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Supplemental Information (Supplemental Cash Flow Information) (Details) - USD ($)
$ in Millions
 9 Months Ended
Sep. 30, 2025
Sep. 30, 2024
Supplemental Cash Flow Information [Line Items]    
Accrued capital expenditures, including nuclear fuel expenditures $ 412 $ 404
Net realized and unrealized gain – nuclear decommissioning trust fund 136 163
Issuance of common stock for stock-based compensation    
Supplemental Cash Flow Information [Line Items]    
Stock Issued 25 16
Issuance of common stock under the DRPlus    
Supplemental Cash Flow Information [Line Items]    
Stock Issued 7 7
Union Electric Company    
Supplemental Cash Flow Information [Line Items]    
Accrued capital expenditures, including nuclear fuel expenditures 277 257
Net realized and unrealized gain – nuclear decommissioning trust fund 136 163
Union Electric Company | Issuance of common stock for stock-based compensation    
Supplemental Cash Flow Information [Line Items]    
Stock Issued 0 0
Union Electric Company | Issuance of common stock under the DRPlus    
Supplemental Cash Flow Information [Line Items]    
Stock Issued 0 0
Ameren Illinois Company    
Supplemental Cash Flow Information [Line Items]    
Accrued capital expenditures, including nuclear fuel expenditures 147 127
Net realized and unrealized gain – nuclear decommissioning trust fund 0 0
Ameren Illinois Company | Issuance of common stock for stock-based compensation    
Supplemental Cash Flow Information [Line Items]    
Stock Issued 0 0
Ameren Illinois Company | Issuance of common stock under the DRPlus    
Supplemental Cash Flow Information [Line Items]    
Stock Issued $ 0 $ 0