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Segment Information (Schedule Of Segment Reporting Information By Segment) (Details)
$ in Millions
 12 Months Ended
Dec. 31, 2025
USD ($)
segment
Dec. 31, 2024
USD ($)
Dec. 31, 2023
USD ($)
Segment Reporting Information [Line Items]      
Number of reportable segments | segment 4    
External revenues $ 8,799 $ 7,623 $ 7,500
Revenues 8,799 7,623 7,500
Fuel and purchased power (2,306) (1,681) (1,812)
Natural gas purchased for resale (348) (320) (355)
Other operations and maintenance (1,974) (1,969) (1,866)
Depreciation and amortization (1,568) (1,590) (1,387)
Taxes other than income taxes (577) (547) (522)
Other Income, Net 347 417 348
Interest Charges (776) (663) (566)
Income tax benefit (136) (83) (183)
Preferred Stock Dividends (5) (5) (5)
Net Income (Loss) Available to Common Stockholders, Basic 1,456 1,182 1,152
Interest income 41 41 33
Capital expenditures $ 4,128 4,319 3,597
Ameren Missouri      
Segment Reporting Information [Line Items]      
Number of reportable segments | segment 1    
Ameren Illinois      
Segment Reporting Information [Line Items]      
Number of reportable segments | segment 3    
External revenues $ 3,844 3,472 3,482
Revenues 3,844 3,472 3,482
Purchased power 781 621 820
Natural gas purchased for resale (283) (260) (276)
Other operations and maintenance (945) (906) (818)
Depreciation and amortization (652) (619) (556)
Taxes other than income taxes (169) (157) (146)
Other Income, Net 136 147 156
Interest Charges (260) (241) (204)
Income tax benefit (153) (193) (209)
Preferred Stock Dividends (2) (2) (2)
Net Income (Loss) Available to Common Stockholders, Basic 735 620 607
Interest income 32 32 21
Capital expenditures 1,481 1,467 1,731
Operating Segments | Ameren Missouri      
Segment Reporting Information [Line Items]      
External revenues 4,763 3,960 3,825
Revenues 4,795 3,993 3,859
Fuel and purchased power (1,538) (1,071) (997)
Natural gas purchased for resale (65) (60) (79)
Other operations and maintenance (1,029) (1,050) (1,003)
Depreciation and amortization (860) (917) (783)
Taxes other than income taxes (393) (372) (360)
Other Income, Net 180 196 130
Interest Charges (297) (244) (227)
Income tax benefit (43) 87 8
Preferred Stock Dividends (3) (3) (3)
Net Income (Loss) Available to Common Stockholders, Basic 747 559 545
Interest income 9 8 11
Capital expenditures 2,502 2,712 1,760
Operating Segments | Ameren Illinois Electric Distribution      
Segment Reporting Information [Line Items]      
External revenues 2,393 2,088 2,217
Revenues 2,399 2,089 2,218
Fuel and purchased power (941) (740) (933)
Natural gas purchased for resale 0 0 0
Other operations and maintenance (656) (619) (532)
Depreciation and amortization (373) (369) (351)
Taxes other than income taxes (82) (75) (75)
Other Income, Net 89 97 103
Interest Charges (107) (98) (89)
Income tax benefit (47) (50) (82)
Preferred Stock Dividends (1) (1) (1)
Net Income (Loss) Available to Common Stockholders, Basic 281 234 258
Interest income 30 28 19
Capital expenditures 635 579 752
Operating Segments | Ameren Illinois Gas      
Segment Reporting Information [Line Items]      
External revenues 968 938 897
Revenues 968 938 897
Fuel and purchased power 0 0 0
Natural gas purchased for resale (283) (260) (276)
Other operations and maintenance (233) (230) (237)
Depreciation and amortization (128) (129) (108)
Taxes other than income taxes (82) (78) (67)
Other Income, Net 19 27 30
Interest Charges (65) (63) (55)
Income tax benefit (38) (56) (50)
Preferred Stock Dividends 0 0 0
Net Income (Loss) Available to Common Stockholders, Basic 158 149 134
Interest income 0 1 1
Capital expenditures 283 264 299
Operating Segments | Ameren Transmission      
Segment Reporting Information [Line Items]      
External revenues 675 637 561
Revenues 862 781 677
Fuel and purchased power 0 0 0
Natural gas purchased for resale 0 0 0
Other operations and maintenance (74) (70) (60)
Depreciation and amortization (199) (167) (138)
Taxes other than income taxes (9) (9) (8)
Other Income, Net 24 26 28
Interest Charges (120) (117) (96)
Income tax benefit (68) (120) (106)
Preferred Stock Dividends (1) (1) (1)
Net Income (Loss) Available to Common Stockholders, Basic 415 323 296
Interest income 3 6 2
Capital expenditures 717 758 804
Operating Segments | Ameren Illinois | Ameren Illinois Electric Distribution      
Segment Reporting Information [Line Items]      
External revenues 2,399 2,089 2,218
Revenues 2,399 2,089 2,218
Purchased power 941 740 933
Other operations and maintenance (656) (619) (532)
Depreciation and amortization (373) (369) (351)
Taxes other than income taxes (82) (75) (75)
Other Income, Net 89 97 103
Interest Charges (107) (98) (89)
Income tax benefit (47) (50) (82)
Preferred Stock Dividends (1) (1) (1)
Net Income (Loss) Available to Common Stockholders, Basic 281 234 258
Interest income 30 28 19
Capital expenditures 635 579 752
Operating Segments | Ameren Illinois | Ameren Illinois Gas      
Segment Reporting Information [Line Items]      
External revenues 968 938 897
Revenues 968 938 897
Natural gas purchased for resale (283) (260) (276)
Other operations and maintenance (233) (230) (237)
Depreciation and amortization (128) (129) (108)
Taxes other than income taxes (82) (78) (67)
Other Income, Net 19 27 30
Interest Charges (65) (63) (55)
Income tax benefit (38) (56) (50)
Net Income (Loss) Available to Common Stockholders, Basic 158 149 134
Interest income 0 1 1
Capital expenditures 283 264 299
Operating Segments | Ameren Illinois | Ameren Illinois Transmission      
Segment Reporting Information [Line Items]      
External revenues 477 445 367
Revenues 637 564 480
Other operations and maintenance (56) (57) (49)
Depreciation and amortization (151) (121) (97)
Taxes other than income taxes (5) (4) (4)
Other Income, Net 28 23 23
Interest Charges (88) (80) (60)
Income tax benefit (68) (87) (77)
Preferred Stock Dividends (1) (1) (1)
Net Income (Loss) Available to Common Stockholders, Basic 296 237 215
Interest income 2 3 1
Capital expenditures 563 624 680
Segment Reconciling Items      
Segment Reporting Information [Line Items]      
Other operations and maintenance (34) (48) (67)
Depreciation and amortization (8) (8) (7)
Taxes other than income taxes (11) (13) (12)
Other Income, Net 42 83 62
Interest Charges (194) (153) (104)
Income tax benefit 60 56 47
Net Income (Loss) Available to Common Stockholders, Basic (145) (83) (81)
Interest income 6 10 5
Capital expenditures 8 7 9
Intersegment Elimination      
Segment Reporting Information [Line Items]      
Intersegment revenues 225 178 151
Revenues (225) (178) (151)
Fuel and purchased power 173 130 118
Natural gas purchased for resale 0 0 0
Other operations and maintenance 52 48 33
Depreciation and amortization 0 0 0
Taxes other than income taxes 0 0 0
Other Income, Net (7) (12) (5)
Interest Charges 7 12 5
Income tax benefit 0 0 0
Preferred Stock Dividends 0 0 0
Interest income (7) (12) (5)
Capital expenditures (17) (1) (27)
Intersegment Elimination | Ameren Missouri      
Segment Reporting Information [Line Items]      
Intersegment revenues 32 33 34
Intersegment Elimination | Ameren Illinois Electric Distribution      
Segment Reporting Information [Line Items]      
Intersegment revenues 6 1 1
Intersegment Elimination | Ameren Illinois Gas      
Segment Reporting Information [Line Items]      
Intersegment revenues 0 0 0
Intersegment Elimination | Ameren Transmission      
Segment Reporting Information [Line Items]      
Intersegment revenues 187 144 116
Intersegment Elimination | Ameren Illinois      
Segment Reporting Information [Line Items]      
Intersegment revenues 160 119 113
Revenues (160) (119) (113)
Purchased power 160 119 113
Intersegment Elimination | Ameren Illinois | Ameren Illinois Electric Distribution      
Segment Reporting Information [Line Items]      
Intersegment revenues 0 0 0
Intersegment Elimination | Ameren Illinois | Ameren Illinois Gas      
Segment Reporting Information [Line Items]      
Intersegment revenues 0 0 0
Intersegment Elimination | Ameren Illinois | Ameren Illinois Transmission      
Segment Reporting Information [Line Items]      
Intersegment revenues $ 160 $ 119 $ 113