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Fair Value Measurements (Schedule Of Valuation Process And Unobservable Inputs) (Detail) (USD $)
In Millions, unless otherwise specified
 9 Months Ended 12 Months Ended
Sep. 30, 2013
Dec. 31, 2012
Natural Gas
    
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]    
Derivative assets $ 1 [1]  
Natural Gas | Discounted Cash Flow | Minimum
    
Fair Value Inputs [Abstract]    
Counterparty credit risk 0.64% [2],[3]  
Nodal basis (0.10) [3]  
Credit risk 2.00% [2],[3]  
Natural Gas | Discounted Cash Flow | Maximum
    
Fair Value Inputs [Abstract]    
Counterparty credit risk 0.83% [2],[3]  
Credit risk 3.00% [2],[3]  
Natural Gas | Discounted Cash Flow | Weighted Average
    
Fair Value Inputs [Abstract]    
Counterparty credit risk 0.77% [2],[3]  
Credit risk 2.00% [2],[3]  
Natural Gas | Option Model | Minimum
    
Fair Value Inputs [Abstract]    
Volatilities 5.00% [4]  
Nodal basis (0.40) [3]  
Natural Gas | Option Model | Maximum
    
Fair Value Inputs [Abstract]    
Volatilities 29.00% [4]  
Nodal basis (0.20) [3]  
Natural Gas | Option Model | Weighted Average
    
Fair Value Inputs [Abstract]    
Volatilities 24.00% [4]  
Nodal basis (0.40) [3]  
Natural Gas | Union Electric Company | Discounted Cash Flow | Minimum
    
Fair Value Inputs [Abstract]    
Counterparty credit risk 0.64% [2],[3]  
Nodal basis (0.10) [3]  
Credit risk 2.00% [2],[3]  
Natural Gas | Union Electric Company | Discounted Cash Flow | Maximum
    
Fair Value Inputs [Abstract]    
Counterparty credit risk 0.83% [2],[3]  
Credit risk 3.00% [2],[3]  
Natural Gas | Union Electric Company | Discounted Cash Flow | Weighted Average
    
Fair Value Inputs [Abstract]    
Counterparty credit risk 0.77% [2],[3]  
Credit risk 2.00% [2],[3]  
Natural Gas | Union Electric Company | Option Model | Minimum
    
Fair Value Inputs [Abstract]    
Escalation rate     
Volatilities 5.00% [4]  
Nodal basis (0.40) [3]  
Natural Gas | Union Electric Company | Option Model | Maximum
    
Fair Value Inputs [Abstract]    
Escalation rate     
Volatilities 29.00% [4]  
Nodal basis (0.20) [3]  
Natural Gas | Union Electric Company | Option Model | Weighted Average
    
Fair Value Inputs [Abstract]    
Escalation rate     
Volatilities 24.00% [4]  
Nodal basis (0.40) [3]  
Natural Gas | Union Electric Company | Derivative Assets
    
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]    
Derivative assets 1 [1]  
Natural Gas | Ameren Illinois Company | Derivative Assets
    
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]    
Derivative assets    [1]  
Natural Gas | Derivative Liabilities
    
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]    
Derivative liabilities (1) [1]  
Natural Gas | Derivative Liabilities | Union Electric Company
    
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]    
Derivative liabilities (1) [1]  
Natural Gas | Derivative Liabilities | Ameren Illinois Company
    
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]    
Derivative liabilities    [1]  
Fuel Oils
    
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]    
Derivative assets 7 [1] 8 [1]
Fuel Oils | Discounted Cash Flow
    
Fair Value Inputs [Abstract]    
Credit risk   2.00% [2],[3]
Fuel Oils | Discounted Cash Flow | Minimum
    
Fair Value Inputs [Abstract]    
Escalation rate   0.21% [4]
Counterparty credit risk 0.26% [2],[3] 0.12% [2],[3]
Fuel Oils | Discounted Cash Flow | Maximum
    
Fair Value Inputs [Abstract]    
Escalation rate   0.60% [4]
Counterparty credit risk 3.00% [2],[3] 1.00% [2],[3]
Fuel Oils | Discounted Cash Flow | Weighted Average
    
Fair Value Inputs [Abstract]    
Escalation rate   0.44% [4]
Counterparty credit risk 2.00% [2],[3] 1.00% [2],[3]
Fuel Oils | Option Model | Minimum
    
Fair Value Inputs [Abstract]    
Volatilities 9.00% [4] 7.00% [4]
Fuel Oils | Option Model | Maximum
    
Fair Value Inputs [Abstract]    
Volatilities 20.00% [4] 27.00% [4]
Fuel Oils | Option Model | Weighted Average
    
Fair Value Inputs [Abstract]    
Volatilities 17.00% [4] 24.00% [4]
Fuel Oils | Union Electric Company | Discounted Cash Flow
    
Fair Value Inputs [Abstract]    
Credit risk   2.00% [2],[3]
Fuel Oils | Union Electric Company | Discounted Cash Flow | Minimum
    
Fair Value Inputs [Abstract]    
Escalation rate   0.21% [4]
Counterparty credit risk 0.26% [2],[3] 0.12% [2],[3]
Fuel Oils | Union Electric Company | Discounted Cash Flow | Maximum
    
Fair Value Inputs [Abstract]    
Escalation rate   0.60% [4]
Counterparty credit risk 3.00% [2],[3] 1.00% [2],[3]
Fuel Oils | Union Electric Company | Discounted Cash Flow | Weighted Average
    
Fair Value Inputs [Abstract]    
Escalation rate   0.44% [4]
Counterparty credit risk 2.00% [2],[3] 1.00% [2],[3]
Fuel Oils | Union Electric Company | Option Model | Minimum
    
Fair Value Inputs [Abstract]    
Volatilities 9.00% [4] 7.00% [4]
Fuel Oils | Union Electric Company | Option Model | Maximum
    
Fair Value Inputs [Abstract]    
Volatilities 20.00% [4] 27.00% [4]
Fuel Oils | Union Electric Company | Option Model | Weighted Average
    
Fair Value Inputs [Abstract]    
Volatilities 17.00% [4] 24.00% [4]
Fuel Oils | Union Electric Company | Derivative Assets
    
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]    
Derivative assets 7 [1] 8 [1]
Fuel Oils | Derivative Liabilities
    
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]    
Derivative liabilities (4) [1] (3) [1]
Fuel Oils | Derivative Liabilities | Union Electric Company
    
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]    
Derivative liabilities (4) [1] (3) [1]
Power | Discounted Cash Flow
    
Fair Value Inputs [Abstract]    
Credit risk 3.00% [2],[3]  
Power | Discounted Cash Flow | Minimum
    
Fair Value Inputs [Abstract]    
Counterparty credit risk 0.22% [2],[3] 0.22% [2],[3]
Nodal basis (5) [3] (5) [3]
Credit risk   2.00% [2],[3]
Average bid/ask consensus peak and offpeak pricing 27 [3] 22 [3]
Estimated auction price (103) [4] (281) [4]
Power | Discounted Cash Flow | Maximum
    
Fair Value Inputs [Abstract]    
Counterparty credit risk 3.00% [2],[3] 1.00% [2],[3]
Nodal basis (1) [3] (1) [3]
Credit risk   5.00% [2],[3]
Average bid/ask consensus peak and offpeak pricing 43 [3] 47 [3]
Estimated auction price 1,790 [4] 1,851 [4]
Power | Discounted Cash Flow | Weighted Average
    
Fair Value Inputs [Abstract]    
Counterparty credit risk 0.73% [2],[3] 1.00% [2],[3]
Nodal basis (3) [3] (3) [3]
Credit risk   5.00% [2],[3]
Average bid/ask consensus peak and offpeak pricing 30 [3] 31 [3]
Estimated auction price 660 [4] 178 [4]
Power | Fundamental Energy Production Model | Minimum
    
Fair Value Inputs [Abstract]    
Escalation rate 4.00% [4],[5]  
Estimated future gas prices 4 [4] 4 [4]
Power | Fundamental Energy Production Model | Maximum
    
Fair Value Inputs [Abstract]    
Escalation rate 5.00% [4],[5]  
Estimated future gas prices 7 [4] 8 [4]
Power | Fundamental Energy Production Model | Weighted Average
    
Fair Value Inputs [Abstract]    
Escalation rate 4.00% [4],[5]  
Estimated future gas prices 5 [4] 6 [4]
Power | Contract Price Allocation | Minimum
    
Fair Value Inputs [Abstract]    
Estimated renewable energy credit costs 5 [4] 5 [4]
Power | Contract Price Allocation | Maximum
    
Fair Value Inputs [Abstract]    
Estimated renewable energy credit costs 7 [4] 7 [4]
Power | Contract Price Allocation | Weighted Average
    
Fair Value Inputs [Abstract]    
Estimated renewable energy credit costs 6 [4] 6 [4]
Power | Derivative Assets
    
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]    
Derivative assets 29 [1],[6] 14 [1],[6]
Power | Union Electric Company | Discounted Cash Flow
    
Fair Value Inputs [Abstract]    
Credit risk 3.00% [2],[3] 2.00% [2],[3]
Power | Union Electric Company | Discounted Cash Flow | Minimum
    
Fair Value Inputs [Abstract]    
Counterparty credit risk 0.22% [2],[3] 0.22% [2],[3]
Nodal basis (4) [3] (5) [3]
Average bid/ask consensus peak and offpeak pricing 27 [3] 24 [3]
Estimated auction price (103) [4] (281) [4]
Power | Union Electric Company | Discounted Cash Flow | Maximum
    
Fair Value Inputs [Abstract]    
Counterparty credit risk 3.00% [2],[3] 1.00% [2],[3]
Nodal basis (1) [3] (1) [3]
Average bid/ask consensus peak and offpeak pricing 43 [3] 56 [3]
Estimated auction price 1,790 [4] 1,851 [4]
Power | Union Electric Company | Discounted Cash Flow | Weighted Average
    
Fair Value Inputs [Abstract]    
Counterparty credit risk 0.73% [2],[3] 1.00% [2],[3]
Nodal basis (4) [3] (2) [3]
Average bid/ask consensus peak and offpeak pricing 33 [3] 36 [3]
Estimated auction price 660 [4] 178 [4]
Power | Union Electric Company | Derivative Assets
    
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]    
Derivative assets 29 [1],[6] 14 [1],[6]
Power | Ameren Illinois Company | Discounted Cash Flow
    
Fair Value Inputs [Abstract]    
Credit risk 3.00% [2],[3] 5.00% [2],[3]
Power | Ameren Illinois Company | Discounted Cash Flow | Minimum
    
Fair Value Inputs [Abstract]    
Nodal basis (5) [4] (5) [4]
Average bid/ask consensus peak and offpeak pricing 27 [4] 22 [4]
Power | Ameren Illinois Company | Discounted Cash Flow | Maximum
    
Fair Value Inputs [Abstract]    
Nodal basis (1) [4] (1) [4]
Average bid/ask consensus peak and offpeak pricing 37 [4] 47 [4]
Power | Ameren Illinois Company | Discounted Cash Flow | Weighted Average
    
Fair Value Inputs [Abstract]    
Nodal basis (3) [4] (3) [4]
Average bid/ask consensus peak and offpeak pricing 30 [4] 30 [4]
Power | Ameren Illinois Company | Fundamental Energy Production Model | Minimum
    
Fair Value Inputs [Abstract]    
Escalation rate 4.00% [4],[5]  
Estimated future gas prices 4 [4] 4 [4]
Power | Ameren Illinois Company | Fundamental Energy Production Model | Maximum
    
Fair Value Inputs [Abstract]    
Escalation rate 5.00% [4],[5]  
Estimated future gas prices 7 [4] 8 [4]
Power | Ameren Illinois Company | Fundamental Energy Production Model | Weighted Average
    
Fair Value Inputs [Abstract]    
Escalation rate 4.00% [4],[5]  
Estimated future gas prices 5 [4] 6 [4]
Power | Ameren Illinois Company | Contract Price Allocation | Minimum
    
Fair Value Inputs [Abstract]    
Estimated renewable energy credit costs 5 [4] 5 [4]
Power | Ameren Illinois Company | Contract Price Allocation | Maximum
    
Fair Value Inputs [Abstract]    
Estimated renewable energy credit costs 7 [4] 7 [4]
Power | Ameren Illinois Company | Contract Price Allocation | Weighted Average
    
Fair Value Inputs [Abstract]    
Estimated renewable energy credit costs 6 [4] 6 [4]
Power | Ameren Illinois Company | Derivative Assets
    
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]    
Derivative assets    [1],[6]    [1],[6]
Power | Derivative Liabilities
    
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]    
Derivative liabilities (95) [1],[6] (114) [1],[6]
Power | Derivative Liabilities | Union Electric Company
    
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]    
Derivative liabilities (1) [1],[6] (3) [1],[6]
Power | Derivative Liabilities | Ameren Illinois Company
    
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]    
Derivative liabilities (94) [1],[6] (111) [1],[6]
Uranium | Discounted Cash Flow | Minimum
    
Fair Value Inputs [Abstract]    
Average bid/ask consensus pricing 35 [4] 43 [4]
Uranium | Discounted Cash Flow | Maximum
    
Fair Value Inputs [Abstract]    
Average bid/ask consensus pricing 43 [4] 46 [4]
Uranium | Discounted Cash Flow | Weighted Average
    
Fair Value Inputs [Abstract]    
Average bid/ask consensus pricing 36 [4] 44 [4]
Uranium | Derivative Assets
    
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]    
Derivative assets 1 [1]    [1]
Uranium | Union Electric Company | Discounted Cash Flow | Minimum
    
Fair Value Inputs [Abstract]    
Average bid/ask consensus pricing 35 [4] 43 [4]
Uranium | Union Electric Company | Discounted Cash Flow | Maximum
    
Fair Value Inputs [Abstract]    
Average bid/ask consensus pricing 43 [4] 46 [4]
Uranium | Union Electric Company | Discounted Cash Flow | Weighted Average
    
Fair Value Inputs [Abstract]    
Average bid/ask consensus pricing 36 [4] 44 [4]
Uranium | Union Electric Company | Derivative Assets
    
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]    
Derivative assets 1 [1]    [1]
Uranium | Derivative Liabilities
    
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]    
Derivative liabilities (6) [1] (2) [1]
Uranium | Derivative Liabilities | Union Electric Company
    
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]    
Derivative liabilities $ (6) [1] $ (2) [1]
[1] The derivative asset and liability balances are presented net of counterparty credit considerations.
[2] Counterparty credit risk is only applied to counterparties with derivative asset balances. Ameren, Ameren Missouri, and Ameren Illinois credit risk is only applied to counterparties with derivative liability balances.
[3] Generally, significant increases (decreases) in this input in isolation would result in a significantly lower (higher) fair value measurement.
[4] Generally, significant increases (decreases) in this input in isolation would result in a significantly higher (lower) fair value measurement.
[5] Escalation rate applies to power prices 2026 and beyond.
[6] Power valuations utilize visible third-party pricing evaluated by month for peak and off-peak demand through 2017. Valuations beyond 2017 utilize fundamentally modeled pricing by month for peak and off-peak demand.