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Related Party Transactions (Schedule of Related Party Transactions) (Detail) - USD ($)
$ in Millions
3 Months Ended 9 Months Ended
Sep. 30, 2017
Sep. 30, 2016
Sep. 30, 2017
Sep. 30, 2016
Related Party Transaction [Line Items]        
Operating Revenues $ 0 $ 0 $ 0 $ 0
Ameren Illinois Company        
Related Party Transaction [Line Items]        
Operating Revenues 0 0 0  
Purchased Power | Ameren Illinois Company        
Related Party Transaction [Line Items]        
Operating Expenses 4 10 22 23
Ameren Missouri Power Supply Agreements with Ameren Illinois | Union Electric Company        
Related Party Transaction [Line Items]        
Operating Revenues 4 9 21 21
Ameren Missouri and Ameren Illinois Rent and Facility Services | Union Electric Company        
Related Party Transaction [Line Items]        
Operating Revenues 7 5 20 18
Ameren Missouri and Ameren Illinois Rent and Facility Services | Ameren Illinois Company        
Related Party Transaction [Line Items]        
Operating Revenues 1 1 3 3
Ameren Missouri and Ameren Illinois miscellaneous support services | Union Electric Company        
Related Party Transaction [Line Items]        
Operating Revenues 1 [1] 1 1 [1] 1
Ameren Missouri and Ameren Illinois miscellaneous support services | Ameren Illinois Company        
Related Party Transaction [Line Items]        
Operating Revenues 1 [1] 1 [1] 1 1 [1]
Total Related Party Operating Revenues [Member] | Union Electric Company        
Related Party Transaction [Line Items]        
Operating Revenues 11 15 41 40
Total Related Party Operating Revenues [Member] | Ameren Illinois Company        
Related Party Transaction [Line Items]        
Operating Revenues 1 1 4 3
Ameren Illinois Power Supply Agreements with Ameren Missouri | Ameren Illinois Company        
Related Party Transaction [Line Items]        
Operating Expenses 4 9 21 21
Ameren Illinois Transmission Agreements with ATXI | Ameren Illinois Company        
Related Party Transaction [Line Items]        
Operating Expenses 1 [1] 1 1 2
Ameren Services Support Services Agreement | Union Electric Company        
Related Party Transaction [Line Items]        
Operating Expenses 34 30 103 96
Ameren Services Support Services Agreement | Ameren Illinois Company        
Related Party Transaction [Line Items]        
Operating Expenses 33 29 99 90
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.