v3.26.3
Related Party Transactions (Effects of Related-party Transactions on the Statement of Income) (Details) - USD ($)
$ in Millions
3 Months Ended 9 Months Ended
Sep. 30, 2026
Sep. 30, 2025
Sep. 30, 2026
Sep. 30, 2025
Related Party Transaction [Line Items]        
Operating Revenues $ 2,882 $ 2,699 $ 7,150 $ 7,017
Other operations and maintenance 535 502 1,547 1,447
Interest Charges 216 208 629 570
Union Electric Company        
Related Party Transaction [Line Items]        
Operating Revenues 1,781 1,705 3,835 4,002
Other operations and maintenance 274 260 820 748
Interest Charges 82 85 247 215
Union Electric Company | Ameren Missouri and Ameren Illinois Rent and Facility Services | Related Party        
Related Party Transaction [Line Items]        
Operating Revenues 11 8 32 23
Other operations and maintenance 2 1 2 1
Union Electric Company | Ameren Missouri and Ameren Illinois Miscellaneous Support Services | Related Party        
Related Party Transaction [Line Items]        
Operating Revenues 1 1 2 1
Union Electric Company | Total Related Party Operating Revenues | Related Party        
Related Party Transaction [Line Items]        
Operating Revenues 11 9 34 24
Union Electric Company | Ameren Missouri and Ameren Illinois Transmission Services from ATXI | Related Party        
Related Party Transaction [Line Items]        
Purchased Power 3 2 8 7
Union Electric Company | Ameren Missouri Interconnection Agreement With Ameren Illinois | Related Party        
Related Party Transaction [Line Items]        
Purchased Power 1 1 1 1
Union Electric Company | Purchased Power | Related Party        
Related Party Transaction [Line Items]        
Purchased Power 4 2 9 7
Union Electric Company | Ameren Services Support Services Agreement | Related Party        
Related Party Transaction [Line Items]        
Other operations and maintenance 46 42 142 130
Union Electric Company | Total Related Party Other Operations and Maintenance | Related Party        
Related Party Transaction [Line Items]        
Other operations and maintenance 48 42 144 130
Union Electric Company | Money pool interest (advances) | Related Party        
Related Party Transaction [Line Items]        
Interest Charges 1 1 1 1
Union Electric Company | Long-Term Debt | Related Party        
Related Party Transaction [Line Items]        
Interest Charges 1 1 3 2
Ameren Illinois Company        
Related Party Transaction [Line Items]        
Operating Revenues 1,053 946 3,180 2,889
Purchased Power 304 284 719 642
Other operations and maintenance 265 241 745 702
Interest Charges 72 67 209 192
Ameren Illinois Company | Ameren Missouri and Ameren Illinois Rent and Facility Services | Related Party        
Related Party Transaction [Line Items]        
Operating Revenues 1 1 1 1
Other operations and maintenance 2 1 2 3
Ameren Illinois Company | Ameren Illinois Interconnection Agreement With Ameren Missouri | Related Party        
Related Party Transaction [Line Items]        
Operating Revenues 1 1 1 1
Ameren Illinois Company | Ameren Missouri and Ameren Illinois Miscellaneous Support Services | Related Party        
Related Party Transaction [Line Items]        
Operating Revenues 2 1 3 5
Ameren Illinois Company | Total Related Party Operating Revenues | Related Party        
Related Party Transaction [Line Items]        
Operating Revenues 3 1 4 6
Ameren Illinois Company | Ameren Missouri and Ameren Illinois Transmission Services from ATXI | Related Party        
Related Party Transaction [Line Items]        
Purchased Power 1 1 2 2
Ameren Illinois Company | Purchased Power | Related Party        
Related Party Transaction [Line Items]        
Purchased Power 1 1 2 2
Ameren Illinois Company | Ameren Services Support Services Agreement | Related Party        
Related Party Transaction [Line Items]        
Other operations and maintenance 43 39 133 119
Ameren Illinois Company | Total Related Party Other Operations and Maintenance | Related Party        
Related Party Transaction [Line Items]        
Other operations and maintenance 45 39 135 122
Ameren Illinois Company | Money pool interest (advances) | Related Party        
Related Party Transaction [Line Items]        
Interest Charges 1 1 1 1
Ameren Illinois Company | Long-Term Debt | Related Party        
Related Party Transaction [Line Items]        
Interest Charges $ 1 $ 1 $ 1 $ 1