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Related Party Transactions (Schedule of Related Party Transactions) (Detail) - USD ($)
$ in Millions
3 Months Ended
Mar. 31, 2016
Mar. 31, 2015
Union Electric Company    
Related Party Transaction [Line Items]    
Operating Revenues $ 15 $ 7
Ameren Illinois Company    
Related Party Transaction [Line Items]    
Operating Revenues 1 1
Purchased Power | Ameren Illinois Company    
Related Party Transaction [Line Items]    
Operating Expenses 9 2
Ameren Missouri Power Supply Agreements with Ameren Illinois | Union Electric Company    
Related Party Transaction [Line Items]    
Operating Revenues 9 1
Ameren Missouri and Ameren Illinois Rent and Facility Services | Union Electric Company    
Related Party Transaction [Line Items]    
Operating Revenues 6 6
Ameren Missouri and Ameren Illinois Rent and Facility Services | Ameren Illinois Company    
Related Party Transaction [Line Items]    
Operating Revenues 1 1
Ameren Missouri and Ameren Illinois miscellaneous support services [Member] | Union Electric Company    
Related Party Transaction [Line Items]    
Operating Revenues [1] 1 1
Ameren Missouri and Ameren Illinois miscellaneous support services [Member] | Ameren Illinois Company    
Related Party Transaction [Line Items]    
Operating Revenues [1] 1 1
Ameren Illinois Power Supply Agreements with Ameren Missouri | Ameren Illinois Company    
Related Party Transaction [Line Items]    
Operating Expenses 9 1
Ameren Illinois Transmission Agreements with ATXI | Ameren Illinois Company    
Related Party Transaction [Line Items]    
Operating Expenses 1 [1] 1
Ameren Services Support Services Agreement | Union Electric Company    
Related Party Transaction [Line Items]    
Operating Expenses 34 34
Ameren Services Support Services Agreement | Ameren Illinois Company    
Related Party Transaction [Line Items]    
Operating Expenses 31 29
Related Party Money Pool Interest | Union Electric Company    
Related Party Transaction [Line Items]    
Interest Charges [1] 1 1
Related Party Money Pool Interest | Ameren Illinois Company    
Related Party Transaction [Line Items]    
Interest Charges [1] $ 1 $ 1
[1] Amount less than $1 million.