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Retirement Benefits (Narrative) (Details)
$ in Millions
 12 Months Ended
Dec. 31, 2026
Dec. 31, 2025
USD ($)
yr
employee
Dec. 31, 2024
USD ($)
Dec. 31, 2023
Defined Benefit Plan Disclosure [Line Items]        
Defined Benefit Plan, Funded (Unfunded) Status of Plan   $ 954 $ 734  
Defined Benefit Plan Assumptions Used Calculating Benefit Obligation, Change in Discount Rate   0.05%    
Defined benefit plan estimated future employer contributions over next five years   $ 240    
Collateralized loan percentage compared to asset's market value   103.00%    
Securities Loaned   $ 517 454  
Minimum        
Defined Benefit Plan Disclosure [Line Items]        
Defined benefit plan estimated future employer contributions over next five years   45    
Maximum        
Defined Benefit Plan Disclosure [Line Items]        
Defined benefit plan estimated future employer contributions over next five years   50    
Pension Benefits        
Defined Benefit Plan Disclosure [Line Items]        
Defined Benefit Plan, Funded (Unfunded) Status of Plan   $ 163 $ 48  
Expected return on plan assets   6.75% 6.75% 6.75%
Defined Benefit Plan, Plan Assets, Increase (Decrease) for Assets Transferred to (from) Plan   $ 240 $ 0  
Plan amendments   $ 240 $ 0  
Defined Benefit Plan Number Of Covered Employees By Plan | employee   4,200    
Defined Benefit Plan Plan Assets Pre Tax Gain   $ 15    
Defined Benefit Plan Plan Assets Pre Tax Gain Amortization Period | yr   10    
Pension Benefits | Forecast        
Defined Benefit Plan Disclosure [Line Items]        
Expected return on plan assets 6.75%      
Ameren Missouri | Pension Benefits        
Defined Benefit Plan Disclosure [Line Items]        
Future funding requirement, percentage   35.00%    
Ameren Illinois | Pension Benefits        
Defined Benefit Plan Disclosure [Line Items]        
Future funding requirement, percentage   45.00%