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Stock-Based Compensation Stock-Based Compensation (Summary of Expense) (Details) - Performance Share Units - USD ($)
$ in Millions
12 Months Ended
Dec. 31, 2016
Dec. 31, 2015
Dec. 31, 2014
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Share-based compensation expense $ 17 $ 19 $ 19
Employee service share-based compensation, tax benefit from compensation expense 6 7 7
Share-based Compensation Expense, Net of Tax 11 12 12
Ameren Missouri [Member]      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Share-based compensation expense 4 5 5
Ameren Illinois Company      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Share-based compensation expense 2 3 2
Other      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Share-based compensation expense [1] $ 11 $ 11 $ 12
[1] (a)Represents compensation expense of employees of Ameren Services. These amounts are not included in the Ameren Missouri and Ameren Illinois amounts above.