Fair Value Measurements (Schedule Of Fair Value Hierarchy Of Assets And Liabilities Measured At Fair Value On Recurring Basis) (Details) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund	$ 702	$ 605
Assets	719	632
Excluded receivables, payables, and accrued income, net	2	2
Quoted Prices in Active Markets for Identical Assets or Liabilities (Level 1)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund	470	409
Assets	474	413
Significant Other Observable Inputs (Level 2)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund	232	196
Assets	233	208
Significant Other Unobservable Inputs (Level 3)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund	0	0
Assets	12	11
Cash And Cash Equivalents
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund	2	1
Cash And Cash Equivalents | Quoted Prices in Active Markets for Identical Assets or Liabilities (Level 1)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund	2	1
Cash And Cash Equivalents | Significant Other Observable Inputs (Level 2)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund	0	0
Cash And Cash Equivalents | Significant Other Unobservable Inputs (Level 3)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund	0	0
Commodity Contract
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets	17	27
Derivative liabilities	226	211
Commodity Contract | Quoted Prices in Active Markets for Identical Assets or Liabilities (Level 1)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets	4	4
Derivative liabilities	1	5
Commodity Contract | Significant Other Observable Inputs (Level 2)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets	1	12
Derivative liabilities	25	14
Commodity Contract | Significant Other Unobservable Inputs (Level 3)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets	12	11
Derivative liabilities	200	192
Fuel Oils
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets	7	3
Derivative liabilities		5
Fuel Oils | Quoted Prices in Active Markets for Identical Assets or Liabilities (Level 1)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets	4	2
Derivative liabilities		5
Fuel Oils | Significant Other Observable Inputs (Level 2)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets	0	0
Derivative liabilities		0
Fuel Oils | Significant Other Unobservable Inputs (Level 3)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets	3	1
Derivative liabilities		0
Natural Gas
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets	1	15
Derivative liabilities	30	14
Natural Gas | Quoted Prices in Active Markets for Identical Assets or Liabilities (Level 1)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets	0	2
Derivative liabilities	1	0
Natural Gas | Significant Other Observable Inputs (Level 2)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets	0	12
Derivative liabilities	25	13
Natural Gas | Significant Other Unobservable Inputs (Level 3)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets	1	1
Derivative liabilities	4	1
Power
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets	9	9
Derivative liabilities	196	188
Power | Quoted Prices in Active Markets for Identical Assets or Liabilities (Level 1)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets	0	0
Derivative liabilities	0	0
Power | Significant Other Observable Inputs (Level 2)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets	1	0
Derivative liabilities	0	1
Power | Significant Other Unobservable Inputs (Level 3)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets	8	9
Derivative liabilities	196	187
Uranium
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative liabilities		4
Uranium | Quoted Prices in Active Markets for Identical Assets or Liabilities (Level 1)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative liabilities		0
Uranium | Significant Other Observable Inputs (Level 2)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative liabilities		0
Uranium | Significant Other Unobservable Inputs (Level 3)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative liabilities		4
Equity Securities | U.S. Large Capitalization
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund	468	408
Equity Securities | U.S. Large Capitalization | Quoted Prices in Active Markets for Identical Assets or Liabilities (Level 1)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund	468	408
Equity Securities | U.S. Large Capitalization | Significant Other Observable Inputs (Level 2)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund	0	0
Equity Securities | U.S. Large Capitalization | Significant Other Unobservable Inputs (Level 3)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund	0	0
Debt Securities | Corporate Bonds
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund	82	67
Debt Securities | Corporate Bonds | Quoted Prices in Active Markets for Identical Assets or Liabilities (Level 1)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund	0	0
Debt Securities | Corporate Bonds | Significant Other Observable Inputs (Level 2)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund	82	67
Debt Securities | Corporate Bonds | Significant Other Unobservable Inputs (Level 3)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund	0	0
Debt Securities | U.S. treasury and agency securities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund	125	112
Debt Securities | U.S. treasury and agency securities | Quoted Prices in Active Markets for Identical Assets or Liabilities (Level 1)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund	0	0
Debt Securities | U.S. treasury and agency securities | Significant Other Observable Inputs (Level 2)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund	125	112
Debt Securities | U.S. treasury and agency securities | Significant Other Unobservable Inputs (Level 3)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund	0	0
Debt Securities | Other debt securities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund	25	17
Debt Securities | Other debt securities | Quoted Prices in Active Markets for Identical Assets or Liabilities (Level 1)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund	0	0
Debt Securities | Other debt securities | Significant Other Observable Inputs (Level 2)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund	25	17
Debt Securities | Other debt securities | Significant Other Unobservable Inputs (Level 3)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund	0	0
Union Electric Company
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund	702	605
Assets	719	619
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]
Nuclear Decommissioning Trust Fund	470	409
Assets	474	411
Union Electric Company | Significant Other Observable Inputs (Level 2)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund	232	196
Assets	233	197
Union Electric Company | Significant Other Unobservable Inputs (Level 3)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund	0	0
Assets	12	11
Union Electric Company | Cash And Cash Equivalents
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund	2	1
Union Electric Company | Cash And Cash Equivalents | Quoted Prices in Active Markets for Identical Assets or Liabilities (Level 1)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund	2	1
Union Electric Company | Cash And Cash Equivalents | Significant Other Observable Inputs (Level 2)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund	0	0
Union Electric Company | Cash And Cash Equivalents | Significant Other Unobservable Inputs (Level 3)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund	0	0
Union Electric Company | Commodity Contract
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets	17	14
Derivative liabilities	9	18
Union Electric Company | Commodity Contract | Quoted Prices in Active Markets for Identical Assets or Liabilities (Level 1)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets	4	2
Derivative liabilities	0	5
Union Electric Company | Commodity Contract | Significant Other Observable Inputs (Level 2)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets	1	1
Derivative liabilities	7	7
Union Electric Company | Commodity Contract | Significant Other Unobservable Inputs (Level 3)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets	12	11
Derivative liabilities	2	6
Union Electric Company | Fuel Oils
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets	7	3
Derivative liabilities		5
Union Electric Company | Fuel Oils | Quoted Prices in Active Markets for Identical Assets or Liabilities (Level 1)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets	4	2
Derivative liabilities		5
Union Electric Company | Fuel Oils | Significant Other Observable Inputs (Level 2)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets	0	0
Derivative liabilities		0
Union Electric Company | Fuel Oils | Significant Other Unobservable Inputs (Level 3)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets	3	1
Derivative liabilities		0
Union Electric Company | Natural Gas
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets	1	2
Derivative liabilities	8	6
Union Electric Company | Natural Gas | Quoted Prices in Active Markets for Identical Assets or Liabilities (Level 1)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets	0	0
Derivative liabilities	0	0
Union Electric Company | Natural Gas | Significant Other Observable Inputs (Level 2)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets	0	1
Derivative liabilities	7	6
Union Electric Company | Natural Gas | Significant Other Unobservable Inputs (Level 3)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets	1	1
Derivative liabilities	1	0
Union Electric Company | Power
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets	9	9
Derivative liabilities	1	3
Union Electric Company | Power | Quoted Prices in Active Markets for Identical Assets or Liabilities (Level 1)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets	0	0
Derivative liabilities	0	0
Union Electric Company | Power | Significant Other Observable Inputs (Level 2)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets	1	0
Derivative liabilities	0	1
Union Electric Company | Power | Significant Other Unobservable Inputs (Level 3)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets	8	9
Derivative liabilities	1	2
Union Electric Company | Uranium
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative liabilities		4
Union Electric Company | Uranium | Quoted Prices in Active Markets for Identical Assets or Liabilities (Level 1)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative liabilities		0
Union Electric Company | Uranium | Significant Other Observable Inputs (Level 2)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative liabilities		0
Union Electric Company | Uranium | Significant Other Unobservable Inputs (Level 3)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative liabilities		4
Union Electric Company | Equity Securities | U.S. Large Capitalization
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund	468	408
Union Electric Company | Equity Securities | U.S. Large Capitalization | Quoted Prices in Active Markets for Identical Assets or Liabilities (Level 1)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund	468	408
Union Electric Company | Equity Securities | U.S. Large Capitalization | Significant Other Observable Inputs (Level 2)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund	0	0
Union Electric Company | Equity Securities | U.S. Large Capitalization | Significant Other Unobservable Inputs (Level 3)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund	0	0
Union Electric Company | Debt Securities | Corporate Bonds
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund	82	67
Union Electric Company | Debt Securities | Corporate Bonds | Quoted Prices in Active Markets for Identical Assets or Liabilities (Level 1)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund	0	0
Union Electric Company | Debt Securities | Corporate Bonds | Significant Other Observable Inputs (Level 2)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund	82	67
Union Electric Company | Debt Securities | Corporate Bonds | Significant Other Unobservable Inputs (Level 3)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund	0	0
Union Electric Company | Debt Securities | U.S. treasury and agency securities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund	125	112
Union Electric Company | Debt Securities | U.S. treasury and agency securities | Quoted Prices in Active Markets for Identical Assets or Liabilities (Level 1)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund	0	0
Union Electric Company | Debt Securities | U.S. treasury and agency securities | Significant Other Observable Inputs (Level 2)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund	125	112
Union Electric Company | Debt Securities | U.S. treasury and agency securities | Significant Other Unobservable Inputs (Level 3)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund	0	0
Union Electric Company | Debt Securities | Other debt securities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund	25	17
Union Electric Company | Debt Securities | Other debt securities | Quoted Prices in Active Markets for Identical Assets or Liabilities (Level 1)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund	0	0
Union Electric Company | Debt Securities | Other debt securities | Significant Other Observable Inputs (Level 2)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund	25	17
Union Electric Company | Debt Securities | Other debt securities | Significant Other Unobservable Inputs (Level 3)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund	0	0
Ameren Illinois Company | Commodity Contract
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative liabilities	217	193
Ameren Illinois Company | Commodity Contract | Quoted Prices in Active Markets for Identical Assets or Liabilities (Level 1)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative liabilities	1	0
Ameren Illinois Company | Commodity Contract | Significant Other Observable Inputs (Level 2)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative liabilities	18	7
Ameren Illinois Company | Commodity Contract | Significant Other Unobservable Inputs (Level 3)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative liabilities	198	186
Ameren Illinois Company | Natural Gas
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets		13
Derivative liabilities	22	8
Ameren Illinois Company | Natural Gas | Quoted Prices in Active Markets for Identical Assets or Liabilities (Level 1)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets		2
Derivative liabilities	1	0
Ameren Illinois Company | Natural Gas | Significant Other Observable Inputs (Level 2)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets		11
Derivative liabilities	18	7
Ameren Illinois Company | Natural Gas | Significant Other Unobservable Inputs (Level 3)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets		0
Derivative liabilities	3	1
Ameren Illinois Company | Power
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative liabilities	195	185
Ameren Illinois Company | Power | Quoted Prices in Active Markets for Identical Assets or Liabilities (Level 1)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative liabilities	0	0
Ameren Illinois Company | Power | Significant Other Observable Inputs (Level 2)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative liabilities	0	0
Ameren Illinois Company | Power | Significant Other Unobservable Inputs (Level 3)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative liabilities	$ 195	$ 185