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Rate And Regulatory Matters (Narrative-Federal) (Detail) (USD $)
In Millions, unless otherwise specified
3 Months Ended 6 Months Ended 3 Months Ended 6 Months Ended 3 Months Ended 6 Months Ended 1 Months Ended 6 Months Ended
Jun. 30, 2012
Jun. 30, 2011
Jun. 30, 2012
Jun. 30, 2011
Dec. 31, 2011
Jun. 30, 2012
Ameren Illinois Company [Member]
Jun. 30, 2011
Ameren Illinois Company [Member]
Jun. 30, 2012
Ameren Illinois Company [Member]
Jun. 30, 2011
Ameren Illinois Company [Member]
Dec. 31, 2011
Ameren Illinois Company [Member]
Jun. 30, 2012
Ameren Missouri [Member]
Jun. 30, 2011
Ameren Missouri [Member]
Jun. 30, 2012
Ameren Missouri [Member]
Year
Jun. 30, 2011
Ameren Missouri [Member]
Jun. 30, 2012
Ameren Missouri [Member]
Entergy Refund [Member]
Jan. 31, 2011
Wholesale Distribution Rate Case [Member]
Ameren Illinois Company [Member]
Customer
Mar. 31, 2012
New Nuclear Energy Center [Member]
Ameren Missouri [Member]
Jun. 30, 2012
New Nuclear Energy Center [Member]
Ameren Missouri [Member]
Jun. 30, 2012
New Nuclear Energy Center [Member]
Ameren Missouri [Member]
Minimum [Member]
Jun. 30, 2012
New Nuclear Energy Center [Member]
Ameren Missouri [Member]
Maximum [Member]
Jun. 30, 2012
FERC Transmission Rate Refund [Member]
Ameren Illinois Company [Member]
Minimum [Member]
Jun. 30, 2012
FERC Transmission Rate Refund [Member]
Ameren Illinois Company [Member]
Maximum [Member]
Rate And Regulatory Matters [Line Items]                                            
Proceeds from Legal Settlements     $ 1                       $ 31              
Purchased power 133 237 296 464   162 196 352 407           26              
Miscellaneous income 20 [1] 17 [1] 37 [1] 33 [1]   2 1 3 3   18 16 33 29 5              
Reduction to under-recovered asset                             2              
Pretax charge on earnings                                         (10) (15)
Utility revenue increase requested                               11            
Number of Customers, Reached Agreements                               4            
Number of Wholesale Customers                               9            
Number of Customers, Remaining on Agreements                               5            
Availability of investment funds                                 452          
Number of years an COL is valid for                         40                  
Capitalized costs relating to construction of new nuclear unit $ 17,690   $ 17,690   $ 18,127 $ 4,869   $ 4,869   $ 4,770               $ 69 $ 80 $ 100    
[1] Includes amounts for Ameren registrant and nonregistrant subsidiaries and intercompany eliminations.