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Related Party Transactions (Schedule of Related Party Transactions) (Detail) - USD ($)
$ in Millions
3 Months Ended 6 Months Ended
Jun. 30, 2017
Jun. 30, 2016
Jun. 30, 2017
Jun. 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 7 4 18 13
Ameren Missouri Power Supply Agreements with Ameren Illinois | Union Electric Company        
Related Party Transaction [Line Items]        
Operating Revenues 6 3 17 12
Ameren Missouri and Ameren Illinois Rent and Facility Services | Union Electric Company        
Related Party Transaction [Line Items]        
Operating Revenues 6 7 13 13
Ameren Missouri and Ameren Illinois Rent and Facility Services | Ameren Illinois Company        
Related Party Transaction [Line Items]        
Operating Revenues 1 1 2 2
Ameren Missouri and Ameren Illinois miscellaneous support services [Member] | Union Electric Company        
Related Party Transaction [Line Items]        
Operating Revenues [1] 1 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] 1 1 [1]
Total Related Party Operating Revenues [Member] | Union Electric Company        
Related Party Transaction [Line Items]        
Operating Revenues 12 10 30 25
Total Related Party Operating Revenues [Member] | Ameren Illinois Company        
Related Party Transaction [Line Items]        
Operating Revenues 2 1 3 2
Ameren Illinois Power Supply Agreements with Ameren Missouri | Ameren Illinois Company        
Related Party Transaction [Line Items]        
Operating Expenses 6 3 17 12
Ameren Illinois Transmission Agreements with ATXI | Ameren Illinois Company        
Related Party Transaction [Line Items]        
Operating Expenses 1 1 1 1
Ameren Services Support Services Agreement | Union Electric Company        
Related Party Transaction [Line Items]        
Operating Expenses 34 32 69 66
Ameren Services Support Services Agreement | Ameren Illinois Company        
Related Party Transaction [Line Items]        
Operating Expenses 34 30 66 61
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.