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Related Party Transactions (Schedule of Related Party Transactions) (Detail) - USD ($)
$ in Millions
3 Months Ended 9 Months Ended
Sep. 30, 2015
Sep. 30, 2014
Sep. 30, 2015
Sep. 30, 2014
Union Electric Company        
Related Party Transaction [Line Items]        
Operating Revenues $ 11 $ 8 $ 30 $ 21
Ameren Illinois Company        
Related Party Transaction [Line Items]        
Operating Revenues 1 1 [1] 3 1
Purchased Power | Ameren Illinois Company        
Related Party Transaction [Line Items]        
Operating Expenses 5 3 11 7
Ameren Missouri Power Supply Agreements with Ameren Illinois | Union Electric Company        
Related Party Transaction [Line Items]        
Operating Revenues 4 2 9 5
Ameren Missouri and Ameren Illinois Rent and Facility Services | Union Electric Company        
Related Party Transaction [Line Items]        
Operating Revenues 6 6 19 15
Ameren Missouri and Ameren Illinois Rent and Facility Services | Ameren Illinois Company        
Related Party Transaction [Line Items]        
Operating Revenues 1 1 [1] 3 1
Ameren Missouri and Ameren Illinois miscellaneous support services [Member] | Union Electric Company        
Related Party Transaction [Line Items]        
Operating Revenues 1 1 [1] 2 1
Ameren Missouri and Ameren Illinois miscellaneous support services [Member] | Ameren Illinois Company        
Related Party Transaction [Line Items]        
Operating Revenues [1] 1 1 1 1
Ameren Illinois Power Supply Agreements with Ameren Missouri | Ameren Illinois Company        
Related Party Transaction [Line Items]        
Operating Expenses 4 2 9 5
Ameren Illinois Transmission Agreements with ATXI | Ameren Illinois Company        
Related Party Transaction [Line Items]        
Operating Expenses 1 1 2 2
Ameren Services Support Services Agreement | Union Electric Company        
Related Party Transaction [Line Items]        
Operating Expenses 30 25 96 90
Ameren Services Support Services Agreement | Ameren Illinois Company        
Related Party Transaction [Line Items]        
Operating Expenses 28 26 87 80
Related Party Money Pool Interest | Union Electric Company        
Related Party Transaction [Line Items]        
Interest Charges [1] 1 1 1 1
Related Party Money Pool Interest | Ameren Illinois Company        
Related Party Transaction [Line Items]        
Interest Charges [1] $ 1 $ 1 $ 1 $ 1
[1] Amount less than $1 million.