v3.26.3
Retirement Benefits (Components Of Net Periodic Benefit Cost) (Detail) - USD ($)
$ in Millions
 3 Months Ended 9 Months Ended
Sep. 30, 2026
Sep. 30, 2025
Sep. 30, 2026
Sep. 30, 2025
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]        
Total non-service cost components $ (45) $ (59) $ (143) $ (187)
Ameren Illinois Company        
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]        
Total non-service cost components (12) (21) (41) (61)
Pension Plan        
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]        
Service Cost 22 21 62 62
Interest cost 56 58 169 175
Defined Benefit Plan, Expected Return (Loss) on Plan Assets (63) (76) (197) (227)
Prior service cost (credit) 0 0 0 0
Actuarial (gain) loss 3 (9) 8 (26)
Total non-service cost components (4) (27) (20) (78)
Defined Benefit Plan, Net Periodic Benefit Cost (Credit), Total 18 (6) $ 42 (16)
Defined Benefit Plan, Amortization Period of Actuarial Gain (Loss)     10 years  
Defined Benefit Plan, Difference Between Actual and Expected Return (Loss) on Plan Assets Amortization Period     4 years  
Pension Plan | Ameren Illinois Company        
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]        
Defined Benefit Plan, Net Periodic Benefit Cost (Credit), Total 10 (1) $ 22 (3)
Pension Plan | Other Affiliated Entities And Intercompany Eliminations        
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]        
Defined Benefit Plan, Net Periodic Benefit Cost (Credit), Total 1 (1) 2 (2)
Other Postretirement Benefit Plan, Defined Benefit        
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]        
Service Cost 2 3 7 8
Interest cost 10 11 33 33
Defined Benefit Plan, Expected Return (Loss) on Plan Assets (24) (24) (71) (71)
Prior service cost (credit) (1) (1) (3) (3)
Actuarial (gain) loss (7) (9) (23) (27)
Total non-service cost components (22) (23) (64) (68)
Defined Benefit Plan, Net Periodic Benefit Cost (Credit), Total (20) (20) $ (57) (60)
Defined Benefit Plan, Amortization Period of Actuarial Gain (Loss)     10 years  
Defined Benefit Plan, Difference Between Actual and Expected Return (Loss) on Plan Assets Amortization Period     4 years  
Other Postretirement Benefit Plan, Defined Benefit | Ameren Illinois Company        
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]        
Defined Benefit Plan, Net Periodic Benefit Cost (Credit), Total (14) (13) $ (40) (39)
Other Postretirement Benefit Plan, Defined Benefit | Other Affiliated Entities And Intercompany Eliminations        
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]        
Defined Benefit Plan, Net Periodic Benefit Cost (Credit), Total $ 1 $ 1 $ 1 $ 1