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Fair Value Measurements (Schedule Of Valuation Process And Unobservable Inputs) (Detail) (USD $)
In Millions, unless otherwise specified
 3 Months Ended 12 Months Ended
Mar. 31, 2014
Dec. 31, 2013
Natural Gas | Discounted Cash Flow | Minimum
    
Fair Value Inputs [Abstract]    
Counterparty credit risk 0.27% [1],[2]  
Nodal basis (0.20) [1]  
Credit risk 0.43% [1],[2]  
Natural Gas | Discounted Cash Flow | Maximum
    
Fair Value Inputs [Abstract]    
Counterparty credit risk 5.00% [1],[2]  
Nodal basis (0.1) [1]  
Credit risk 0.43% [1],[2]  
Natural Gas | Discounted Cash Flow | Weighted Average
    
Fair Value Inputs [Abstract]    
Counterparty credit risk 1.00% [1],[2]  
Nodal basis (0.1) [1]  
Credit risk 0.43% [1],[2]  
Natural Gas | Option Model | Minimum
    
Fair Value Inputs [Abstract]    
Volatilities 27.00% [3]  
Nodal basis (0.30) [1]  
Natural Gas | Option Model | Maximum
    
Fair Value Inputs [Abstract]    
Volatilities 102.00% [3]  
Nodal basis (0.20) [1]  
Natural Gas | Option Model | Weighted Average
    
Fair Value Inputs [Abstract]    
Volatilities 40.00% [3]  
Nodal basis (0.20) [1]  
Natural Gas | Derivative Assets
    
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]    
Derivative assets $ 1 [4]  
Natural Gas | Union Electric Company | Discounted Cash Flow | Minimum
    
Fair Value Inputs [Abstract]    
Counterparty credit risk 0.27% [1],[2]  
Nodal basis (0.20) [1]  
Credit risk 0.43% [1],[2]  
Natural Gas | Union Electric Company | Discounted Cash Flow | Maximum
    
Fair Value Inputs [Abstract]    
Counterparty credit risk 5.00% [1],[2]  
Nodal basis (0.1) [1]  
Credit risk 0.43% [1],[2]  
Natural Gas | Union Electric Company | Discounted Cash Flow | Weighted Average
    
Fair Value Inputs [Abstract]    
Counterparty credit risk 1.00% [1],[2]  
Nodal basis (0.1) [1]  
Credit risk 0.43% [1],[2]  
Natural Gas | Union Electric Company | Option Model | Minimum
    
Fair Value Inputs [Abstract]    
Escalation rate     
Volatilities 27.00% [3]  
Nodal basis (0.30) [1]  
Natural Gas | Union Electric Company | Option Model | Maximum
    
Fair Value Inputs [Abstract]    
Escalation rate     
Volatilities 102.00% [3]  
Nodal basis (0.20) [1]  
Natural Gas | Union Electric Company | Option Model | Weighted Average
    
Fair Value Inputs [Abstract]    
Escalation rate     
Volatilities 40.00% [3]  
Nodal basis (0.20) [1]  
Natural Gas | Union Electric Company | Derivative Assets
    
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]    
Derivative assets 1 [4]  
Natural Gas | Ameren Illinois Company | Derivative Assets
    
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]    
Derivative assets    [4]  
Natural Gas | Derivative Liabilities
    
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]    
Derivative liabilities (1) [4]  
Natural Gas | Derivative Liabilities | Union Electric Company
    
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]    
Derivative liabilities (1) [4]  
Natural Gas | Derivative Liabilities | Ameren Illinois Company
    
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]    
Derivative liabilities    [4]  
Fuel Oils | Discounted Cash Flow | Minimum
    
Fair Value Inputs [Abstract]    
Counterparty credit risk 0.25% [1],[2] 0.26% [1],[2]
Fuel Oils | Discounted Cash Flow | Maximum
    
Fair Value Inputs [Abstract]    
Counterparty credit risk 0.99% [1],[2] 2.00% [1],[2]
Fuel Oils | Discounted Cash Flow | Weighted Average
    
Fair Value Inputs [Abstract]    
Counterparty credit risk 0.63% [1],[2] 1.00% [1],[2]
Fuel Oils | Option Model | Minimum
    
Fair Value Inputs [Abstract]    
Volatilities 13.00% [3] 10.00% [3]
Fuel Oils | Option Model | Maximum
    
Fair Value Inputs [Abstract]    
Volatilities 34.00% [3] 35.00% [3]
Fuel Oils | Option Model | Weighted Average
    
Fair Value Inputs [Abstract]    
Volatilities 16.00% [3] 16.00% [3]
Fuel Oils | Derivative Assets
    
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]    
Derivative assets 5 [4] 8 [4]
Fuel Oils | Union Electric Company | Discounted Cash Flow | Minimum
    
Fair Value Inputs [Abstract]    
Counterparty credit risk 0.25% [1],[2] 0.26% [1],[2]
Fuel Oils | Union Electric Company | Discounted Cash Flow | Maximum
    
Fair Value Inputs [Abstract]    
Counterparty credit risk 0.99% [1],[2] 2.00% [1],[2]
Fuel Oils | Union Electric Company | Discounted Cash Flow | Weighted Average
    
Fair Value Inputs [Abstract]    
Counterparty credit risk 0.63% [1],[2] 1.00% [1],[2]
Fuel Oils | Union Electric Company | Option Model | Minimum
    
Fair Value Inputs [Abstract]    
Volatilities 13.00% [3] 10.00% [3]
Fuel Oils | Union Electric Company | Option Model | Maximum
    
Fair Value Inputs [Abstract]    
Volatilities 34.00% [3] 35.00% [3]
Fuel Oils | Union Electric Company | Option Model | Weighted Average
    
Fair Value Inputs [Abstract]    
Volatilities 16.00% [3] 16.00% [3]
Fuel Oils | Union Electric Company | Derivative Assets
    
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]    
Derivative assets 5 [4] 8 [4]
Fuel Oils | Derivative Liabilities
    
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]    
Derivative liabilities (4) [4] (3) [4]
Fuel Oils | Derivative Liabilities | Union Electric Company
    
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]    
Derivative liabilities (4) [4] (3) [4]
Power | Discounted Cash Flow | Minimum
    
Fair Value Inputs [Abstract]    
Counterparty credit risk 0.35% [1],[2] 0.39% [1],[2]
Nodal basis (6) [1] (3) [1]
Credit risk 0.43% [1],[2] 2.00% [1],[2]
Average forward peak and off-peak pricing 27 [1] 25 [1]
Estimated auction price (1,109) [3] (1,594) [3]
Power | Discounted Cash Flow | Maximum
    
Fair Value Inputs [Abstract]    
Counterparty credit risk 0.35% [1],[2] 0.50% [1],[2]
Nodal basis (1) [1] (1) [1]
Credit risk 0.43% [1],[2] 2.00% [1],[2]
Average forward peak and off-peak pricing 57 [1] 51 [1]
Estimated auction price 2,981 [3] 945 [3]
Power | Discounted Cash Flow | Weighted Average
    
Fair Value Inputs [Abstract]    
Counterparty credit risk 0.35% [1],[2] 0.42% [1],[2]
Nodal basis (3) [1] (2) [1]
Credit risk 0.43% [1],[2] 2.00% [1],[2]
Average forward peak and off-peak pricing 33 [1] 32 [1]
Estimated auction price 474 [3] 305 [3]
Power | Fundamental Energy Production Model | Minimum
    
Fair Value Inputs [Abstract]    
Escalation rate 2.00% [3],[5] 3.00% [3],[5]
Estimated future gas prices 4 [3] 4 [3]
Power | Fundamental Energy Production Model | Maximum
    
Fair Value Inputs [Abstract]    
Escalation rate 3.00% [3],[5] 4.00% [3],[5]
Estimated future gas prices 6 [3] 5 [3]
Power | Fundamental Energy Production Model | Weighted Average
    
Fair Value Inputs [Abstract]    
Escalation rate 3.00% [3],[5] 4.00% [3],[5]
Estimated future gas prices 5 [3] 5 [3]
Power | Contract Price Allocation | Minimum
    
Fair Value Inputs [Abstract]    
Estimated renewable energy credit costs 5 [3] 5 [3]
Power | Contract Price Allocation | Maximum
    
Fair Value Inputs [Abstract]    
Estimated renewable energy credit costs 7 [3] 7 [3]
Power | Contract Price Allocation | Weighted Average
    
Fair Value Inputs [Abstract]    
Estimated renewable energy credit costs 6 [3] 6 [3]
Power | Derivative Assets
    
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]    
Derivative assets 14 [4],[6] 21 [4],[7]
Power | Union Electric Company | Discounted Cash Flow | Minimum
    
Fair Value Inputs [Abstract]    
Counterparty credit risk 0.35% [1],[2] 0.39% [1],[2]
Nodal basis (3) [1] (3) [1]
Credit risk 0.43% [1],[2] 2.00% [1],[2]
Average forward peak and off-peak pricing 30 [1] 25 [1]
Estimated auction price (1,109) [3] (1,594) [3]
Power | Union Electric Company | Discounted Cash Flow | Maximum
    
Fair Value Inputs [Abstract]    
Counterparty credit risk 0.35% [1],[2] 0.50% [1],[2]
Nodal basis (1) [1] (1) [1]
Credit risk 0.43% [1],[2] 2.00% [1],[2]
Average forward peak and off-peak pricing 57 [1] 51 [1]
Estimated auction price 2,981 [3] 945 [3]
Power | Union Electric Company | Discounted Cash Flow | Weighted Average
    
Fair Value Inputs [Abstract]    
Counterparty credit risk 0.35% [1],[2] 0.42% [1],[2]
Nodal basis (2) [1] (2) [1]
Credit risk 0.43% [1],[2] 2.00% [1],[2]
Average forward peak and off-peak pricing 45 [1] 40 [1]
Estimated auction price 474 [3] 305 [3]
Power | Union Electric Company | Derivative Assets
    
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]    
Derivative assets 14 [4],[6] 21 [4],[7]
Power | Ameren Illinois Company | Discounted Cash Flow | Minimum
    
Fair Value Inputs [Abstract]    
Nodal basis (6) [3] (4) [3]
Credit risk 0.43% [1],[2] 2.00% [1],[2]
Average forward peak and off-peak pricing 27 [3] 27 [3]
Power | Ameren Illinois Company | Discounted Cash Flow | Maximum
    
Fair Value Inputs [Abstract]    
Nodal basis (1) [3] 0 [3]
Credit risk 0.43% [1],[2] 2.00% [1],[2]
Average forward peak and off-peak pricing 40 [3] 36 [3]
Power | Ameren Illinois Company | Discounted Cash Flow | Weighted Average
    
Fair Value Inputs [Abstract]    
Nodal basis (3) [3] (2) [3]
Credit risk 0.43% [1],[2] 2.00% [1],[2]
Average forward peak and off-peak pricing 32 [3] 30 [3]
Power | Ameren Illinois Company | Fundamental Energy Production Model | Minimum
    
Fair Value Inputs [Abstract]    
Escalation rate 2.00% [3],[5] 3.00% [3],[5]
Estimated future gas prices 4 [3] 4 [3]
Power | Ameren Illinois Company | Fundamental Energy Production Model | Maximum
    
Fair Value Inputs [Abstract]    
Escalation rate 3.00% [3],[5] 4.00% [3],[5]
Estimated future gas prices 6 [3] 5 [3]
Power | Ameren Illinois Company | Fundamental Energy Production Model | Weighted Average
    
Fair Value Inputs [Abstract]    
Escalation rate 3.00% [3],[5] 4.00% [3],[5]
Estimated future gas prices 5 [3] 5 [3]
Power | Ameren Illinois Company | Contract Price Allocation | Minimum
    
Fair Value Inputs [Abstract]    
Estimated renewable energy credit costs 5 [3] 5 [3]
Power | Ameren Illinois Company | Contract Price Allocation | Maximum
    
Fair Value Inputs [Abstract]    
Estimated renewable energy credit costs 7 [3] 7 [3]
Power | Ameren Illinois Company | Contract Price Allocation | Weighted Average
    
Fair Value Inputs [Abstract]    
Estimated renewable energy credit costs 6 [3] 6 [3]
Power | Ameren Illinois Company | Derivative Assets
    
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]    
Derivative assets    [4],[6]    [4],[7]
Power | Derivative Liabilities
    
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]    
Derivative liabilities (124) [4],[6] (110) [4],[7]
Power | Derivative Liabilities | Union Electric Company
    
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]    
Derivative liabilities (4) [4],[6] (2) [4],[7]
Power | Derivative Liabilities | Ameren Illinois Company
    
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]    
Derivative liabilities (120) [4],[6] (108) [4],[7]
Uranium | Discounted Cash Flow | Minimum
    
Fair Value Inputs [Abstract]    
Average forward pricing 34 [3] 34 [3]
Uranium | Discounted Cash Flow | Maximum
    
Fair Value Inputs [Abstract]    
Average forward pricing 40 [3] 41 [3]
Uranium | Discounted Cash Flow | Weighted Average
    
Fair Value Inputs [Abstract]    
Average forward pricing 35 [3] 36 [3]
Uranium | Derivative Assets
    
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]    
Derivative assets    [4]    [4]
Uranium | Union Electric Company | Discounted Cash Flow | Minimum
    
Fair Value Inputs [Abstract]    
Average forward pricing 34 [3] 34 [3]
Uranium | Union Electric Company | Discounted Cash Flow | Maximum
    
Fair Value Inputs [Abstract]    
Average forward pricing 40 [3] 41 [3]
Uranium | Union Electric Company | Discounted Cash Flow | Weighted Average
    
Fair Value Inputs [Abstract]    
Average forward pricing 35 [3] 36 [3]
Uranium | Union Electric Company | Derivative Assets
    
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]    
Derivative assets    [4]    [4]
Uranium | Derivative Liabilities
    
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]    
Derivative liabilities (5) [4] (6) [4]
Uranium | Derivative Liabilities | Union Electric Company
    
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]    
Derivative liabilities $ (5) [4] $ (6) [4]
[1] Generally, significant increases (decreases) in this input in isolation would result in a significantly lower (higher) fair value measurement.
[2] Counterparty credit risk is applied only to counterparties with derivative asset balances. Ameren Missouri and Ameren Illinois credit risk is applied only to counterparties with derivative liability balances.
[3] Generally, significant increases (decreases) in this input in isolation would result in a significantly higher (lower) fair value measurement.
[4] The derivative asset and liability balances are presented net of counterparty credit considerations.
[5] Escalation rate applies to power prices 2026 and beyond.
[6] Power valuations use visible third-party pricing evaluated by month for peak and off-peak demand through 2018. Valuations beyond 2018 use fundamentally modeled pricing by month for peak and off-peak demand.
[7] Power valuations use visible third-party pricing evaluated by month for peak and off-peak demand through 2017. Valuations beyond 2017 use fundamentally modeled pricing by month for peak and off-peak demand.