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Summary Of Significant Accounting Policies (Narrative) (Details) (USD $)
In Millions, unless otherwise specified
12 Months Ended 0 Months Ended 12 Months Ended
Dec. 31, 2012
Dec. 31, 2011
Dec. 31, 2010
Dec. 31, 2012
Ameren Illinois Company [Member]
sqmi
people
Dec. 31, 2012
Union Electric Company [Member]
sqmi
people
Dec. 31, 2012
Electric Energy Inc [Member]
Mar. 14, 2013
Ameren Energy Generating Company [Member]
Dec. 31, 2011
Shutdown Of Meredosia And Hutsonville Energy Centers [Member]
Dec. 31, 2012
Voluntary Separation Offer [Member]
Dec. 31, 2011
SO2 Emission Allowances [Member]
Dec. 31, 2012
Minimum [Member]
Dec. 31, 2011
Minimum [Member]
Dec. 31, 2010
Minimum [Member]
Dec. 31, 2012
Maximum [Member]
Dec. 31, 2011
Maximum [Member]
Dec. 31, 2010
Maximum [Member]
Dec. 31, 2012
Power [Member]
Ameren Illinois Company [Member]
customer
Dec. 31, 2012
Power [Member]
Union Electric Company [Member]
customer
Dec. 31, 2012
Natural Gas [Member]
Ameren Illinois Company [Member]
customer
Dec. 31, 2012
Natural Gas [Member]
Union Electric Company [Member]
customer
Dec. 31, 2012
FAC [Member]
Union Electric Company [Member]
Accounting Policies [Line Items]                                          
Public utilities, Area serviced       40,000 24,000                                
Public utilities, Estimated population of service territory       3,100,000 2,800,000                                
Public uitilities, Number of customers                                 1,200,000 1,200,000 806,000 127,000  
Ownership percentage by parent           80.00%                              
Initial payment received    $ 3 $ 7       $ 100                            
Percent of average depreciable cost                     3.00% 3.00% 3.00% 4.00% 4.00% 4.00%          
Allowance for funds used during construction, rate                     8.00% 8.00% 8.00% 9.00% 9.00% 9.00%          
Book value 14 7                                      
Amortization expense 4 3 13                                    
Pretax impairment charge                   1                      
Sharing level for FAC                                         95.00%
Excise tax expense 193 194 189                                    
Tax grants received related to renewable energy properties 18         18                                
Percentage of EEI not owned by Ameren           20.00%                              
Number of employee positions eliminated                 340                        
Severance costs               $ 4 $ 28