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Fair Value Measurements (Schedule Of Valuation Process And Unobservable Inputs) (Detail) (USD $)
In Millions, unless otherwise specified
 3 Months Ended 3 Months Ended 3 Months Ended
Jun. 30, 2012
Dec. 31, 2011
Jun. 30, 2012
Ameren Missouri [Member]
Dec. 31, 2011
Ameren Missouri [Member]
Jun. 30, 2012
Ameren Illinois Company [Member]
Dec. 31, 2011
Ameren Illinois Company [Member]
Jun. 30, 2012
Genco [Member]
Dec. 31, 2011
Genco [Member]
Jun. 30, 2012
Commodity Contracts [Member]
Dec. 31, 2011
Commodity Contracts [Member]
Jun. 30, 2012
Commodity Contracts [Member]
Ameren Missouri [Member]
Dec. 31, 2011
Commodity Contracts [Member]
Ameren Missouri [Member]
Jun. 30, 2012
Commodity Contracts [Member]
Fuel Oils [Member]
Dec. 31, 2011
Commodity Contracts [Member]
Fuel Oils [Member]
Jun. 30, 2012
Commodity Contracts [Member]
Fuel Oils [Member]
Ameren Missouri [Member]
Dec. 31, 2011
Commodity Contracts [Member]
Fuel Oils [Member]
Ameren Missouri [Member]
Jun. 30, 2012
Commodity Contracts [Member]
Fuel Oils [Member]
Genco [Member]
Dec. 31, 2011
Commodity Contracts [Member]
Fuel Oils [Member]
Genco [Member]
Jun. 30, 2012
Commodity Contracts [Member]
Fuel Oils [Member]
Derivative Liabilities [Member]
Jun. 30, 2012
Commodity Contracts [Member]
Fuel Oils [Member]
Derivative Liabilities [Member]
Ameren Missouri [Member]
Jun. 30, 2012
Commodity Contracts [Member]
Fuel Oils [Member]
Discounted Cash Flow [Member]
Minimum [Member]
Jun. 30, 2012
Commodity Contracts [Member]
Fuel Oils [Member]
Discounted Cash Flow [Member]
Maximum [Member]
Jun. 30, 2012
Commodity Contracts [Member]
Fuel Oils [Member]
Discounted Cash Flow [Member]
Weighted Average [Member]
Jun. 30, 2012
Commodity Contracts [Member]
Fuel Oils [Member]
Discounted Cash Flow [Member]
Ameren Missouri [Member]
Jun. 30, 2012
Commodity Contracts [Member]
Fuel Oils [Member]
Discounted Cash Flow [Member]
Ameren Missouri [Member]
Minimum [Member]
Jun. 30, 2012
Commodity Contracts [Member]
Fuel Oils [Member]
Discounted Cash Flow [Member]
Ameren Missouri [Member]
Maximum [Member]
Jun. 30, 2012
Commodity Contracts [Member]
Fuel Oils [Member]
Discounted Cash Flow [Member]
Ameren Missouri [Member]
Weighted Average [Member]
Jun. 30, 2012
Commodity Contracts [Member]
Fuel Oils [Member]
Discounted Cash Flow [Member]
Genco [Member]
Jun. 30, 2012
Commodity Contracts [Member]
Fuel Oils [Member]
Discounted Cash Flow [Member]
Genco [Member]
Minimum [Member]
Jun. 30, 2012
Commodity Contracts [Member]
Fuel Oils [Member]
Discounted Cash Flow [Member]
Genco [Member]
Maximum [Member]
Jun. 30, 2012
Commodity Contracts [Member]
Fuel Oils [Member]
Discounted Cash Flow [Member]
Genco [Member]
Weighted Average [Member]
Jun. 30, 2012
Commodity Contracts [Member]
Fuel Oils [Member]
Option Model [Member]
Minimum [Member]
Jun. 30, 2012
Commodity Contracts [Member]
Fuel Oils [Member]
Option Model [Member]
Maximum [Member]
Jun. 30, 2012
Commodity Contracts [Member]
Fuel Oils [Member]
Option Model [Member]
Weighted Average [Member]
Jun. 30, 2012
Commodity Contracts [Member]
Fuel Oils [Member]
Option Model [Member]
Ameren Missouri [Member]
Minimum [Member]
Jun. 30, 2012
Commodity Contracts [Member]
Fuel Oils [Member]
Option Model [Member]
Ameren Missouri [Member]
Maximum [Member]
Jun. 30, 2012
Commodity Contracts [Member]
Fuel Oils [Member]
Option Model [Member]
Ameren Missouri [Member]
Weighted Average [Member]
Jun. 30, 2012
Commodity Contracts [Member]
Fuel Oils [Member]
Option Model [Member]
Genco [Member]
Minimum [Member]
Jun. 30, 2012
Commodity Contracts [Member]
Fuel Oils [Member]
Option Model [Member]
Genco [Member]
Maximum [Member]
Jun. 30, 2012
Commodity Contracts [Member]
Fuel Oils [Member]
Option Model [Member]
Genco [Member]
Weighted Average [Member]
Jun. 30, 2012
Commodity Contracts [Member]
Power [Member]
Dec. 31, 2011
Commodity Contracts [Member]
Power [Member]
Jun. 30, 2012
Commodity Contracts [Member]
Power [Member]
Ameren Missouri [Member]
Dec. 31, 2011
Commodity Contracts [Member]
Power [Member]
Ameren Missouri [Member]
Jun. 30, 2012
Commodity Contracts [Member]
Power [Member]
Ameren Illinois Company [Member]
Dec. 31, 2011
Commodity Contracts [Member]
Power [Member]
Ameren Illinois Company [Member]
Jun. 30, 2012
Commodity Contracts [Member]
Power [Member]
Derivative Liabilities [Member]
Jun. 30, 2012
Commodity Contracts [Member]
Power [Member]
Derivative Liabilities [Member]
Ameren Missouri [Member]
Jun. 30, 2012
Commodity Contracts [Member]
Power [Member]
Derivative Liabilities [Member]
Ameren Illinois Company [Member]
Jun. 30, 2012
Commodity Contracts [Member]
Power [Member]
Discounted Cash Flow [Member]
Minimum [Member]
Jun. 30, 2012
Commodity Contracts [Member]
Power [Member]
Discounted Cash Flow [Member]
Maximum [Member]
Jun. 30, 2012
Commodity Contracts [Member]
Power [Member]
Discounted Cash Flow [Member]
Weighted Average [Member]
Jun. 30, 2012
Commodity Contracts [Member]
Power [Member]
Discounted Cash Flow [Member]
Ameren Missouri [Member]
Jun. 30, 2012
Commodity Contracts [Member]
Power [Member]
Discounted Cash Flow [Member]
Ameren Missouri [Member]
Minimum [Member]
Jun. 30, 2012
Commodity Contracts [Member]
Power [Member]
Discounted Cash Flow [Member]
Ameren Missouri [Member]
Maximum [Member]
Jun. 30, 2012
Commodity Contracts [Member]
Power [Member]
Discounted Cash Flow [Member]
Ameren Missouri [Member]
Weighted Average [Member]
Jun. 30, 2012
Commodity Contracts [Member]
Power [Member]
Discounted Cash Flow [Member]
Ameren Illinois Company [Member]
Jun. 30, 2012
Commodity Contracts [Member]
Power [Member]
Discounted Cash Flow [Member]
Ameren Illinois Company [Member]
Minimum [Member]
Jun. 30, 2012
Commodity Contracts [Member]
Power [Member]
Discounted Cash Flow [Member]
Ameren Illinois Company [Member]
Maximum [Member]
Jun. 30, 2012
Commodity Contracts [Member]
Power [Member]
Discounted Cash Flow [Member]
Ameren Illinois Company [Member]
Weighted Average [Member]
Jun. 30, 2012
Commodity Contracts [Member]
Power [Member]
Option Model [Member]
Minimum [Member]
Jun. 30, 2012
Commodity Contracts [Member]
Power [Member]
Option Model [Member]
Maximum [Member]
Jun. 30, 2012
Commodity Contracts [Member]
Power [Member]
Option Model [Member]
Weighted Average [Member]
Jun. 30, 2012
Commodity Contracts [Member]
Power [Member]
Option Model [Member]
Ameren Missouri [Member]
Minimum [Member]
Jun. 30, 2012
Commodity Contracts [Member]
Power [Member]
Option Model [Member]
Ameren Missouri [Member]
Maximum [Member]
Jun. 30, 2012
Commodity Contracts [Member]
Power [Member]
Option Model [Member]
Ameren Missouri [Member]
Weighted Average [Member]
Jun. 30, 2012
Commodity Contracts [Member]
Power [Member]
Fundamental Energy Production Model [Member]
Minimum [Member]
Jun. 30, 2012
Commodity Contracts [Member]
Power [Member]
Fundamental Energy Production Model [Member]
Maximum [Member]
Jun. 30, 2012
Commodity Contracts [Member]
Power [Member]
Fundamental Energy Production Model [Member]
Weighted Average [Member]
Jun. 30, 2012
Commodity Contracts [Member]
Power [Member]
Fundamental Energy Production Model [Member]
Ameren Illinois Company [Member]
Minimum [Member]
Jun. 30, 2012
Commodity Contracts [Member]
Power [Member]
Fundamental Energy Production Model [Member]
Ameren Illinois Company [Member]
Maximum [Member]
Jun. 30, 2012
Commodity Contracts [Member]
Power [Member]
Fundamental Energy Production Model [Member]
Ameren Illinois Company [Member]
Weighted Average [Member]
Jun. 30, 2012
Commodity Contracts [Member]
Power [Member]
Contract Price Allocation [Member]
Minimum [Member]
Jun. 30, 2012
Commodity Contracts [Member]
Power [Member]
Contract Price Allocation [Member]
Maximum [Member]
Jun. 30, 2012
Commodity Contracts [Member]
Power [Member]
Contract Price Allocation [Member]
Weighted Average [Member]
Jun. 30, 2012
Commodity Contracts [Member]
Power [Member]
Contract Price Allocation [Member]
Ameren Illinois Company [Member]
Minimum [Member]
Jun. 30, 2012
Commodity Contracts [Member]
Power [Member]
Contract Price Allocation [Member]
Ameren Illinois Company [Member]
Maximum [Member]
Jun. 30, 2012
Commodity Contracts [Member]
Power [Member]
Contract Price Allocation [Member]
Ameren Illinois Company [Member]
Weighted Average [Member]
Jun. 30, 2012
Commodity Contracts [Member]
Uranium [Member]
Dec. 31, 2011
Commodity Contracts [Member]
Uranium [Member]
Jun. 30, 2012
Commodity Contracts [Member]
Uranium [Member]
Ameren Missouri [Member]
Dec. 31, 2011
Commodity Contracts [Member]
Uranium [Member]
Ameren Missouri [Member]
Jun. 30, 2012
Commodity Contracts [Member]
Uranium [Member]
Derivative Liabilities [Member]
Jun. 30, 2012
Commodity Contracts [Member]
Uranium [Member]
Derivative Liabilities [Member]
Ameren Missouri [Member]
Jun. 30, 2012
Commodity Contracts [Member]
Uranium [Member]
Discounted Cash Flow [Member]
Minimum [Member]
Jun. 30, 2012
Commodity Contracts [Member]
Uranium [Member]
Discounted Cash Flow [Member]
Maximum [Member]
Jun. 30, 2012
Commodity Contracts [Member]
Uranium [Member]
Discounted Cash Flow [Member]
Weighted Average [Member]
Jun. 30, 2012
Commodity Contracts [Member]
Uranium [Member]
Discounted Cash Flow [Member]
Ameren Missouri [Member]
Minimum [Member]
Jun. 30, 2012
Commodity Contracts [Member]
Uranium [Member]
Discounted Cash Flow [Member]
Ameren Missouri [Member]
Maximum [Member]
Jun. 30, 2012
Commodity Contracts [Member]
Uranium [Member]
Discounted Cash Flow [Member]
Ameren Missouri [Member]
Weighted Average [Member]
Jun. 30, 2012
Commodity Contracts [Member]
Derivative Assets [Member]
Fuel Oils [Member]
Jun. 30, 2012
Commodity Contracts [Member]
Derivative Assets [Member]
Fuel Oils [Member]
Ameren Missouri [Member]
Jun. 30, 2012
Commodity Contracts [Member]
Derivative Assets [Member]
Fuel Oils [Member]
Genco [Member]
Jun. 30, 2012
Commodity Contracts [Member]
Derivative Assets [Member]
Power [Member]
Jun. 30, 2012
Commodity Contracts [Member]
Derivative Assets [Member]
Power [Member]
Ameren Missouri [Member]
Derivatives, Fair Value [Line Items]                                                                                                                                                                                              
Escalation rate                                         0.50% [1],[2],[3] 0.78% [1],[2],[3] 0.72% [1],[2],[3]   0.50% [2],[3] 0.75% [2],[3] 0.65% [2],[3]   0.50% [2],[3] 0.78% [2],[3] 0.71% [2],[3]                                                                                                                                
Nodal basis                                                                                                   6 [1],[2],[3],[4] 0.5 [1],[2],[3],[4] 3 [1],[2],[3],[4]   3 [2],[3],[4] 0.48 [2],[3],[4] 2 [2],[3],[4]   4 [2],[4],[5] 1 [2],[4],[5] 2 [2],[4],[5]                                                                      
Volatalities                                                               23.00% [1],[2],[3] 33.00% [1],[2],[3] 26.00% [1],[2],[3] 23.00% [2],[3] 33.00% [2],[3] 25.00% [2],[3] 23.00% [2],[3] 33.00% [2],[3] 24.00% [2],[3]                                         17.00% [1],[2],[4],[5] 143.00% [1],[2],[4],[5] 34.00% [1],[2],[4],[5] 43.00% [2],[4],[5] 143.00% [2],[4],[5] 76.00% [2],[4],[5]                                                          
Estimated future gas prices                                                                                                                                     4 [1],[2],[3],[4] 6 [1],[2],[3],[4] 5 [1],[2],[3],[4] 4 [2],[3],[4] 6 [2],[3],[4] 5 [2],[3],[4]                                              
Average bid/ask consensus peak and offpeak pricing                                                                                                   18 [1],[2],[4],[5] 51 [1],[2],[4],[5] 34 [1],[2],[4],[5]   21 [2],[4],[5] 46 [2],[4],[5] 25 [2],[4],[5]   19 [2],[3],[4] 45 [2],[3],[4] 26 [2],[3] 21 [1],[2],[4],[5] 44 [1],[2],[4],[5] 36 [1],[2],[4],[5] 24 [2],[4],[5] 32 [2],[4],[5] 27 [2],[4],[5]                                                          
Estimated renewable energy credit costs                                                                                                                                                 5 [1],[2],[3],[4] 7 [1],[2],[3],[4] 6 [1],[2],[3],[4] 5 [2],[3],[4] 7 [2],[3],[4] 6 [2],[3],[4]                                  
Estimated auction price                                                                                                   (672) [1],[2],[3],[4] 7,200 [1],[2],[3],[4] 138 [1],[2],[3],[4]   (149) [2],[3],[4] 1,851 [2],[3],[4] 140 [2],[3],[4]                                                                              
Average bid/ask consensus pricing                                                                                                                                                                         62 [1],[2],[3] 63 [1],[2],[3] 62 [1],[2],[3] 62 [2],[3],[4] 63 [2],[3],[4] 62 [2],[3],[4]          
Counterparty credit risk                                         0.12% [1],[2],[5],[6] 4.00% [1],[2],[5],[6] 2.00% [1],[2],[5],[6]   0.12% [2],[5],[6] 2.00% [2],[5],[6] 2.00% [2],[5],[6] 2.00% [2],[5],[6]                                           0.06% [1],[2],[4],[5],[6] 12.00% [1],[2],[4],[5],[6] 4.00% [1],[2],[4],[5],[6]   0.42% [2],[4],[5],[6] 12.00% [2],[4],[5],[6] 7.00% [2],[4],[5],[6]                                                                              
Credit risk                                         4.00% [1],[2],[5],[6] 23.00% [1],[2],[5],[6] 9.00% [1],[2],[5],[6] 5.00% [2],[5],[6]       23.00% [2],[5],[6]                                           4.00% [1],[2],[4],[5],[6] 5.00% [1],[2],[4],[5],[6] 5.00% [1],[2],[4],[5],[6] 5.00% [2],[4],[5],[6]       5.00% [2],[4],[5],[6]                                                                            
Derivative assets           $ 79 [2] $ 10 [2] $ 13 [2] $ 227 [1],[2] $ 238 [1],[2] $ 55 [2] $ 55 [2] $ 18 [1],[2] $ 37 [1],[2] $ 12 [2] $ 23 [2] $ 5 [2] $ 11 [2]                                             $ 199 [1],[2] $ 195 [1],[2] $ 41 [2] $ 30 [2]   $ 77 [2]                                                                                         $ 6 [1],[2] $ 5 [2] $ 1 [2] $ 182 [1],[2],[4] $ 31 [2],[4]
Derivative liabilities $ 404 [1],[2] $ 281 [1],[2] $ 45 [2] $ 37 [2] $ 365 [2] $ 386 [2] $ 14 [2] $ 3 [2]         $ 11 [1],[2] $ 2 [1],[2] $ 4 [2] $ 1 [2] $ 5 [2] $ 1 [2] $ (2) [1],[2] $ (2) [2]                                         $ 214 [1],[2] $ 80 [1],[2] $ 17 [2] $ 9 [2] $ 221 [2] $ 217 [2] $ (192) [1],[2],[4] $ (5) [2],[4] $ (221) [2],[4]                                                           $ 1 [1],[2] $ 1 [1],[2] $ 1 [2] $ 1 [2] $ (1) [1],[2] $ (1) [2]                      
[1] Includes amounts for Ameren registrant and nonregistrant subsidiaries and intercompany eliminations.
[2] The derivative asset and liability balances are presented net of counterparty credit considerations.
[3] Generally, significant increases (decreases) in this input in isolation would result in a significantly higher (lower) fair value measurement.
[4] Power valuations utilize visible third party pricing evaluated by month for peak and off-peak through 2015. Valuations beyond 2015 utilize fundamentally modeled pricing by month for peak and off-peak.
[5] Generally, significant increases (decreases) in this input in isolation would result in a significantly lower (higher) fair value measurement.
[6] Counterparty credit risk is only applied to derivative asset balances. Ameren, Ameren Missouri, Ameren Illinois, and Genco credit risk is only applied to derivative liability balances.