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Segment Information (Schedule Of Segment Reporting Information By Segment) (Details) - USD ($)
$ in Millions
 3 Months Ended 9 Months Ended
Sep. 30, 2025
Sep. 30, 2024
Sep. 30, 2025
Sep. 30, 2024
Segment Reporting Information [Line Items]        
External revenues $ 2,699 $ 2,173 $ 7,017 $ 5,682
Revenues 2,699 2,173 7,017 5,682
Fuel and purchased power (768) (499) (2,064) (1,154)
Natural gas purchased for resale (25) (30) (233) (214)
Other operations and maintenance expenses (502) (520) (1,447) (1,455)
Depreciation and amortization (415) (388) (1,168) (1,125)
Taxes other than income taxes (164) (150) (439) (416)
Other Income, Net 76 101 257 293
Interest charges (208) (173) (570) (492)
Income taxes (52) (57) (145) (140)
Noncontrolling interests - preferred stock dividends (1) (1) (4) (4)
Net income attributable to Ameren common shareholders 640 456 1,204 975
Interest income 10 8 31 29
Capital expenditures 988 1,137 3,118 3,029
Union Electric Company        
Segment Reporting Information [Line Items]        
External revenues 1,705 1,342 4,002 3,005
Fuel and purchased power (487) (334) (1,431) (689)
Natural gas purchased for resale (6) (4) (45) (41)
Other operations and maintenance expenses (260) (288) (748) (789)
Taxes other than income taxes (122) (109) (300) (287)
Other Income, Net 45 51 135 144
Interest charges (85) (62) (215) (187)
Income taxes (32) 6 (43) 4
Noncontrolling interests - preferred stock dividends (1) (1) (3) (3)
Net income attributable to Ameren common shareholders 518 381 710 534
Ameren Illinois Company        
Segment Reporting Information [Line Items]        
External revenues 946 793 2,889 2,559
Revenues 946 793 2,889 2,559
Purchased Power (284) (169) (642) (474)
Natural gas purchased for resale (19) (26) (188) (173)
Other operations and maintenance expenses (241) (229) (702) (663)
Depreciation and amortization (162) (155) (484) (462)
Taxes other than income taxes (38) (37) (126) (116)
Other Income, Net 30 37 101 105
Interest charges (67) (63) (192) (178)
Income taxes (17) (37) (134) (144)
Noncontrolling interests - preferred stock dividends 0 0 (1) (1)
Net Income Available to Common Shareholder 148 114 521 453
Net income attributable to Ameren common shareholders 148 114 521 453
Interest income 7 7 24 22
Capital expenditures 385 374 1,127 1,091
Operating Segments | Union Electric Company        
Segment Reporting Information [Line Items]        
External revenues 1,696 1,333 3,978 2,980
Revenues 1,705 1,342 4,002 3,005
Fuel and purchased power (487) (334) (1,431) (689)
Natural gas purchased for resale (6) (4) (45) (41)
Other operations and maintenance expenses (260) (288) (748) (789)
Depreciation and amortization (239) (220) (642) (623)
Taxes other than income taxes (122) (109) (300) (287)
Other Income, Net 45 51 135 144
Interest charges (85) (62) (215) (187)
Income taxes (32) 6 (43) 4
Noncontrolling interests - preferred stock dividends (1) (1) (3) (3)
Net Income Available to Common Shareholder 518 381 710 534
Interest income 3 2 7 7
Capital expenditures 548 735 1,873 1,839
Operating Segments | Ameren Illinois Electric Distribution        
Segment Reporting Information [Line Items]        
External revenues 698 551 1,838 1,566
Revenues 699 552 1,844 1,567
Fuel and purchased power (330) (204) (765) (564)
Natural gas purchased for resale 0 0 0 0
Other operations and maintenance expenses (171) (163) (493) (452)
Depreciation and amortization (92) (92) (276) (278)
Taxes other than income taxes (22) (21) (63) (57)
Other Income, Net 20 23 66 72
Interest charges (28) (25) (80) (73)
Income taxes (19) (14) (48) (41)
Noncontrolling interests - preferred stock dividends 0 0 (1) (1)
Net Income Available to Common Shareholder 57 56 184 173
Interest income 6 6 22 21
Capital expenditures 163 152 483 468
Operating Segments | Ameren Illinois Natural Gas        
Segment Reporting Information [Line Items]        
External revenues 117 121 686 660
Revenues 117 121 686 660
Fuel and purchased power 0 0 0 0
Natural gas purchased for resale (19) (26) (188) (173)
Other operations and maintenance expenses (55) (51) (165) (168)
Depreciation and amortization (33) (32) (98) (98)
Taxes other than income taxes (15) (14) (60) (55)
Other Income, Net 4 6 14 20
Interest charges (16) (16) (47) (46)
Income taxes 4 2 (37) (38)
Noncontrolling interests - preferred stock dividends 0 0 0 0
Net Income Available to Common Shareholder (13) (10) 105 102
Interest income 0 1 0 1
Capital expenditures 78 83 225 202
Operating Segments | Ameren Transmission        
Segment Reporting Information [Line Items]        
External revenues 188 168 515 476
Revenues 240 210 658 586
Fuel and purchased power 0 0 0 0
Natural gas purchased for resale 0 0 0 0
Other operations and maintenance expenses (19) (19) (56) (54)
Depreciation and amortization (49) (42) (146) (121)
Taxes other than income taxes (3) (2) (7) (6)
Other Income, Net (3) 8 14 14
Interest charges (31) (31) (89) (89)
Income taxes 16 (34) (48) (89)
Noncontrolling interests - preferred stock dividends 0 0 0 0
Net Income Available to Common Shareholder 151 90 326 241
Interest income 1 0 2 1
Capital expenditures 174 174 505 515
Operating Segments | Ameren Illinois Company | Ameren Illinois Electric Distribution        
Segment Reporting Information [Line Items]        
External revenues 699 552 1,844 1,567
Revenues 699 552 1,844 1,567
Purchased Power (330) (204) (765) (564)
Natural gas purchased for resale 0 0 0 0
Other operations and maintenance expenses (171) (163) (493) (452)
Depreciation and amortization (92) (92) (276) (278)
Taxes other than income taxes (22) (21) (63) (57)
Other Income, Net 20 23 66 72
Interest charges (28) (25) (80) (73)
Income taxes (19) (14) (48) (41)
Noncontrolling interests - preferred stock dividends     (1) (1)
Net Income Available to Common Shareholder 57 56 184 173
Interest income 6 6 22 21
Capital expenditures 163 152 483 468
Operating Segments | Ameren Illinois Company | Ameren Illinois Natural Gas        
Segment Reporting Information [Line Items]        
External revenues 117 121 686 660
Revenues 117 121 686 660
Purchased Power 0 0 0 0
Natural gas purchased for resale (19) (26) (188) (173)
Other operations and maintenance expenses (55) (51) (165) (168)
Depreciation and amortization (33) (32) (98) (98)
Taxes other than income taxes (15) (14) (60) (55)
Other Income, Net 4 6 14 20
Interest charges (16) (16) (47) (46)
Income taxes 4 2 (37) (38)
Noncontrolling interests - preferred stock dividends     0 0
Net Income Available to Common Shareholder (13) (10) 105 102
Interest income 0 1 0 1
Capital expenditures 78 83 225 202
Operating Segments | Ameren Illinois Company | Ameren Illinois Transmission        
Segment Reporting Information [Line Items]        
External revenues 130 120 359 332
Revenues 176 155 482 422
Purchased Power 0 0 0 0
Natural gas purchased for resale 0 0 0 0
Other operations and maintenance expenses (15) (15) (44) (43)
Depreciation and amortization (37) (31) (110) (86)
Taxes other than income taxes (1) (2) (3) (4)
Other Income, Net 6 8 21 13
Interest charges (23) (22) (65) (59)
Income taxes (2) (25) (49) (65)
Noncontrolling interests - preferred stock dividends     0 0
Net Income Available to Common Shareholder 104 68 232 178
Interest income 1 0 2 0
Capital expenditures 144 139 419 421
Other        
Segment Reporting Information [Line Items]        
Other operations and maintenance expenses (10) (12) (26) (29)
Depreciation and amortization (2) (2) (6) (5)
Taxes other than income taxes (2) (4) (9) (11)
Other Income, Net 11 16 31 54
Interest charges (49) (42) (142) (108)
Income taxes (21) (17) 31 24
Noncontrolling interests - preferred stock dividends 0 0 0 0
Net Income Available to Common Shareholder (73) (61) (121) (75)
Interest income 1 2 3 10
Capital expenditures 1 2 6 6
Intersegment Eliminations        
Segment Reporting Information [Line Items]        
Intersegment revenues (62) (52) (173) (136)
Revenues (62) (52) (173) (136)
Fuel and purchased power 49 39 132 99
Natural gas purchased for resale 0 0 0 0
Other operations and maintenance expenses 13 13 41 37
Depreciation and amortization 0 0 0 0
Taxes other than income taxes 0 0 0 0
Other Income, Net (1) (3) (3) (11)
Interest charges 1 3 3 11
Income taxes 0 0 0 0
Noncontrolling interests - preferred stock dividends 0 0 0 0
Interest income (1) (3) (3) (11)
Capital expenditures 24 (9) 26 (1)
Intersegment Eliminations | Union Electric Company        
Segment Reporting Information [Line Items]        
Intersegment revenues (9) (9) (24) (25)
Intersegment Eliminations | Ameren Illinois Electric Distribution        
Segment Reporting Information [Line Items]        
Intersegment revenues (1) (1) (6) (1)
Intersegment Eliminations | Ameren Illinois Natural Gas        
Segment Reporting Information [Line Items]        
Intersegment revenues 0 0 0 0
Intersegment Eliminations | Ameren Transmission        
Segment Reporting Information [Line Items]        
Intersegment revenues (52) (42) (143) (110)
Intersegment Eliminations | Ameren Illinois Company        
Segment Reporting Information [Line Items]        
Intersegment revenues (46) (35) (123) (90)
Revenues (46) (35) (123) (90)
Purchased Power (46) (35) (123) (90)
Intersegment Eliminations | Ameren Illinois Company | Ameren Illinois Electric Distribution        
Segment Reporting Information [Line Items]        
Intersegment revenues 0 0 0 0
Intersegment Eliminations | Ameren Illinois Company | Ameren Illinois Natural Gas        
Segment Reporting Information [Line Items]        
Intersegment revenues 0 0 0 0
Intersegment Eliminations | Ameren Illinois Company | Ameren Illinois Transmission        
Segment Reporting Information [Line Items]        
Intersegment revenues $ (46) $ (35) $ (123) $ (90)