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Summary Of Significant Accounting Policies (Narrative) (Detail) (USD $)
In Millions, except Per Share data, unless otherwise specified
3 Months Ended 6 Months Ended 12 Months Ended 3 Months Ended 6 Months Ended 3 Months Ended 6 Months Ended 3 Months Ended 6 Months Ended 3 Months Ended 6 Months Ended 3 Months Ended 6 Months Ended 1 Months Ended
Jun. 30, 2012
Jun. 30, 2012
Employee
Jun. 30, 2011
Dec. 31, 2011
Dec. 31, 2011
Common Stock [Member]
Jun. 30, 2012
Emission Allowances [Member]
Jun. 30, 2011
Emission Allowances [Member]
Jun. 30, 2012
Emission Allowances [Member]
Jun. 30, 2011
Emission Allowances [Member]
Jun. 30, 2012
Ameren Missouri [Member]
Jun. 30, 2012
Ameren Missouri [Member]
Emission Allowances [Member]
Jun. 30, 2012
Ameren Missouri [Member]
Emission Allowances [Member]
Jun. 30, 2012
Ameren Illinois Company [Member]
Jun. 30, 2012
Ameren Illinois Company [Member]
Emission Allowances [Member]
Jun. 30, 2011
Ameren Illinois Company [Member]
Emission Allowances [Member]
Jun. 30, 2012
Ameren Illinois Company [Member]
Emission Allowances [Member]
Jun. 30, 2011
Ameren Illinois Company [Member]
Emission Allowances [Member]
Jun. 30, 2012
Genco [Member]
Jun. 30, 2012
Genco [Member]
Jun. 30, 2012
Genco [Member]
Emission Allowances [Member]
Jun. 30, 2011
Genco [Member]
Emission Allowances [Member]
Jun. 30, 2012
Genco [Member]
Emission Allowances [Member]
Jun. 30, 2011
Genco [Member]
Emission Allowances [Member]
Feb. 29, 2012
Medina Valley Energy Center [Member]
Basis Of Presentation And Significant Accounting Policies [Line Items]                                                
Ownership interest                                   80.00% 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 $ 12 $ 12   $ 7           $ 11                            
Amortization expense   41 34     1 1 1 3   1 1   1 1 1 1     1 1 1 2  
Pretax impairment charge 2                 1               1            
Unrecognized tax benefits 158 158               132     11         11 11          
Unrecognized tax benefits that would impact effective tax rate 1 1               1     (1)         1 1          
ARO Incurred   1                                 1          
Percentage of EEI not owned by Ameren 20.00% 20.00%                               20.00% 20.00%          
Proceeds from sales of properties                                               16
Additional contingent proceeds from sale of properties                                               1
Pretax gain recognized on sale                                               10
Reduction in workforce   44                                            
Severance $ 1                                 $ 1