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Related Party Transactions (Schedule of Related Party Transactions) (Detail) - USD ($)
$ in Millions
3 Months Ended
Mar. 31, 2018
Mar. 31, 2017
Related Party Transaction [Line Items]    
Operating Revenues $ 0 $ 0
Ameren Illinois Company    
Related Party Transaction [Line Items]    
Operating Revenues 0 0
Purchased Power | Ameren Illinois Company    
Related Party Transaction [Line Items]    
Operating Expenses 3 11
Ameren Missouri Power Supply Agreements with Ameren Illinois | Union Electric Company    
Related Party Transaction [Line Items]    
Operating Revenues 3 11
Ameren Missouri and Ameren Illinois Rent and Facility Services | Union Electric Company    
Related Party Transaction [Line Items]    
Operating Revenues 5 7
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 | Union Electric Company    
Related Party Transaction [Line Items]    
Operating Revenues [1] 1 1
Ameren Missouri and Ameren Illinois miscellaneous support services | Ameren Illinois Company    
Related Party Transaction [Line Items]    
Operating Revenues [1] 1 1
Total Related Party Operating Revenues [Member] | Union Electric Company    
Related Party Transaction [Line Items]    
Operating Revenues 8 18
Total Related Party Operating Revenues [Member] | Ameren Illinois Company    
Related Party Transaction [Line Items]    
Operating Revenues 1 1
Ameren Illinois Power Supply Agreements with Ameren Missouri | Ameren Illinois Company    
Related Party Transaction [Line Items]    
Operating Expenses 3 11
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 33 35
Ameren Services Support Services Agreement | Ameren Illinois Company    
Related Party Transaction [Line Items]    
Operating Expenses 30 32
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.