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Fair Value Measurements (Schedule Of Fair Value Hierarchy Of Assets And Liabilities Measured At Fair Value On Recurring Basis) (Details) (USD $)
In Millions, unless otherwise specified
Dec. 31, 2012
Dec. 31, 2011
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Nuclear Decommissioning Trust Fund $ 406 [1],[2] $ 358 [1],[3]
Assets 568 [1] 596 [1]
Excluded receivables, payables, and accrued income, net 2 (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 265 [1],[2] 237 [1],[3]
Assets 276 [1] 274 [1]
Significant Other Observable Inputs (Level 2) [Member]
    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Nuclear Decommissioning Trust Fund 141 [1],[2] 121 [1],[3]
Assets 152 [1] 123 [1]
Significant Other Unobservable Inputs (Level 3) [Member]
    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Nuclear Decommissioning Trust Fund    [1],[2]    [1],[3]
Assets 140 [1] 199 [1]
Fuel Oils [Member]
    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Derivative assets 9 [4]  
Cash And Cash Equivalents [Member]
    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Nuclear Decommissioning Trust Fund 1 [1],[2] 3 [1],[3]
Cash And Cash Equivalents [Member] | 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 1 [1],[2] 3 [1],[3]
Cash And Cash Equivalents [Member] | Significant Other Observable Inputs (Level 2) [Member]
    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Nuclear Decommissioning Trust Fund    [1],[2]    [1],[3]
Cash And Cash Equivalents [Member] | Significant Other Unobservable Inputs (Level 3) [Member]
    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Nuclear Decommissioning Trust Fund    [1],[2]    [1],[3]
Commodity Contracts [Member]
    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Derivative assets 162 [1],[4] 238 [1],[4]
Derivative liabilities 315 [4] 281 [4]
Commodity Contracts [Member] | Quoted Prices In Active Markets For Identical Assets Or Liabilities (Level 1) [Member]
    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Derivative assets 11 [1],[4] 37 [1],[4]
Derivative liabilities 27 [4] 24 [4]
Commodity Contracts [Member] | Significant Other Observable Inputs (Level 2) [Member]
    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Derivative assets 11 [1],[4] 2 [1],[4]
Derivative liabilities 111 [4] 2 [4]
Commodity Contracts [Member] | Significant Other Unobservable Inputs (Level 3) [Member]
    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Derivative assets 140 [1],[4] 199 [1],[4]
Derivative liabilities 177 [4] 255 [4]
Commodity Contracts [Member] | Coal [Member]
    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Derivative assets 1 [1],[4]  
Derivative liabilities 13 [4]  
Commodity Contracts [Member] | Coal [Member] | Quoted Prices In Active Markets For Identical Assets Or Liabilities (Level 1) [Member]
    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Derivative assets 1 [1],[4]  
Derivative liabilities 13 [4]  
Commodity Contracts [Member] | Coal [Member] | Significant Other Observable Inputs (Level 2) [Member]
    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Derivative assets    [1],[4]  
Derivative liabilities    [4]  
Commodity Contracts [Member] | Coal [Member] | Significant Other Unobservable Inputs (Level 3) [Member]
    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Derivative assets    [1],[4]  
Derivative liabilities    [4]  
Commodity Contracts [Member] | Fuel Oils [Member]
    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Derivative assets 15 [1],[4] 37 [1],[4]
Derivative liabilities 6 [4] 2 [4]
Commodity Contracts [Member] | Fuel Oils [Member] | Quoted Prices In Active Markets For Identical Assets Or Liabilities (Level 1) [Member]
    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Derivative assets 6 [1],[4] 33 [1],[4]
Derivative liabilities 3 [4] 2 [4]
Commodity Contracts [Member] | Fuel Oils [Member] | Significant Other Observable Inputs (Level 2) [Member]
    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Derivative assets    [1],[4]    [1],[4]
Derivative liabilities    [4]    [4]
Commodity Contracts [Member] | Fuel Oils [Member] | Significant Other Unobservable Inputs (Level 3) [Member]
    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Derivative assets 9 [1],[4] 4 [1],[4]
Derivative liabilities 3 [4]    [4]
Commodity Contracts [Member] | Natural Gas [Member]
    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Derivative assets 6 [1],[4] 6 [1],[4]
Derivative liabilities 113 [4] 198 [4]
Commodity Contracts [Member] | Natural Gas [Member] | Quoted Prices In Active Markets For Identical Assets Or Liabilities (Level 1) [Member]
    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Derivative assets 4 [1],[4] 4 [1],[4]
Derivative liabilities 11 [4] 22 [4]
Commodity Contracts [Member] | Natural Gas [Member] | Significant Other Observable Inputs (Level 2) [Member]
    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Derivative assets 2 [1],[4]    [1],[4]
Derivative liabilities 102 [4]    [4]
Commodity Contracts [Member] | Natural Gas [Member] | Significant Other Unobservable Inputs (Level 3) [Member]
    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Derivative assets    [1],[4] 2 [1],[4]
Derivative liabilities    [4] 176 [4]
Commodity Contracts [Member] | Power [Member]
    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Derivative assets 140 [1],[4] 195 [1],[4]
Derivative liabilities 181 [4] 80 [4]
Commodity Contracts [Member] | Power [Member] | Quoted Prices In Active Markets For Identical Assets Or Liabilities (Level 1) [Member]
    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Derivative assets    [1],[4]    [1],[4]
Derivative liabilities    [4]    [4]
Commodity Contracts [Member] | Power [Member] | Significant Other Observable Inputs (Level 2) [Member]
    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Derivative assets 9 [1],[4] 2 [1],[4]
Derivative liabilities 9 [4] 2 [4]
Commodity Contracts [Member] | Power [Member] | Significant Other Unobservable Inputs (Level 3) [Member]
    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Derivative assets 131 [1],[4] 193 [1],[4]
Derivative liabilities 172 [4] 78 [4]
Commodity Contracts [Member] | Uranium [Member]
    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Derivative liabilities 2 [4] 1 [4]
Commodity Contracts [Member] | Uranium [Member] | Quoted Prices In Active Markets For Identical Assets Or Liabilities (Level 1) [Member]
    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Derivative liabilities    [4]    [4]
Commodity Contracts [Member] | Uranium [Member] | Significant Other Observable Inputs (Level 2) [Member]
    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Derivative liabilities    [4]    [4]
Commodity Contracts [Member] | Uranium [Member] | Significant Other Unobservable Inputs (Level 3) [Member]
    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Derivative liabilities 2 [4] 1 [4]
Equity Securities [Member] | U.S. Large Capitalization [Member]
    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Nuclear Decommissioning Trust Fund 264 [1],[2] 234 [1],[3]
Equity Securities [Member] | U.S. Large Capitalization [Member] | 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 264 [1],[2] 234 [1],[3]
Equity Securities [Member] | U.S. Large Capitalization [Member] | Significant Other Observable Inputs (Level 2) [Member]
    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Nuclear Decommissioning Trust Fund    [1],[2]    [1],[3]
Equity Securities [Member] | U.S. Large Capitalization [Member] | Significant Other Unobservable Inputs (Level 3) [Member]
    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Nuclear Decommissioning Trust Fund    [1],[2]    [1],[3]
Debt Securities [Member] | Corporate Bonds [Member]
    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Nuclear Decommissioning Trust Fund 47 [1],[2] 44 [1],[3]
Debt Securities [Member] | Corporate Bonds [Member] | 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    [1],[2]    [1],[3]
Debt Securities [Member] | Corporate Bonds [Member] | Significant Other Observable Inputs (Level 2) [Member]
    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Nuclear Decommissioning Trust Fund 47 [1],[2] 44 [1],[3]
Debt Securities [Member] | Corporate Bonds [Member] | Significant Other Unobservable Inputs (Level 3) [Member]
    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Nuclear Decommissioning Trust Fund    [1],[2]    [1],[3]
Debt Securities [Member] | Municipal Bonds [Member]
    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Nuclear Decommissioning Trust Fund 1 [1],[2] 1 [1],[3]
Debt Securities [Member] | Municipal Bonds [Member] | 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    [1],[2]    [1],[3]
Debt Securities [Member] | Municipal Bonds [Member] | Significant Other Observable Inputs (Level 2) [Member]
    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Nuclear Decommissioning Trust Fund 1 [1],[2] 1 [1],[3]
Debt Securities [Member] | Municipal Bonds [Member] | Significant Other Unobservable Inputs (Level 3) [Member]
    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Nuclear Decommissioning Trust Fund    [1],[2]    [1],[3]
Debt Securities [Member] | U.S. treasury and agency securities [Member]
    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Nuclear Decommissioning Trust Fund 81 [1],[2] 65 [1],[3]
Debt Securities [Member] | U.S. treasury and agency securities [Member] | 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    [1],[2]    [1],[3]
Debt Securities [Member] | U.S. treasury and agency securities [Member] | Significant Other Observable Inputs (Level 2) [Member]
    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Nuclear Decommissioning Trust Fund 81 [1],[2] 65 [1],[3]
Debt Securities [Member] | U.S. treasury and agency securities [Member] | Significant Other Unobservable Inputs (Level 3) [Member]
    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Nuclear Decommissioning Trust Fund    [1],[2]    [1],[3]
Debt Securities [Member] | Asset-Backed Securities [Member]
    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Nuclear Decommissioning Trust Fund 11 [1],[2] 10 [1],[3]
Debt Securities [Member] | Asset-Backed Securities [Member] | 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    [1],[2]    [1],[3]
Debt Securities [Member] | Asset-Backed Securities [Member] | Significant Other Observable Inputs (Level 2) [Member]
    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Nuclear Decommissioning Trust Fund 11 [1],[2] 10 [1],[3]
Debt Securities [Member] | Asset-Backed Securities [Member] | Significant Other Unobservable Inputs (Level 3) [Member]
    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Nuclear Decommissioning Trust Fund    [1],[2]    [1],[3]
Debt Securities [Member] | Other debt securities [Member]
    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Nuclear Decommissioning Trust Fund 1 [1],[2] 1 [1],[3]
Debt Securities [Member] | Other debt securities [Member] | 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    [1],[2]    [1],[3]
Debt Securities [Member] | Other debt securities [Member] | Significant Other Observable Inputs (Level 2) [Member]
    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Nuclear Decommissioning Trust Fund 1 [1],[2] 1 [1],[3]
Debt Securities [Member] | Other debt securities [Member] | Significant Other Unobservable Inputs (Level 3) [Member]
    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Nuclear Decommissioning Trust Fund    [1],[2]    [1],[3]
Union Electric Company [Member]
    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Nuclear Decommissioning Trust Fund 406 [2] 358 [3]
Assets 434 413
Union Electric Company [Member] | 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 265 [2] 237 [3]
Assets 269 259
Union Electric Company [Member] | Significant Other Observable Inputs (Level 2) [Member]
    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Nuclear Decommissioning Trust Fund 141 [2] 121 [3]
Assets 143 122
Union Electric Company [Member] | Significant Other Unobservable Inputs (Level 3) [Member]
    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Nuclear Decommissioning Trust Fund    [2]    [3]
Assets 22 32
Union Electric Company [Member] | Cash And Cash Equivalents [Member]
    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Nuclear Decommissioning Trust Fund 1 [2] 3 [3]
Union Electric Company [Member] | Cash And Cash Equivalents [Member] | 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 1 [2] 3 [3]
Union Electric Company [Member] | Cash And Cash Equivalents [Member] | Significant Other Observable Inputs (Level 2) [Member]
    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Nuclear Decommissioning Trust Fund    [2]    [3]
Union Electric Company [Member] | Cash And Cash Equivalents [Member] | Significant Other Unobservable Inputs (Level 3) [Member]
    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Nuclear Decommissioning Trust Fund    [2]    [3]
Union Electric Company [Member] | Commodity Contracts [Member]
    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Derivative assets 28 [4] 55 [4]
Derivative liabilities 25 [4] 37 [4]
Union Electric Company [Member] | Commodity Contracts [Member] | Quoted Prices In Active Markets For Identical Assets Or Liabilities (Level 1) [Member]
    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Derivative assets 4 [4] 22 [4]
Derivative liabilities 8 [4] 13 [4]
Union Electric Company [Member] | Commodity Contracts [Member] | Significant Other Observable Inputs (Level 2) [Member]
    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Derivative assets 2 [4] 1 [4]
Derivative liabilities 9 [4] 1 [4]
Union Electric Company [Member] | Commodity Contracts [Member] | Significant Other Unobservable Inputs (Level 3) [Member]
    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Derivative assets 22 [4] 32 [4]
Derivative liabilities 8 [4] 23 [4]
Union Electric Company [Member] | Commodity Contracts [Member] | Fuel Oils [Member]
    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Derivative assets 12 [4] 23 [4]
Derivative liabilities 4 [4] 1 [4]
Union Electric Company [Member] | Commodity Contracts [Member] | Fuel Oils [Member] | Quoted Prices In Active Markets For Identical Assets Or Liabilities (Level 1) [Member]
    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Derivative assets 4 [4] 20 [4]
Derivative liabilities 1 [4] 1 [4]
Union Electric Company [Member] | Commodity Contracts [Member] | Fuel Oils [Member] | Significant Other Observable Inputs (Level 2) [Member]
    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Derivative assets    [4]    [4]
Derivative liabilities    [4]    [4]
Union Electric Company [Member] | Commodity Contracts [Member] | Fuel Oils [Member] | Significant Other Unobservable Inputs (Level 3) [Member]
    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Derivative assets 8 [4] 3 [4]
Derivative liabilities 3 [4]    [4]
Union Electric Company [Member] | Commodity Contracts [Member] | Natural Gas [Member]
    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Derivative assets 1 [4] 2 [4]
Derivative liabilities 15 [4] 26 [4]
Union Electric Company [Member] | Commodity Contracts [Member] | Natural Gas [Member] | Quoted Prices In Active Markets For Identical Assets Or Liabilities (Level 1) [Member]
    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Derivative assets    [4] 2 [4]
Derivative liabilities 7 [4] 12 [4]
Union Electric Company [Member] | Commodity Contracts [Member] | Natural Gas [Member] | Significant Other Observable Inputs (Level 2) [Member]
    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Derivative assets 1 [4]    [4]
Derivative liabilities 8 [4]    [4]
Union Electric Company [Member] | Commodity Contracts [Member] | Natural Gas [Member] | Significant Other Unobservable Inputs (Level 3) [Member]
    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Derivative assets    [4]    [4]
Derivative liabilities    [4] 14 [4]
Union Electric Company [Member] | Commodity Contracts [Member] | Power [Member]
    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Derivative assets 15 [4] 30 [4]
Derivative liabilities 4 [4] 9 [4]
Union Electric Company [Member] | Commodity Contracts [Member] | Power [Member] | Quoted Prices In Active Markets For Identical Assets Or Liabilities (Level 1) [Member]
    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Derivative assets    [4]    [4]
Derivative liabilities    [4]    [4]
Union Electric Company [Member] | Commodity Contracts [Member] | Power [Member] | Significant Other Observable Inputs (Level 2) [Member]
    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Derivative assets 1 [4] 1 [4]
Derivative liabilities 1 [4] 1 [4]
Union Electric Company [Member] | Commodity Contracts [Member] | Power [Member] | Significant Other Unobservable Inputs (Level 3) [Member]
    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Derivative assets 14 [4] 29 [4]
Derivative liabilities 3 [4] 8 [4]
Union Electric Company [Member] | Commodity Contracts [Member] | Uranium [Member]
    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Derivative liabilities 2 [4] 1 [4]
Union Electric Company [Member] | Commodity Contracts [Member] | Uranium [Member] | Quoted Prices In Active Markets For Identical Assets Or Liabilities (Level 1) [Member]
    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Derivative liabilities    [4]    [4]
Union Electric Company [Member] | Commodity Contracts [Member] | Uranium [Member] | Significant Other Observable Inputs (Level 2) [Member]
    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Derivative liabilities    [4]    [4]
Union Electric Company [Member] | Commodity Contracts [Member] | Uranium [Member] | Significant Other Unobservable Inputs (Level 3) [Member]
    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Derivative liabilities 2 [4] 1 [4]
Union Electric Company [Member] | Equity Securities [Member] | U.S. Large Capitalization [Member]
    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Nuclear Decommissioning Trust Fund 264 [2] 234 [3]
Union Electric Company [Member] | Equity Securities [Member] | U.S. Large Capitalization [Member] | 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 264 [2] 234 [3]
Union Electric Company [Member] | Equity Securities [Member] | U.S. Large Capitalization [Member] | Significant Other Observable Inputs (Level 2) [Member]
    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Nuclear Decommissioning Trust Fund    [2]    [3]
Union Electric Company [Member] | Equity Securities [Member] | U.S. Large Capitalization [Member] | Significant Other Unobservable Inputs (Level 3) [Member]
    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Nuclear Decommissioning Trust Fund    [2]    [3]
Union Electric Company [Member] | Debt Securities [Member] | Corporate Bonds [Member]
    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Nuclear Decommissioning Trust Fund 47 [2] 44 [3]
Union Electric Company [Member] | Debt Securities [Member] | Corporate Bonds [Member] | 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    [2]    [3]
Union Electric Company [Member] | Debt Securities [Member] | Corporate Bonds [Member] | Significant Other Observable Inputs (Level 2) [Member]
    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Nuclear Decommissioning Trust Fund 47 [2] 44 [3]
Union Electric Company [Member] | Debt Securities [Member] | Corporate Bonds [Member] | Significant Other Unobservable Inputs (Level 3) [Member]
    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Nuclear Decommissioning Trust Fund    [2]    [3]
Union Electric Company [Member] | Debt Securities [Member] | Municipal Bonds [Member]
    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Nuclear Decommissioning Trust Fund 1 [2] 1 [3]
Union Electric Company [Member] | Debt Securities [Member] | Municipal Bonds [Member] | 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    [2]    [3]
Union Electric Company [Member] | Debt Securities [Member] | Municipal Bonds [Member] | Significant Other Observable Inputs (Level 2) [Member]
    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Nuclear Decommissioning Trust Fund 1 [2] 1 [3]
Union Electric Company [Member] | Debt Securities [Member] | Municipal Bonds [Member] | Significant Other Unobservable Inputs (Level 3) [Member]
    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Nuclear Decommissioning Trust Fund    [2]    [3]
Union Electric Company [Member] | Debt Securities [Member] | U.S. treasury and agency securities [Member]
    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Nuclear Decommissioning Trust Fund 81 [2] 65 [3]
Union Electric Company [Member] | Debt Securities [Member] | U.S. treasury and agency securities [Member] | 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    [2]    [3]
Union Electric Company [Member] | Debt Securities [Member] | U.S. treasury and agency securities [Member] | Significant Other Observable Inputs (Level 2) [Member]
    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Nuclear Decommissioning Trust Fund 81 [2] 65 [3]
Union Electric Company [Member] | Debt Securities [Member] | U.S. treasury and agency securities [Member] | Significant Other Unobservable Inputs (Level 3) [Member]
    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Nuclear Decommissioning Trust Fund    [2]    [3]
Union Electric Company [Member] | Debt Securities [Member] | Asset-Backed Securities [Member]
    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Nuclear Decommissioning Trust Fund 11 [2] 10 [3]
Union Electric Company [Member] | Debt Securities [Member] | Asset-Backed Securities [Member] | 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    [2]    [3]
Union Electric Company [Member] | Debt Securities [Member] | Asset-Backed Securities [Member] | Significant Other Observable Inputs (Level 2) [Member]
    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Nuclear Decommissioning Trust Fund 11 [2] 10 [3]
Union Electric Company [Member] | Debt Securities [Member] | Asset-Backed Securities [Member] | Significant Other Unobservable Inputs (Level 3) [Member]
    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Nuclear Decommissioning Trust Fund    [2]    [3]
Union Electric Company [Member] | Debt Securities [Member] | Other debt securities [Member]
    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Nuclear Decommissioning Trust Fund 1 [2] 1 [3]
Union Electric Company [Member] | Debt Securities [Member] | Other debt securities [Member] | 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    [2]    [3]
Union Electric Company [Member] | Debt Securities [Member] | Other debt securities [Member] | Significant Other Observable Inputs (Level 2) [Member]
    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Nuclear Decommissioning Trust Fund 1 [2] 1 [3]
Union Electric Company [Member] | Debt Securities [Member] | Other debt securities [Member] | Significant Other Unobservable Inputs (Level 3) [Member]
    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Nuclear Decommissioning Trust Fund    [2]    [3]
Ameren Illinois Company [Member] | Commodity Contracts [Member]
    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Derivative assets 1 [4] 79 [4]
Derivative liabilities 205 [4] 386 [4]
Ameren Illinois Company [Member] | Commodity Contracts [Member] | Quoted Prices In Active Markets For Identical Assets Or Liabilities (Level 1) [Member]
    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Derivative assets    [4]    [4]
Derivative liabilities    [4] 7 [4]
Ameren Illinois Company [Member] | Commodity Contracts [Member] | Significant Other Observable Inputs (Level 2) [Member]
    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Derivative assets 1 [4]    [4]
Derivative liabilities 94 [4]    [4]
Ameren Illinois Company [Member] | Commodity Contracts [Member] | Significant Other Unobservable Inputs (Level 3) [Member]
    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Derivative assets    [4] 79 [4]
Derivative liabilities 111 [4] 379 [4]
Ameren Illinois Company [Member] | Commodity Contracts [Member] | Natural Gas [Member]
    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Derivative assets 1 [4] 2 [4]
Derivative liabilities 94 [4] 169 [4]
Ameren Illinois Company [Member] | Commodity Contracts [Member] | Natural Gas [Member] | Quoted Prices In Active Markets For Identical Assets Or Liabilities (Level 1) [Member]
    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Derivative assets    [4]    [4]
Derivative liabilities    [4] 7 [4]
Ameren Illinois Company [Member] | Commodity Contracts [Member] | Natural Gas [Member] | Significant Other Observable Inputs (Level 2) [Member]
    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Derivative assets 1 [4]    [4]
Derivative liabilities 94 [4]    [4]
Ameren Illinois Company [Member] | Commodity Contracts [Member] | Natural Gas [Member] | Significant Other Unobservable Inputs (Level 3) [Member]
    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Derivative assets    [4] 2 [4]
Derivative liabilities    [4] 162 [4]
Ameren Illinois Company [Member] | Commodity Contracts [Member] | Power [Member]
    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Derivative assets    [4] 77 [4]
Derivative liabilities 111 [4] 217 [4]
Ameren Illinois Company [Member] | Commodity Contracts [Member] | Power [Member] | Quoted Prices In Active Markets For Identical Assets Or Liabilities (Level 1) [Member]
    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Derivative assets    [4]    [4]
Derivative liabilities    [4]    [4]
Ameren Illinois Company [Member] | Commodity Contracts [Member] | Power [Member] | Significant Other Observable Inputs (Level 2) [Member]
    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Derivative assets    [4]    [4]
Derivative liabilities    [4]    [4]
Ameren Illinois Company [Member] | Commodity Contracts [Member] | Power [Member] | Significant Other Unobservable Inputs (Level 3) [Member]
    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Derivative assets    [4] 77 [4]
Derivative liabilities $ 111 [4] $ 217 [4]
[1] Includes amounts for Ameren registrant and nonregistrant subsidiaries and intercompany eliminations.
[2] Balance excludes $2 million of receivables, payables, and accrued income, net.
[3] Balance excludes $(1) million of receivables, payables, and accrued income, net.
[4] The derivative asset and liability balances are presented net of counterparty credit considerations.