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Fair Value Measurements (Schedule of Valuation Process and Unobservable Inputs) (Details)
$ in Millions
 12 Months Ended
Dec. 31, 2016
USD ($)
$ / MMBTU
$ / credit
$ / MW
$ / MWh
$ / lb
Dec. 31, 2015
USD ($)
$ / MMBTU
$ / credit
$ / MW
$ / MWh
$ / lb
Fuel Oils    
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]    
Derivative assets [1] $ 3  
Derivative liabilities (5) [1] $ (29) [2]
Natural Gas    
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]    
Derivative assets 15 [1] 2 [2]
Derivative liabilities (14) [1] (64) [2]
Power    
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]    
Derivative assets 9 [1] 16 [2]
Derivative liabilities (188) [1] (170) [2]
Uranium    
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]    
Derivative liabilities (4) [1] (1) [2]
Union Electric Company | Fuel Oils    
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]    
Derivative assets [1] 3  
Derivative liabilities (5) [1] (29) [2]
Union Electric Company | Natural Gas    
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]    
Derivative assets 2 [1] 1 [2]
Derivative liabilities (6) [1] (14) [2]
Union Electric Company | Power    
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]    
Derivative assets 9 [1] 16 [2]
Derivative liabilities [1] (3)  
Union Electric Company | Uranium    
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]    
Derivative liabilities (4) [1] (1) [2]
Ameren Illinois Company | Natural Gas    
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]    
Derivative assets 13 [1] 1 [2]
Derivative liabilities (8) [1] (50) [2]
Ameren Illinois Company | Power    
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]    
Derivative liabilities $ (185) [1] $ (170) [2]
Discounted Cash Flow | Minimum | Fuel Oils    
Fair Value Inputs [Abstract]    
Escalation rate [3],[4] (2.00%)  
Counterparty credit risk [5],[6] 0.13%  
Credit risk [5],[6] 0.38%  
Discounted Cash Flow | Minimum | Natural Gas    
Fair Value Inputs [Abstract]    
Counterparty credit risk [5],[6] 0.13% 0.40%
Credit risk [5],[6] 0.38% 0.40%
Nodal basis | $ / MWh [4] (0.80) (0.10)
Discounted Cash Flow | Minimum | Power    
Fair Value Inputs [Abstract]    
Counterparty credit risk [5],[6]   0.86%
Credit risk [5],[6] 0.38% 0.40%
Average bid/ask consensus peak and off-peak pricing | $ / MWh [7] 26 22
Estimated auction price for FTRs | $ / MW [4] (71) (270)
Nodal basis | $ / MWh [7] (6) (10)
Discounted Cash Flow | Minimum | Uranium    
Fair Value Inputs [Abstract]    
Credit risk [5],[6] 0.38% 0.40%
Average bid/ask consensus pricing | $ / lb [4] 22 35
Discounted Cash Flow | Maximum | Fuel Oils    
Fair Value Inputs [Abstract]    
Escalation rate [3],[4] 2.00%  
Counterparty credit risk [5],[6] 0.22%  
Credit risk [5],[6] 0.38%  
Discounted Cash Flow | Maximum | Natural Gas    
Fair Value Inputs [Abstract]    
Counterparty credit risk [5],[6] 8.00% 12.00%
Credit risk [5],[6] 0.38% 0.40%
Nodal basis | $ / MWh [4] 0 0
Discounted Cash Flow | Maximum | Power    
Fair Value Inputs [Abstract]    
Counterparty credit risk [5],[6]   0.86%
Credit risk [5],[6] 0.38% 0.40%
Average bid/ask consensus peak and off-peak pricing | $ / MWh [7] 44 39
Estimated auction price for FTRs | $ / MW [4] 5,270 2,057
Nodal basis | $ / MWh [7] 0 (1)
Discounted Cash Flow | Maximum | Uranium    
Fair Value Inputs [Abstract]    
Credit risk [5],[6] 0.38% 0.40%
Average bid/ask consensus pricing | $ / lb [4] 24 42
Discounted Cash Flow | Weighted Average | Fuel Oils    
Fair Value Inputs [Abstract]    
Escalation rate [3],[4] 0.00%  
Counterparty credit risk [5],[6] 0.15%  
Credit risk [5],[6] 0.38%  
Discounted Cash Flow | Weighted Average | Natural Gas    
Fair Value Inputs [Abstract]    
Counterparty credit risk [5],[6] 1.00% 7.00%
Credit risk [5],[6] 0.38% 0.40%
Nodal basis | $ / MWh [4] (0.50) (0.10)
Discounted Cash Flow | Weighted Average | Power    
Fair Value Inputs [Abstract]    
Counterparty credit risk [5],[6]   0.86%
Credit risk [5],[6] 0.38% 0.40%
Average bid/ask consensus peak and off-peak pricing | $ / MWh [7] 29 29
Estimated auction price for FTRs | $ / MW [4] 125 211
Nodal basis | $ / MWh [7] (2) (3)
Discounted Cash Flow | Weighted Average | Uranium    
Fair Value Inputs [Abstract]    
Credit risk [5],[6] 0.38% 0.40%
Average bid/ask consensus pricing | $ / lb [4] 22 37
Option Model | Minimum | Fuel Oils    
Fair Value Inputs [Abstract]    
Volatilities [4] 24.00%  
Option Model | Minimum | Natural Gas    
Fair Value Inputs [Abstract]    
Volatilities [4] 31.00% 35.00%
Nodal basis | $ / MWh (0.40) [4] (0.30) [6]
Option Model | Minimum | Uranium    
Fair Value Inputs [Abstract]    
Volatilities [4] 24.00% 20.00%
Option Model | Maximum | Fuel Oils    
Fair Value Inputs [Abstract]    
Volatilities [4] 66.00%  
Option Model | Maximum | Natural Gas    
Fair Value Inputs [Abstract]    
Volatilities [4] 66.00% 55.00%
Nodal basis | $ / MWh (0.10) [4] 0 [6]
Option Model | Maximum | Uranium    
Fair Value Inputs [Abstract]    
Volatilities [4] 24.00% 20.00%
Option Model | Weighted Average | Fuel Oils    
Fair Value Inputs [Abstract]    
Volatilities [4] 28.00%  
Option Model | Weighted Average | Natural Gas    
Fair Value Inputs [Abstract]    
Volatilities [4] 36.00% 45.00%
Nodal basis | $ / MWh (0.20) [4] (0.20) [6]
Option Model | Weighted Average | Uranium    
Fair Value Inputs [Abstract]    
Volatilities [4] 24.00% 20.00%
Fundamental Energy Production Model | Minimum | Power    
Fair Value Inputs [Abstract]    
Escalation rate [4],[8] 5.00% 3.00%
Estimated future gas prices | $ / MMBTU [4] 3 3
Fundamental Energy Production Model | Maximum | Power    
Fair Value Inputs [Abstract]    
Escalation rate [4],[8] 5.00% 3.00%
Estimated future gas prices | $ / MMBTU [4] 4 4
Fundamental Energy Production Model | Weighted Average | Power    
Fair Value Inputs [Abstract]    
Escalation rate [4],[8] 5.00% 3.00%
Estimated future gas prices | $ / MMBTU [4] 3 4
Contract Price Allocation | Minimum | Power    
Fair Value Inputs [Abstract]    
Estimated renewable energy credit costs | $ / credit [4] 5 5
Contract Price Allocation | Maximum | Power    
Fair Value Inputs [Abstract]    
Estimated renewable energy credit costs | $ / credit [4] 7 7
Contract Price Allocation | Weighted Average | Power    
Fair Value Inputs [Abstract]    
Estimated renewable energy credit costs | $ / credit [4] 6 6
Derivative liabilities | Fuel Oils    
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]    
Derivative liabilities [9] $ 0  
Derivative liabilities | Natural Gas    
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]    
Derivative liabilities [9] (1) $ (1)
Derivative liabilities | Power    
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]    
Derivative liabilities [9],[10] (187) (170)
Derivative liabilities | Uranium    
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]    
Derivative liabilities [9] (4) (1)
Derivative assets | Fuel Oils    
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]    
Derivative assets [9] 1  
Derivative assets | Natural Gas    
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]    
Derivative assets [9] 1 1
Derivative assets | Power    
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]    
Derivative assets [9],[10] $ 9 $ 16
[1] The derivative asset and liability balances are presented net of counterparty credit considerations.
[2] The derivative asset and liability balances are presented net of counterparty credit considerations.
[3] Escalation rate applies to fuel oil prices 2019 and beyond.
[4] Generally, significant increases (decreases) in this input in isolation would result in a significantly higher (lower) fair value measurement.
[5] 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.
[6] Generally, significant increases (decreases) in this input in isolation would result in a significantly lower (higher) fair value measurement.
[7] 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.
[8] Escalation rate applies to power prices in 2031 and beyond for December 31, 2016, and to power prices in 2026 and beyond for December 31, 2015.
[9] The derivative asset and liability balances are presented net of counterparty credit considerations.
[10] Power valuations use visible third-party pricing evaluated by month for peak and off-peak demand through 2020. Valuations beyond 2020 use fundamentally modeled pricing by month for peak and off-peak demand.