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Segment Information Segment Information (Disaggregation of Revenue) (Details) - USD ($)
$ in Millions
 3 Months Ended 6 Months Ended
Jun. 30, 2018
Jun. 30, 2017
Jun. 30, 2018
Jun. 30, 2017
Disaggregation of Revenue [Line Items]        
Revenue from Contract with Customer, Including Assessed Tax $ 1,563 $ 1,537 $ 3,148 $ 3,052
Revenues from alternative revenue programs 0 12 20 54
Other revenues not from contracts with customers 8 5 33 12
TCJA - Excess Amounts Collected in Rates        
Disaggregation of Revenue [Line Items]        
TCJA Revenue Reduction 37   47  
Intersegment Eliminations        
Disaggregation of Revenue [Line Items]        
Revenue from Contract with Customer, Including Assessed Tax (24) (27) (46) (52)
Electricity        
Disaggregation of Revenue [Line Items]        
Revenue from Contract with Customer, Including Assessed Tax 1,396 1,382 2,619 2,589
Electricity | Residential        
Disaggregation of Revenue [Line Items]        
Revenue from Contract with Customer, Including Assessed Tax 653 566 1,204 1,071
Electricity | Commercial        
Disaggregation of Revenue [Line Items]        
Revenue from Contract with Customer, Including Assessed Tax 490 461 866 824
Electricity | Industrial        
Disaggregation of Revenue [Line Items]        
Revenue from Contract with Customer, Including Assessed Tax 120 112 216 198
Electricity | Other        
Disaggregation of Revenue [Line Items]        
Revenue from Contract with Customer, Including Assessed Tax 133 243 333 496
Electricity | Intersegment Eliminations        
Disaggregation of Revenue [Line Items]        
Revenue from Contract with Customer, Including Assessed Tax (24) (26) (46) (51)
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 (24) (26) (46) (51)
Natural Gas        
Disaggregation of Revenue [Line Items]        
Revenue from Contract with Customer, Including Assessed Tax 167 155 529 463
Natural Gas | Residential        
Disaggregation of Revenue [Line Items]        
Revenue from Contract with Customer, Including Assessed Tax 110 94 394 327
Natural Gas | Commercial        
Disaggregation of Revenue [Line Items]        
Revenue from Contract with Customer, Including Assessed Tax 32 28 115 95
Natural Gas | Industrial        
Disaggregation of Revenue [Line Items]        
Revenue from Contract with Customer, Including Assessed Tax 5 3 13 7
Natural Gas | Other        
Disaggregation of Revenue [Line Items]        
Revenue from Contract with Customer, Including Assessed Tax 20 30 7 34
Natural Gas | Intersegment Eliminations        
Disaggregation of Revenue [Line Items]        
Revenue from Contract with Customer, Including Assessed Tax 0 (1) 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 (1) 0 (1)
Ameren Illinois Company        
Disaggregation of Revenue [Line Items]        
Revenue from Contract with Customer, Including Assessed Tax 578 576 1,338 1,279
Ameren Illinois Company | Intersegment Eliminations        
Disaggregation of Revenue [Line Items]        
Revenue from Contract with Customer, Including Assessed Tax (13) (12) (26) (18)
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 (13) (12) (26) (18)
Ameren Illinois Company | Ameren Illinois Company        
Disaggregation of Revenue [Line Items]        
Revenue from Contract with Customer, Including Assessed Tax 578 576 1,338 1,279
Revenues from alternative revenue programs 5 19 29 66
Other revenues not from contracts with customers 3 2 14 5
Ameren Illinois Company | Ameren Illinois Company | Residential        
Disaggregation of Revenue [Line Items]        
Revenue from Contract with Customer, Including Assessed Tax 318 292 780 714
Ameren Illinois Company | Ameren Illinois Company | Commercial        
Disaggregation of Revenue [Line Items]        
Revenue from Contract with Customer, Including Assessed Tax 152 153 343 341
Ameren Illinois Company | Ameren Illinois Company | Industrial        
Disaggregation of Revenue [Line Items]        
Revenue from Contract with Customer, Including Assessed Tax 38 30 79 61
Ameren Illinois Company | Ameren Illinois Company | Other        
Disaggregation of Revenue [Line Items]        
Revenue from Contract with Customer, Including Assessed Tax 70 101 136 163
Ameren Illinois Company | Electricity        
Disaggregation of Revenue [Line Items]        
Revenue from Contract with Customer, Including Assessed Tax 436 442 885 881
Ameren Illinois Company | Natural Gas        
Disaggregation of Revenue [Line Items]        
Revenue from Contract with Customer, Including Assessed Tax 142 134 453 398
Operating Segments | Union Electric Company        
Disaggregation of Revenue [Line Items]        
Revenue from Contract with Customer, Including Assessed Tax 955 934 1,747 1,725
Revenues from alternative revenue programs (5) (7) (9) (14)
Other revenues not from contracts with customers 5 3 19 7
Operating Segments | Ameren Illinois Electric Distribution        
Disaggregation of Revenue [Line Items]        
Revenue from Contract with Customer, Including Assessed Tax 387 389 787 774
Revenues from alternative revenue programs 15 16 46 49
Other revenues not from contracts with customers 3 1 13 3
Operating Segments | Ameren Illinois Gas        
Disaggregation of Revenue [Line Items]        
Revenue from Contract with Customer, Including Assessed Tax 142 134 453 398
Revenues from alternative revenue programs (5) 1 (8) 12
Other revenues not from contracts with customers 0 1 1 2
Operating Segments | Ameren Transmission        
Disaggregation of Revenue [Line Items]        
Revenue from Contract with Customer, Including Assessed Tax 103 105 207 207
Revenues from alternative revenue programs (5) 2 (9) 7
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 0
Operating Segments | Electricity | Union Electric Company        
Disaggregation of Revenue [Line Items]        
Revenue from Contract with Customer, Including Assessed Tax 930 912 1,671 1,659
Operating Segments | Electricity | Union Electric Company | Residential        
Disaggregation of Revenue [Line Items]        
Revenue from Contract with Customer, Including Assessed Tax 432 358 764 644
Operating Segments | Electricity | Union Electric Company | Commercial        
Disaggregation of Revenue [Line Items]        
Revenue from Contract with Customer, Including Assessed Tax 364 332 616 562
Operating Segments | Electricity | Union Electric Company | Industrial        
Disaggregation of Revenue [Line Items]        
Revenue from Contract with Customer, Including Assessed Tax 87 84 148 142
Operating Segments | Electricity | Union Electric Company | Other        
Disaggregation of Revenue [Line Items]        
Revenue from Contract with Customer, Including Assessed Tax 47 138 143 311
Operating Segments | Electricity | Ameren Illinois Electric Distribution        
Disaggregation of Revenue [Line Items]        
Revenue from Contract with Customer, Including Assessed Tax 387 389 787 774
Operating Segments | Electricity | Ameren Illinois Electric Distribution | Residential        
Disaggregation of Revenue [Line Items]        
Revenue from Contract with Customer, Including Assessed Tax 221 208 440 427
Operating Segments | Electricity | Ameren Illinois Electric Distribution | Commercial        
Disaggregation of Revenue [Line Items]        
Revenue from Contract with Customer, Including Assessed Tax 126 129 250 262
Operating Segments | Electricity | Ameren Illinois Electric Distribution | Industrial        
Disaggregation of Revenue [Line Items]        
Revenue from Contract with Customer, Including Assessed Tax 33 28 68 56
Operating Segments | Electricity | Ameren Illinois Electric Distribution | Other        
Disaggregation of Revenue [Line Items]        
Revenue from Contract with Customer, Including Assessed Tax 7 24 29 29
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 103 105 207 207
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 103 105 207 207
Operating Segments | Electricity | Other        
Disaggregation of Revenue [Line Items]        
Revenue from Contract with Customer, Including Assessed Tax 0 2 0 0
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 0
Operating Segments | Natural Gas | Union Electric Company        
Disaggregation of Revenue [Line Items]        
Revenue from Contract with Customer, Including Assessed Tax 25 22 76 66
Operating Segments | Natural Gas | Union Electric Company | Residential        
Disaggregation of Revenue [Line Items]        
Revenue from Contract with Customer, Including Assessed Tax 13 10 54 40
Operating Segments | Natural Gas | Union Electric Company | Commercial        
Disaggregation of Revenue [Line Items]        
Revenue from Contract with Customer, Including Assessed Tax 6 4 22 16
Operating Segments | Natural Gas | Union Electric Company | Industrial        
Disaggregation of Revenue [Line Items]        
Revenue from Contract with Customer, Including Assessed Tax 0 1 2 2
Operating Segments | Natural Gas | Union Electric Company | Other        
Disaggregation of Revenue [Line Items]        
Revenue from Contract with Customer, Including Assessed Tax 6 7 (2) 8
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 142 134 453 398
Operating Segments | Natural Gas | Ameren Illinois Gas | Residential        
Disaggregation of Revenue [Line Items]        
Revenue from Contract with Customer, Including Assessed Tax 97 84 340 287
Operating Segments | Natural Gas | Ameren Illinois Gas | Commercial        
Disaggregation of Revenue [Line Items]        
Revenue from Contract with Customer, Including Assessed Tax 26 24 93 79
Operating Segments | Natural Gas | Ameren Illinois Gas | Industrial        
Disaggregation of Revenue [Line Items]        
Revenue from Contract with Customer, Including Assessed Tax 5 2 11 5
Operating Segments | Natural Gas | Ameren Illinois Gas | Other        
Disaggregation of Revenue [Line Items]        
Revenue from Contract with Customer, Including Assessed Tax 14 24 9 27
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 15 16 46 49
Other revenues not from contracts with customers 3 1 13 3
Operating Segments | Ameren Illinois Company | Ameren Illinois Gas        
Disaggregation of Revenue [Line Items]        
Revenues from alternative revenue programs (5) 1 (8) 12
Other revenues not from contracts with customers 0 1 1 2
Operating Segments | Ameren Illinois Company | Ameren Illinois Transmission        
Disaggregation of Revenue [Line Items]        
Revenue from Contract with Customer, Including Assessed Tax 62 65 124 125
Revenues from alternative revenue programs (5) 2 (9) 5
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 62 65 124 125
Operating Segments | Ameren Illinois Company | Electricity | Ameren Illinois Electric Distribution        
Disaggregation of Revenue [Line Items]        
Revenue from Contract with Customer, Including Assessed Tax 387 389 787 774
Operating Segments | Ameren Illinois Company | Electricity | Ameren Illinois Electric Distribution | Residential        
Disaggregation of Revenue [Line Items]        
Revenue from Contract with Customer, Including Assessed Tax 221 208 440 427
Operating Segments | Ameren Illinois Company | Electricity | Ameren Illinois Electric Distribution | Commercial        
Disaggregation of Revenue [Line Items]        
Revenue from Contract with Customer, Including Assessed Tax 126 129 250 262
Operating Segments | Ameren Illinois Company | Electricity | Ameren Illinois Electric Distribution | Industrial        
Disaggregation of Revenue [Line Items]        
Revenue from Contract with Customer, Including Assessed Tax 33 28 68 56
Operating Segments | Ameren Illinois Company | Electricity | Ameren Illinois Electric Distribution | Other        
Disaggregation of Revenue [Line Items]        
Revenue from Contract with Customer, Including Assessed Tax 7 24 29 29
Operating Segments | Ameren Illinois Company | Natural Gas | Ameren Illinois Gas        
Disaggregation of Revenue [Line Items]        
Revenue from Contract with Customer, Including Assessed Tax 142 134 453 398
Operating Segments | Ameren Illinois Company | Natural Gas | Ameren Illinois Gas | Residential        
Disaggregation of Revenue [Line Items]        
Revenue from Contract with Customer, Including Assessed Tax 97 84 340 287
Operating Segments | Ameren Illinois Company | Natural Gas | Ameren Illinois Gas | Commercial        
Disaggregation of Revenue [Line Items]        
Revenue from Contract with Customer, Including Assessed Tax 26 24 93 79
Operating Segments | Ameren Illinois Company | Natural Gas | Ameren Illinois Gas | Industrial        
Disaggregation of Revenue [Line Items]        
Revenue from Contract with Customer, Including Assessed Tax 5 2 11 5
Operating Segments | Ameren Illinois Company | Natural Gas | Ameren Illinois Gas | Other        
Disaggregation of Revenue [Line Items]        
Revenue from Contract with Customer, Including Assessed Tax $ 14 $ 24 $ 9 $ 27