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Summary Of Significant Accounting Policies (Narrative) (Detail) (USD $)
In Millions, except Per Share data, unless otherwise specified
1 Months Ended 3 Months Ended 9 Months Ended 1 Months Ended 3 Months Ended 12 Months Ended 1 Months Ended 3 Months Ended 9 Months Ended 1 Months Ended 3 Months Ended 3 Months Ended 9 Months Ended 12 Months Ended
Jun. 30, 2012
Employees
Jun. 30, 2011
Sep. 30, 2012
Sep. 30, 2011
Dec. 31, 2011
Sep. 30, 2012
EEI [Member]
Feb. 29, 2012
Medina Valley Energy Center [Member]
Sep. 30, 2012
AERG [Member]
Sep. 30, 2012
AERG [Member]
Land Sales [Member]
Dec. 31, 2011
Common Stock [Member]
May 31, 2012
Genco [Member]
Jun. 30, 2011
Genco [Member]
Sep. 30, 2012
Genco [Member]
May 31, 2012
Genco [Member]
Building Sale [Member]
Jun. 30, 2011
Ameren Missouri [Member]
Sep. 30, 2012
Ameren Missouri [Member]
Sep. 30, 2012
Ameren Illinois Company [Member]
Sep. 30, 2012
Maximum [Member]
Genco [Member]
Sep. 30, 2011
Maximum [Member]
Genco [Member]
Sep. 30, 2012
Maximum [Member]
Genco [Member]
Aug. 31, 2012
Maximum [Member]
Ameren Missouri [Member]
Basis Of Presentation And Significant Accounting Policies [Line Items]                                          
Ownership interest           80.00%                              
Fair value of each share unit, per share                   $ 35.68                      
Closing common share price         $ 33.13                                
Three-year risk-free rate     0.41%                                    
Volatility rate, minimum     17.00%                                    
Volatility rate, maximum     31.00%                                    
Book value     $ 14   $ 7                     $ 12          
Amortization expense     63 51                           1 1 1 1
Pretax impairment charge   2                   1     1            
Unrecognized tax benefits     167                   12     143 11        
Unrecognized tax benefits that would impact effective tax rate     1                   1     4 (1)        
Percentage of EEI not owned by Ameren     20.00%                   20.00%                
Proceeds from sales of properties     22 50     16   4         1              
Additional contingent proceeds from sale of properties             1                            
Pretax gain (loss) recognized on sale     11 12     10 2     (1)                    
Reduction in workforce 44                                        
Severance     $ 1                   $ 1