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Fair Value Measurements (Schedule of Valuation Process and Unobservable Inputs) (Details)
$ in Millions
 12 Months Ended
Dec. 31, 2015
USD ($)
$ / MMBTU
$ / credit
$ / MW
$ / MWh
$ / lb
Dec. 31, 2014
USD ($)
$ / MMBTU
$ / credit
$ / MW
$ / MWh
$ / lb
Discounted Cash Flow | Minimum | Fuel Oils    
Fair Value Inputs [Abstract]    
Escalation rate [1],[2]   5.00%
Counterparty credit risk [3],[4]   0.43%
Discounted Cash Flow | Minimum | Natural Gas    
Fair Value Inputs [Abstract]    
Counterparty credit risk [3],[4] 0.40% 0.43%
Credit risk [3],[4] 0.40% 0.43%
Nodal basis [2] (0.10) (0.40)
Discounted Cash Flow | Minimum | Power    
Fair Value Inputs [Abstract]    
Counterparty credit risk [3],[4] 0.86% 0.26%
Credit risk [3],[4] 0.40% 0.43%
Average bid/ask consensus peak and off-peak pricing [5] 22 27
Estimated auction price for FTRs | $ / MW [2] (270) (1,833)
Nodal basis (10) [5] (6) [2]
Discounted Cash Flow | Minimum | Uranium    
Fair Value Inputs [Abstract]    
Credit risk [3],[4] 0.40%  
Average bid/ask consensus pricing | $ / lb [2] 35 35
Discounted Cash Flow | Maximum | Fuel Oils    
Fair Value Inputs [Abstract]    
Escalation rate [1],[2]   5.00%
Counterparty credit risk [3],[4]   0.43%
Discounted Cash Flow | Maximum | Natural Gas    
Fair Value Inputs [Abstract]    
Counterparty credit risk [3],[4] 12.00% 13.00%
Credit risk [3],[4] 0.40% 0.43%
Nodal basis [2] 0 0.10
Discounted Cash Flow | Maximum | Power    
Fair Value Inputs [Abstract]    
Counterparty credit risk [3],[4] 0.86% 0.26%
Credit risk [3],[4] 0.40% 0.43%
Average bid/ask consensus peak and off-peak pricing [5] 39 50
Estimated auction price for FTRs | $ / MW [2] 2,057 2,743
Nodal basis (1) [5] 0 [2]
Discounted Cash Flow | Maximum | Uranium    
Fair Value Inputs [Abstract]    
Credit risk [3],[4] 0.40%  
Average bid/ask consensus pricing | $ / lb [2] 42 40
Discounted Cash Flow | Weighted Average | Fuel Oils    
Fair Value Inputs [Abstract]    
Escalation rate [1],[2]   5.00%
Counterparty credit risk [3],[4]   0.43%
Discounted Cash Flow | Weighted Average | Natural Gas    
Fair Value Inputs [Abstract]    
Counterparty credit risk [3],[4] 7.00% 3.00%
Credit risk [3],[4] 0.40% 0.43%
Nodal basis [2] (0.10) (0.20)
Discounted Cash Flow | Weighted Average | Power    
Fair Value Inputs [Abstract]    
Counterparty credit risk [3],[4] 0.86% 0.26%
Credit risk [3],[4] 0.40% 0.43%
Average bid/ask consensus peak and off-peak pricing [5] 29 32
Estimated auction price for FTRs | $ / MW [2] 211 171
Nodal basis (3) [5] (2) [2]
Discounted Cash Flow | Weighted Average | Uranium    
Fair Value Inputs [Abstract]    
Credit risk [3],[4] 0.40%  
Average bid/ask consensus pricing | $ / lb [2] 37 36
Option Model | Minimum | Fuel Oils    
Fair Value Inputs [Abstract]    
Volatilities [4]   3.00%
Option Model | Minimum | Natural Gas    
Fair Value Inputs [Abstract]    
Volatilities 35.00% [2] 31.00% [4]
Nodal basis (0.30) [4] (0.40) [2]
Option Model | Minimum | Uranium    
Fair Value Inputs [Abstract]    
Volatilities [2] 20.00%  
Option Model | Maximum | Fuel Oils    
Fair Value Inputs [Abstract]    
Volatilities [4]   39.00%
Option Model | Maximum | Natural Gas    
Fair Value Inputs [Abstract]    
Volatilities 55.00% [2] 144.00% [4]
Nodal basis 0 [4] 0 [2]
Option Model | Maximum | Uranium    
Fair Value Inputs [Abstract]    
Volatilities [2] 20.00%  
Option Model | Weighted Average | Fuel Oils    
Fair Value Inputs [Abstract]    
Volatilities [4]   32.00%
Option Model | Weighted Average | Natural Gas    
Fair Value Inputs [Abstract]    
Volatilities 45.00% [2] 63.00% [4]
Nodal basis (0.20) [4] (0.20) [2]
Option Model | Weighted Average | Uranium    
Fair Value Inputs [Abstract]    
Volatilities [2] 20.00%  
Fundamental Energy Production Model | Minimum | Power    
Fair Value Inputs [Abstract]    
Escalation rate [2],[6] 3.00% 0.00%
Estimated future gas prices | $ / MMBTU [2] 3 4
Fundamental Energy Production Model | Maximum | Power    
Fair Value Inputs [Abstract]    
Escalation rate [2],[6] 3.00% 1.00%
Estimated future gas prices | $ / MMBTU [2] 4 5
Fundamental Energy Production Model | Weighted Average | Power    
Fair Value Inputs [Abstract]    
Escalation rate [2],[6] 3.00% 1.00%
Estimated future gas prices | $ / MMBTU [2] 4 4
Contract Price Allocation | Minimum | Power    
Fair Value Inputs [Abstract]    
Estimated renewable energy credit costs | $ / credit [2] 5 5
Contract Price Allocation | Maximum | Power    
Fair Value Inputs [Abstract]    
Estimated renewable energy credit costs | $ / credit [2] 7 7
Contract Price Allocation | Weighted Average | Power    
Fair Value Inputs [Abstract]    
Estimated renewable energy credit costs | $ / credit [2] 6 6
Derivative liabilities | Fuel Oils    
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]    
Derivative liabilities | $ [7]   $ (8)
Derivative liabilities | Natural Gas    
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]    
Derivative liabilities | $ [7] $ (1) (2)
Derivative liabilities | Power    
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]    
Derivative liabilities | $ [7],[8] (170) (144)
Derivative liabilities | Uranium    
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]    
Derivative liabilities | $ [7] (1) $ (2)
Derivative assets | Natural Gas    
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]    
Derivative assets | $ [7] 1  
Derivative assets | Power    
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]    
Derivative assets | $ [7],[8] $ 16  
[1] Escalation rate applies to fuel oil prices 2017 and beyond.
[2] Generally, significant increases (decreases) in this input in isolation would result in a significantly higher (lower) fair value measurement.
[3] 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.
[4] Generally, significant increases (decreases) in this input in isolation would result in a significantly lower (higher) fair value measurement.
[5] The balance at Ameren is comprised of Ameren Missouri and Ameren Illinois power contracts, which respond differently to unobservable input changes because of their opposing positions.
[6] Escalation rate applies to power prices 2026 and beyond.
[7] The derivative asset and liability balances are presented net of counterparty credit considerations.
[8] Power valuations use visible third-party pricing evaluated by month for peak and off-peak demand through 2019. Valuations beyond 2019 use fundamentally modeled pricing by month for peak and off-peak demand