Member]
    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Derivative assets 1 [3] 1 [3]
Derivative liabilities 6 [3] 8 [3]
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 3 [3] 1 [3]
Derivative liabilities 67 [3] 94 [3]
Significant Other Observable Inputs (Level 2) [Member] | Commodity Contracts [Member] | Power [Member]
    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Derivative assets 7 [3] 1 [3]
Derivative liabilities 8 [3] 1 [3]
Significant Other Observable Inputs (Level 2) [Member] | Commodity Contracts [Member] | Power [Member] | Union Electric Company [Member]
    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Derivative assets 7 [3] 1 [3]
Derivative liabilities 8 [3] 1 [3]
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 assets      [3]
Derivative liabilities      [3]
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 48 [1] 47 [2]
Significant Other Observable Inputs (Level 2) [Member] | Debt Securities [Member] | Corporate bonds [Member] | Union Electric Company [Member]
    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Nuclear Decommissioning Trust Fund 48 [1] 47 [2]
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 [2]
Significant Other Observable Inputs (Level 2) [Member] | Debt Securities [Member] | Municipal bonds [Member] | Union Electric Company [Member]
    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Nuclear Decommissioning Trust Fund 1 [1] 1 [2]
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 77 [1] 81 [2]
Significant Other Observable Inputs (Level 2) [Member] | Debt Securities [Member] | US treasury and government securities [Member] | Union Electric Company [Member]
    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Nuclear Decommissioning Trust Fund 77 [1] 81 [2]
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 11 [1] 11 [2]
Significant Other Observable Inputs (Level 2) [Member] | Debt Securities [Member] | Asset-backed Securities [Member] | Union Electric Company [Member]
    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Nuclear Decommissioning Trust Fund 11 [1] 11 [2]
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 [2]
Significant Other Observable Inputs (Level 2) [Member] | Debt Securities [Member] | Other Debt Securities [Member] | Union Electric Company [Member]
    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Nuclear Decommissioning Trust Fund 1 [1] 1 [2]
Significant Other Unobservable Inputs (Level 3) [Member]
    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Assets fair value 22 22 [3]
Significant Other Unobservable Inputs (Level 3) [Member] | Union Electric Company [Member]
    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Assets fair value 17 22
Significant Other Unobservable Inputs (Level 3) [Member] | Ameren Illinois Company [Member]
    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Derivative assets     
Significant Other Unobservable Inputs (Level 3) [Member] | Commodity Contracts [Member]
    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Derivative assets 22 [3] 22 [3]
Derivative liabilities 96 [3] 119 [3]
Significant Other Unobservable Inputs (Level 3) [Member] | Commodity Contracts [Member] | Union Electric Company [Member]
    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Derivative assets 17 [3] 22 [3]
Derivative liabilities 12 [3] 8 [3]
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 5  
Derivative liabilities 84 [3] 111 [3]
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 [3] 8 [3]
Derivative liabilities 2 [3] 3 [3]
Significant Other Unobservable Inputs (Level 3) [Member] | Commodity Contracts [Member] | Fuel Oils [Member] | Union Electric Company [Member]
    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Derivative assets 7 [3] 8 [3]
Derivative liabilities 2 [3] 3 [3]
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 3 [3]    [3]
Derivative liabilities 1 [3]    [3]
Significant Other Unobservable Inputs (Level 3) [Member] | Commodity Contracts [Member] | Natural Gas [Member] | Union Electric Company [Member]
    
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) [Member] | Commodity Contracts [Member] | Natural Gas [Member] | Ameren Illinois Company [Member]
    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Derivative assets 2 [3]    [3]
Derivative liabilities      [3]
Significant Other Unobservable Inputs (Level 3) [Member] | Commodity Contracts [Member] | Power [Member]
    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Derivative assets 12 [3] 14 [3]
Derivative liabilities 91 [3] 114 [3]
Significant Other Unobservable Inputs (Level 3) [Member] | Commodity Contracts [Member] | Power [Member] | Union Electric Company [Member]
    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Derivative assets 9 [3] 14 [3]
Derivative liabilities 7 [3] 3 [3]
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 3 [3]    [3]
Derivative liabilities 84 [3] 111 [3]
Significant Other Unobservable Inputs (Level 3) [Member] | Commodity Contracts [Member] | Uranium [Member]
    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Derivative liabilities 2 [3] 2 [3]
Significant Other Unobservable Inputs (Level 3) [Member] | Commodity Contracts [Member] | Uranium [Member] | Union Electric Company [Member]
    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Derivative liabilities $ 2 [3] $ 2 [3]
[1] Balance excludes $3 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.