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Segment Information (Disaggregation of Revenue) (Details) - USD ($)
$ in Millions
 3 Months Ended 9 Months Ended
Sep. 30, 2018
Sep. 30, 2017
Sep. 30, 2018
Sep. 30, 2017
Disaggregation of Revenue [Line Items]        
Revenue from Contract with Customer, Including Assessed Tax $ 1,724 $ 1,723 $ 4,872 $ 4,775
Revenues from alternative revenue programs (111) (105) (91) (51)
Other revenues not from contracts with customers 5 6 38 18
TCJA - Excess Amounts Collected in Rates        
Disaggregation of Revenue [Line Items]        
TCJA Revenue Reduction 13   60  
Intersegment Eliminations        
Disaggregation of Revenue [Line Items]        
Revenue from Contract with Customer, Including Assessed Tax (26) (26) (72) (78)
Electricity        
Disaggregation of Revenue [Line Items]        
Revenue from Contract with Customer, Including Assessed Tax 1,590 1,594 4,209 4,183
Electricity | Residential        
Disaggregation of Revenue [Line Items]        
Revenue from Contract with Customer, Including Assessed Tax 731 715 1,935 1,786
Electricity | Commercial        
Disaggregation of Revenue [Line Items]        
Revenue from Contract with Customer, Including Assessed Tax 548 542 1,414 1,366
Electricity | Industrial        
Disaggregation of Revenue [Line Items]        
Revenue from Contract with Customer, Including Assessed Tax 129 127 345 325
Electricity | Other        
Disaggregation of Revenue [Line Items]        
Revenue from Contract with Customer, Including Assessed Tax 182 210 515 706
Electricity | Intersegment Eliminations        
Disaggregation of Revenue [Line Items]        
Revenue from Contract with Customer, Including Assessed Tax (26) (26) (72) (77)
Electricity | Intersegment Eliminations | Residential        
Disaggregation of Revenue [Line Items]        
Revenue from Contract with Customer, Including Assessed Tax 0 0 0 0
Electricity | Intersegment Eliminations | Commercial        
Disaggregation of Revenue [Line Items]        
Revenue from Contract with Customer, Including Assessed Tax 0 0 0 0
Electricity | Intersegment Eliminations | Industrial        
Disaggregation of Revenue [Line Items]        
Revenue from Contract with Customer, Including Assessed Tax 0 0 0 0
Electricity | Intersegment Eliminations | Other        
Disaggregation of Revenue [Line Items]        
Revenue from Contract with Customer, Including Assessed Tax (26) (26) (72) (77)
Natural Gas        
Disaggregation of Revenue [Line Items]        
Revenue from Contract with Customer, Including Assessed Tax 134 129 663 592
Natural Gas | Residential        
Disaggregation of Revenue [Line Items]        
Revenue from Contract with Customer, Including Assessed Tax 76 81 470 408
Natural Gas | Commercial        
Disaggregation of Revenue [Line Items]        
Revenue from Contract with Customer, Including Assessed Tax 23 25 138 120
Natural Gas | Industrial        
Disaggregation of Revenue [Line Items]        
Revenue from Contract with Customer, Including Assessed Tax 2 3 15 10
Natural Gas | Other        
Disaggregation of Revenue [Line Items]        
Revenue from Contract with Customer, Including Assessed Tax 33 20 40 54
Natural Gas | Intersegment Eliminations        
Disaggregation of Revenue [Line Items]        
Revenue from Contract with Customer, Including Assessed Tax 0 0 0 (1)
Natural Gas | Intersegment Eliminations | Residential        
Disaggregation of Revenue [Line Items]        
Revenue from Contract with Customer, Including Assessed Tax 0 0 0 0
Natural Gas | Intersegment Eliminations | Commercial        
Disaggregation of Revenue [Line Items]        
Revenue from Contract with Customer, Including Assessed Tax 0 0 0 0
Natural Gas | Intersegment Eliminations | Industrial        
Disaggregation of Revenue [Line Items]        
Revenue from Contract with Customer, Including Assessed Tax 0 0 0 0
Natural Gas | Intersegment Eliminations | Other        
Disaggregation of Revenue [Line Items]        
Revenue from Contract with Customer, Including Assessed Tax 0 0 0 (1)
Ameren Illinois Company        
Disaggregation of Revenue [Line Items]        
Revenue from Contract with Customer, Including Assessed Tax 564 574 1,902 1,853
Ameren Illinois Company | Intersegment Eliminations        
Disaggregation of Revenue [Line Items]        
Revenue from Contract with Customer, Including Assessed Tax (15) (14) (41) (32)
Ameren Illinois Company | Intersegment Eliminations | Residential        
Disaggregation of Revenue [Line Items]        
Revenue from Contract with Customer, Including Assessed Tax 0 0 0 0
Ameren Illinois Company | Intersegment Eliminations | Commercial        
Disaggregation of Revenue [Line Items]        
Revenue from Contract with Customer, Including Assessed Tax 0 0 0 0
Ameren Illinois Company | Intersegment Eliminations | Industrial        
Disaggregation of Revenue [Line Items]        
Revenue from Contract with Customer, Including Assessed Tax 0 0 0 0
Ameren Illinois Company | Intersegment Eliminations | Other        
Disaggregation of Revenue [Line Items]        
Revenue from Contract with Customer, Including Assessed Tax (15) (14) (41) (32)
Ameren Illinois Company | Ameren Illinois Company        
Disaggregation of Revenue [Line Items]        
Revenue from Contract with Customer, Including Assessed Tax 564 574 1,902 1,853
Revenues from alternative revenue programs (110) (99) (81) (33)
Other revenues not from contracts with customers 2 2 16 7
Ameren Illinois Company | Ameren Illinois Company | Residential        
Disaggregation of Revenue [Line Items]        
Revenue from Contract with Customer, Including Assessed Tax 291 296 1,071 1,010
Ameren Illinois Company | Ameren Illinois Company | Commercial        
Disaggregation of Revenue [Line Items]        
Revenue from Contract with Customer, Including Assessed Tax 151 154 494 495
Ameren Illinois Company | Ameren Illinois Company | Industrial        
Disaggregation of Revenue [Line Items]        
Revenue from Contract with Customer, Including Assessed Tax 29 29 108 90
Ameren Illinois Company | Ameren Illinois Company | Other        
Disaggregation of Revenue [Line Items]        
Revenue from Contract with Customer, Including Assessed Tax 93 95 229 258
Ameren Illinois Company | Electricity        
Disaggregation of Revenue [Line Items]        
Revenue from Contract with Customer, Including Assessed Tax 448 462 1,333 1,343
Ameren Illinois Company | Natural Gas        
Disaggregation of Revenue [Line Items]        
Revenue from Contract with Customer, Including Assessed Tax 116 112 569 510
Operating Segments | Union Electric Company        
Disaggregation of Revenue [Line Items]        
Revenue from Contract with Customer, Including Assessed Tax 1,129 1,116 2,876 2,841
Revenues from alternative revenue programs 1 (6) (8) (20)
Other revenues not from contracts with customers 3 4 22 11
Operating Segments | Ameren Illinois Electric Distribution        
Disaggregation of Revenue [Line Items]        
Revenue from Contract with Customer, Including Assessed Tax 392 404 1,179 1,178
Revenues from alternative revenue programs (98) (96) (52) (47)
Other revenues not from contracts with customers 1 2 14 5
Operating Segments | Ameren Illinois Gas        
Disaggregation of Revenue [Line Items]        
Revenue from Contract with Customer, Including Assessed Tax 116 112 569 510
Revenues from alternative revenue programs (2) (1) (10) 11
Other revenues not from contracts with customers 1 0 2 2
Operating Segments | Ameren Transmission        
Disaggregation of Revenue [Line Items]        
Revenue from Contract with Customer, Including Assessed Tax 113 119 320 326
Revenues from alternative revenue programs (12) (2) (21) 5
Other revenues not from contracts with customers 0 0 0 0
Operating Segments | Other        
Disaggregation of Revenue [Line Items]        
Revenue from Contract with Customer, Including Assessed Tax 0 (2) 0 (2)
Operating Segments | Electricity | Union Electric Company        
Disaggregation of Revenue [Line Items]        
Revenue from Contract with Customer, Including Assessed Tax 1,111 1,099 2,782 2,758
Operating Segments | Electricity | Union Electric Company | Residential        
Disaggregation of Revenue [Line Items]        
Revenue from Contract with Customer, Including Assessed Tax 508 491 1,272 1,135
Operating Segments | Electricity | Union Electric Company | Commercial        
Disaggregation of Revenue [Line Items]        
Revenue from Contract with Customer, Including Assessed Tax 417 409 1,033 971
Operating Segments | Electricity | Union Electric Company | Industrial        
Disaggregation of Revenue [Line Items]        
Revenue from Contract with Customer, Including Assessed Tax 101 100 249 242
Operating Segments | Electricity | Union Electric Company | Other        
Disaggregation of Revenue [Line Items]        
Revenue from Contract with Customer, Including Assessed Tax 85 99 228 410
Operating Segments | Electricity | Ameren Illinois Electric Distribution        
Disaggregation of Revenue [Line Items]        
Revenue from Contract with Customer, Including Assessed Tax 392 404 1,179 1,178
Operating Segments | Electricity | Ameren Illinois Electric Distribution | Residential        
Disaggregation of Revenue [Line Items]        
Revenue from Contract with Customer, Including Assessed Tax 223 224 663 651
Operating Segments | Electricity | Ameren Illinois Electric Distribution | Commercial        
Disaggregation of Revenue [Line Items]        
Revenue from Contract with Customer, Including Assessed Tax 131 133 381 395
Operating Segments | Electricity | Ameren Illinois Electric Distribution | Industrial        
Disaggregation of Revenue [Line Items]        
Revenue from Contract with Customer, Including Assessed Tax 28 27 96 83
Operating Segments | Electricity | Ameren Illinois Electric Distribution | Other        
Disaggregation of Revenue [Line Items]        
Revenue from Contract with Customer, Including Assessed Tax 10 20 39 49
Operating Segments | Electricity | Ameren Illinois Gas        
Disaggregation of Revenue [Line Items]        
Revenue from Contract with Customer, Including Assessed Tax 0 0 0 0
Operating Segments | Electricity | Ameren Illinois Gas | Residential        
Disaggregation of Revenue [Line Items]        
Revenue from Contract with Customer, Including Assessed Tax 0 0 0 0
Operating Segments | Electricity | Ameren Illinois Gas | Commercial        
Disaggregation of Revenue [Line Items]        
Revenue from Contract with Customer, Including Assessed Tax 0 0 0 0
Operating Segments | Electricity | Ameren Illinois Gas | Industrial        
Disaggregation of Revenue [Line Items]        
Revenue from Contract with Customer, Including Assessed Tax 0 0 0 0
Operating Segments | Electricity | Ameren Illinois Gas | Other        
Disaggregation of Revenue [Line Items]        
Revenue from Contract with Customer, Including Assessed Tax 0 0 0 0
Operating Segments | Electricity | Ameren Transmission        
Disaggregation of Revenue [Line Items]        
Revenue from Contract with Customer, Including Assessed Tax 113 119 320 326
Operating Segments | Electricity | Ameren Transmission | Residential        
Disaggregation of Revenue [Line Items]        
Revenue from Contract with Customer, Including Assessed Tax 0 0 0 0
Operating Segments | Electricity | Ameren Transmission | Commercial        
Disaggregation of Revenue [Line Items]        
Revenue from Contract with Customer, Including Assessed Tax 0 0 0 0
Operating Segments | Electricity | Ameren Transmission | Industrial        
Disaggregation of Revenue [Line Items]        
Revenue from Contract with Customer, Including Assessed Tax 0 0 0 0
Operating Segments | Electricity | Ameren Transmission | Other        
Disaggregation of Revenue [Line Items]        
Revenue from Contract with Customer, Including Assessed Tax 113 119 320 326
Operating Segments | Electricity | Other        
Disaggregation of Revenue [Line Items]        
Revenue from Contract with Customer, Including Assessed Tax 0 (2) 0 (2)
Operating Segments | Electricity | Other | Residential        
Disaggregation of Revenue [Line Items]        
Revenue from Contract with Customer, Including Assessed Tax 0 0 0 0
Operating Segments | Electricity | Other | Commercial        
Disaggregation of Revenue [Line Items]        
Revenue from Contract with Customer, Including Assessed Tax 0 0 0 0
Operating Segments | Electricity | Other | Industrial        
Disaggregation of Revenue [Line Items]        
Revenue from Contract with Customer, Including Assessed Tax 0 0 0 0
Operating Segments | Electricity | Other | Other        
Disaggregation of Revenue [Line Items]        
Revenue from Contract with Customer, Including Assessed Tax 0 (2) 0 (2)
Operating Segments | Natural Gas | Union Electric Company        
Disaggregation of Revenue [Line Items]        
Revenue from Contract with Customer, Including Assessed Tax 18 17 94 83
Operating Segments | Natural Gas | Union Electric Company | Residential        
Disaggregation of Revenue [Line Items]        
Revenue from Contract with Customer, Including Assessed Tax 8 9 62 49
Operating Segments | Natural Gas | Union Electric Company | Commercial        
Disaggregation of Revenue [Line Items]        
Revenue from Contract with Customer, Including Assessed Tax 3 4 25 20
Operating Segments | Natural Gas | Union Electric Company | Industrial        
Disaggregation of Revenue [Line Items]        
Revenue from Contract with Customer, Including Assessed Tax 1 1 3 3
Operating Segments | Natural Gas | Union Electric Company | Other        
Disaggregation of Revenue [Line Items]        
Revenue from Contract with Customer, Including Assessed Tax 6 3 4 11
Operating Segments | Natural Gas | Ameren Illinois Electric Distribution        
Disaggregation of Revenue [Line Items]        
Revenue from Contract with Customer, Including Assessed Tax 0 0 0 0
Operating Segments | Natural Gas | Ameren Illinois Electric Distribution | Residential        
Disaggregation of Revenue [Line Items]        
Revenue from Contract with Customer, Including Assessed Tax 0 0 0 0
Operating Segments | Natural Gas | Ameren Illinois Electric Distribution | Commercial        
Disaggregation of Revenue [Line Items]        
Revenue from Contract with Customer, Including Assessed Tax 0 0 0 0
Operating Segments | Natural Gas | Ameren Illinois Electric Distribution | Industrial        
Disaggregation of Revenue [Line Items]        
Revenue from Contract with Customer, Including Assessed Tax 0 0 0 0
Operating Segments | Natural Gas | Ameren Illinois Electric Distribution | Other        
Disaggregation of Revenue [Line Items]        
Revenue from Contract with Customer, Including Assessed Tax 0 0 0 0
Operating Segments | Natural Gas | Ameren Illinois Gas        
Disaggregation of Revenue [Line Items]        
Revenue from Contract with Customer, Including Assessed Tax 116 112 569 510
Operating Segments | Natural Gas | Ameren Illinois Gas | Residential        
Disaggregation of Revenue [Line Items]        
Revenue from Contract with Customer, Including Assessed Tax 68 72 408 359
Operating Segments | Natural Gas | Ameren Illinois Gas | Commercial        
Disaggregation of Revenue [Line Items]        
Revenue from Contract with Customer, Including Assessed Tax 20 21 113 100
Operating Segments | Natural Gas | Ameren Illinois Gas | Industrial        
Disaggregation of Revenue [Line Items]        
Revenue from Contract with Customer, Including Assessed Tax 1 2 12 7
Operating Segments | Natural Gas | Ameren Illinois Gas | Other        
Disaggregation of Revenue [Line Items]        
Revenue from Contract with Customer, Including Assessed Tax 27 17 36 44
Operating Segments | Natural Gas | Ameren Transmission        
Disaggregation of Revenue [Line Items]        
Revenue from Contract with Customer, Including Assessed Tax 0 0 0 0
Operating Segments | Natural Gas | Ameren Transmission | Residential        
Disaggregation of Revenue [Line Items]        
Revenue from Contract with Customer, Including Assessed Tax 0 0 0 0
Operating Segments | Natural Gas | Ameren Transmission | Commercial        
Disaggregation of Revenue [Line Items]        
Revenue from Contract with Customer, Including Assessed Tax 0 0 0 0
Operating Segments | Natural Gas | Ameren Transmission | Industrial        
Disaggregation of Revenue [Line Items]        
Revenue from Contract with Customer, Including Assessed Tax 0 0 0 0
Operating Segments | Natural Gas | Ameren Transmission | Other        
Disaggregation of Revenue [Line Items]        
Revenue from Contract with Customer, Including Assessed Tax 0 0 0 0
Operating Segments | Natural Gas | Other        
Disaggregation of Revenue [Line Items]        
Revenue from Contract with Customer, Including Assessed Tax 0 0 0 0
Operating Segments | Natural Gas | Other | Residential        
Disaggregation of Revenue [Line Items]        
Revenue from Contract with Customer, Including Assessed Tax 0 0 0 0
Operating Segments | Natural Gas | Other | Commercial        
Disaggregation of Revenue [Line Items]        
Revenue from Contract with Customer, Including Assessed Tax 0 0 0 0
Operating Segments | Natural Gas | Other | Industrial        
Disaggregation of Revenue [Line Items]        
Revenue from Contract with Customer, Including Assessed Tax 0 0 0 0
Operating Segments | Natural Gas | Other | Other        
Disaggregation of Revenue [Line Items]        
Revenue from Contract with Customer, Including Assessed Tax 0 0 0 0
Operating Segments | Ameren Illinois Company | Ameren Illinois Electric Distribution        
Disaggregation of Revenue [Line Items]        
Revenues from alternative revenue programs (98) (96) (52) (47)
Other revenues not from contracts with customers 1 2 14 5
Operating Segments | Ameren Illinois Company | Ameren Illinois Gas        
Disaggregation of Revenue [Line Items]        
Revenues from alternative revenue programs (2) (1) (10) 11
Other revenues not from contracts with customers 1 0 2 2
Operating Segments | Ameren Illinois Company | Ameren Illinois Transmission        
Disaggregation of Revenue [Line Items]        
Revenue from Contract with Customer, Including Assessed Tax 71 72 195 197
Revenues from alternative revenue programs (10) (2) (19) 3
Other revenues not from contracts with customers 0 0 0 0
Operating Segments | Ameren Illinois Company | Ameren Illinois Transmission | Residential        
Disaggregation of Revenue [Line Items]        
Revenue from Contract with Customer, Including Assessed Tax 0 0 0 0
Operating Segments | Ameren Illinois Company | Ameren Illinois Transmission | Commercial        
Disaggregation of Revenue [Line Items]        
Revenue from Contract with Customer, Including Assessed Tax 0 0 0 0
Operating Segments | Ameren Illinois Company | Ameren Illinois Transmission | Industrial        
Disaggregation of Revenue [Line Items]        
Revenue from Contract with Customer, Including Assessed Tax 0 0 0 0
Operating Segments | Ameren Illinois Company | Ameren Illinois Transmission | Other        
Disaggregation of Revenue [Line Items]        
Revenue from Contract with Customer, Including Assessed Tax 71 72 195 197
Operating Segments | Ameren Illinois Company | Electricity | Ameren Illinois Electric Distribution        
Disaggregation of Revenue [Line Items]        
Revenue from Contract with Customer, Including Assessed Tax 392 404 1,179 1,178
Operating Segments | Ameren Illinois Company | Electricity | Ameren Illinois Electric Distribution | Residential        
Disaggregation of Revenue [Line Items]        
Revenue from Contract with Customer, Including Assessed Tax 223 224 663 651
Operating Segments | Ameren Illinois Company | Electricity | Ameren Illinois Electric Distribution | Commercial        
Disaggregation of Revenue [Line Items]        
Revenue from Contract with Customer, Including Assessed Tax 131 133 381 395
Operating Segments | Ameren Illinois Company | Electricity | Ameren Illinois Electric Distribution | Industrial        
Disaggregation of Revenue [Line Items]        
Revenue from Contract with Customer, Including Assessed Tax 28 27 96 83
Operating Segments | Ameren Illinois Company | Electricity | Ameren Illinois Electric Distribution | Other        
Disaggregation of Revenue [Line Items]        
Revenue from Contract with Customer, Including Assessed Tax 10 20 39 49
Operating Segments | Ameren Illinois Company | Natural Gas | Ameren Illinois Gas        
Disaggregation of Revenue [Line Items]        
Revenue from Contract with Customer, Including Assessed Tax 116 112 569 510
Operating Segments | Ameren Illinois Company | Natural Gas | Ameren Illinois Gas | Residential        
Disaggregation of Revenue [Line Items]        
Revenue from Contract with Customer, Including Assessed Tax 68 72 408 359
Operating Segments | Ameren Illinois Company | Natural Gas | Ameren Illinois Gas | Commercial        
Disaggregation of Revenue [Line Items]        
Revenue from Contract with Customer, Including Assessed Tax 20 21 113 100
Operating Segments | Ameren Illinois Company | Natural Gas | Ameren Illinois Gas | Industrial        
Disaggregation of Revenue [Line Items]        
Revenue from Contract with Customer, Including Assessed Tax 1 2 12 7
Operating Segments | Ameren Illinois Company | Natural Gas | Ameren Illinois Gas | Other        
Disaggregation of Revenue [Line Items]        
Revenue from Contract with Customer, Including Assessed Tax $ 27 $ 17 $ 36 $ 44