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Fair Value Measurements (Schedule Of Carrying Amounts And Estimated Fair Values Of Financial Assets and Liabilities) (Detail) - USD ($)
$ in Millions
Mar. 31, 2018
Dec. 31, 2017
Mar. 31, 2017
Dec. 31, 2016
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]        
Cash and Cash Equivalents and Restricted Cash and Cash Equivalents $ 89 $ 68 $ 51 $ 52
Short-term Debt 960 484    
Union Electric Company        
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]        
Cash and Cash Equivalents and Restricted Cash and Cash Equivalents 10 7 5 5
Short-term Debt 282 39    
Ameren Illinois Company        
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]        
Cash and Cash Equivalents and Restricted Cash and Cash Equivalents 52 41 $ 28 $ 28
Short-term Debt 224 62    
Carrying Amount        
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]        
Cash and Cash Equivalents and Restricted Cash and Cash Equivalents 89 68    
Short-term Debt 960 484    
Long-term debt (including current portion) 7,936 7,935    
Preferred stock 142 142    
Debt Issuance Costs, Net 49 50    
Carrying Amount | Union Electric Company        
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]        
Cash and Cash Equivalents and Restricted Cash and Cash Equivalents 10 7    
Available-for-sale Securities and Held-to-maturity Securities 276 276    
Short-term Debt 282 39    
Long-term debt (including current portion) 3,962 3,961    
Preferred stock 80 80    
Debt Issuance Costs, Net 20 20    
Carrying Amount | Ameren Illinois Company        
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]        
Cash and Cash Equivalents and Restricted Cash and Cash Equivalents   41    
Short-term Debt 224 62    
Long-term debt (including current portion) 2,830 2,830    
Preferred stock 62 62    
Fair Value, Concentration of Risk, Cash and Cash Equivalents 52      
Debt Issuance Costs, Net 24 24    
Fair Value        
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]        
Cash and Cash Equivalents and Restricted Cash and Cash Equivalents 89 68    
Short-term Debt, Fair Value 960 484    
Investments, Fair Value Disclosure 276 276    
Long-term Debt, Fair Value 8,259 8,531    
Preferred stock 130 131    
Fair Value | Quoted Prices In Active Markets For Identical Assets or Liabilities (Level 1)        
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]        
Cash and Cash Equivalents and Restricted Cash and Cash Equivalents 89 68    
Short-term Debt, Fair Value 0 0    
Investments, Fair Value Disclosure 0 0    
Long-term Debt, Fair Value 0 0    
Preferred stock 0 0    
Fair Value | Significant Other Observable Inputs (Level 2)        
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]        
Cash and Cash Equivalents and Restricted Cash and Cash Equivalents 0 0    
Short-term Debt, Fair Value 960 484    
Investments, Fair Value Disclosure 276 276    
Long-term Debt, Fair Value 7,820 8,531    
Preferred stock 130 131    
Fair Value | Significant Other Unobservable Inputs (Level 3)        
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]        
Cash and Cash Equivalents and Restricted Cash and Cash Equivalents 0 0    
Short-term Debt, Fair Value 0 0    
Investments, Fair Value Disclosure 0 0    
Long-term Debt, Fair Value 439 0    
Preferred stock 0 0    
Fair Value | Union Electric Company        
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]        
Cash and Cash Equivalents and Restricted Cash and Cash Equivalents 10 7    
Short-term Debt, Fair Value 282 39    
Investments, Fair Value Disclosure 276 276    
Long-term Debt, Fair Value 4,201 4,348    
Preferred stock 78 80    
Fair Value | Union Electric Company | Quoted Prices In Active Markets For Identical Assets or Liabilities (Level 1)        
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]        
Cash and Cash Equivalents and Restricted Cash and Cash Equivalents 10 7    
Short-term Debt, Fair Value 0 0    
Investments, Fair Value Disclosure 0 0    
Long-term Debt, Fair Value 0 0    
Preferred stock 0 0    
Fair Value | Union Electric Company | Significant Other Observable Inputs (Level 2)        
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]        
Cash and Cash Equivalents and Restricted Cash and Cash Equivalents 0 0    
Short-term Debt, Fair Value 282 39    
Investments, Fair Value Disclosure 276 276    
Long-term Debt, Fair Value 4,201 4,348    
Preferred stock 78 80    
Fair Value | Union Electric Company | Significant Other Unobservable Inputs (Level 3)        
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]        
Cash and Cash Equivalents and Restricted Cash and Cash Equivalents 0 0    
Short-term Debt, Fair Value 0 0    
Investments, Fair Value Disclosure 0 0    
Long-term Debt, Fair Value 0 0    
Preferred stock 0 0    
Fair Value | Ameren Illinois Company        
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]        
Cash and Cash Equivalents and Restricted Cash and Cash Equivalents 52 41    
Short-term Debt, Fair Value 224 62    
Long-term Debt, Fair Value 2,928 3,028    
Preferred stock 52 51    
Fair Value | Ameren Illinois Company | Quoted Prices In Active Markets For Identical Assets or Liabilities (Level 1)        
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]        
Cash and Cash Equivalents and Restricted Cash and Cash Equivalents 52 41    
Short-term Debt, Fair Value 0 0    
Long-term Debt, Fair Value 0 0    
Preferred stock 0 0    
Fair Value | Ameren Illinois Company | Significant Other Observable Inputs (Level 2)        
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]        
Cash and Cash Equivalents and Restricted Cash and Cash Equivalents 0 0    
Short-term Debt, Fair Value 224 62    
Long-term Debt, Fair Value 2,928 3,028    
Preferred stock 52 51    
Fair Value | Ameren Illinois Company | Significant Other Unobservable Inputs (Level 3)        
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]        
Cash and Cash Equivalents and Restricted Cash and Cash Equivalents 0 0    
Short-term Debt, Fair Value 0 0    
Long-term Debt, Fair Value 0 0    
Preferred stock $ 0 $ 0