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Short-Term Debt And Liquidity (Narrative) (Details) (USD $)
In Millions, unless otherwise specified
 12 Months Ended 12 Months Ended 12 Months Ended 12 Months Ended
Dec. 31, 2011
Ameren Revolving Credit Facility [Member]
Dec. 31, 2010
Ameren Revolving Credit Facility [Member]
Jun. 02, 2010
Ameren Revolving Credit Facility [Member]
Dec. 31, 2011
Multiyear Credit Facility [Member]
Dec. 31, 2011
2010 Missouri Credit Agreement [Member]
Dec. 31, 2010
2010 Missouri Credit Agreement [Member]
Sep. 10, 2010
2010 Missouri Credit Agreement [Member]
Dec. 31, 2011
2010 Genco Credit Agreement [Member]
Dec. 31, 2010
2010 Genco Credit Agreement [Member]
Sep. 10, 2010
2010 Genco Credit Agreement [Member]
Dec. 31, 2011
2010 Illinois Credit Agreement [Member]
Sep. 10, 2010
2010 Illinois Credit Agreement [Member]
Dec. 31, 2011
Commercial Paper [Member]
Dec. 31, 2010
Commercial Paper [Member]
Dec. 31, 2011
2010 Credit Agreements [Member]
Line of Credit Facility [Line Items]                              
Commercial paper maximum issuance                         $ 500    
Line of credit facility, maximum borrowing capacity     20 2,100 500   800 500   500 300 800     1,900
Number of lenders       25                      
Line of credit facility, maximum borrowing capacity, per lender       125                      
Commercial paper outstanding                         148 269  
Reductions of borrowing capacity of letters of credit issued       15                      
Average daily commercial paper borrowings outstanding                         311 185  
Average daily borrowings outstanding 20 20     105 195   41 90            
Weighted average interest rate 2.48% 2.54%                     0.87% 0.94%  
Optional increase to facility size, maximum amount         1,000     625     1,000        
Letters of credit portion of aggregate commitment       25.00%                      
Debt instrument, interest rate, effective percentage     2.25%                        
Peak short-term borrowings                         435 366  
Peak short-term borrowings interest rate                         1.46%    
Maximum consolidated indebtedness as a percent of total capitalization       65.00%                      
Allowable debt default amount       $ 25                      
Actual debt-to-capital ratio       47                      
Minimum ratio of consolidated funds from operations plus interest expense to consolidated interest expense as of balance sheet date       2.0to 1.0                      
Current ratio of consolidated funds from operations plus interest expense to consolidated interest expense as of balance sheet date       5.1 to 1.0