Fair Value Measurements (Schedule Of Fair Value Hierarchy Of Assets And Liabilities Measured At Fair Value On Recurring Basis) (Detail) - USD ($) $ in Millions	Sep. 30, 2019	Dec. 31, 2018
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund	$ 790	$ 679
Assets fair value	811	694
Excluded receivables, payables, and accrued income, net	8	5
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	518	427
Assets fair value	519	428
Significant Other Observable Inputs (Level 2)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund	272	252
Assets fair value	272	255
Significant Other Unobservable Inputs (Level 3)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund	0	0
Assets fair value	20	11
Commodity Contract
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets	21	15
Derivative liabilities	243	219
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	1	1
Derivative liabilities	6	2
Commodity Contract | Significant Other Observable Inputs (Level 2)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets	0	3
Derivative liabilities	18	16
Commodity Contract | Significant Other Unobservable Inputs (Level 3)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets	20	11
Derivative liabilities	219	201
Fuel Oils | Commodity Contract
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets	6	8
Derivative liabilities	12	13
Fuel Oils | 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	0	1
Derivative liabilities	4	2
Fuel Oils | Commodity Contract | Significant Other Observable Inputs (Level 2)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets	0	0
Derivative liabilities	0	0
Fuel Oils | Commodity Contract | Significant Other Unobservable Inputs (Level 3)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets	6	7
Derivative liabilities	8	11
Power | Commodity Contract
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets	12	4
Derivative liabilities	208	187
Power | 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	1	0
Derivative liabilities	0	0
Power | Commodity Contract | Significant Other Observable Inputs (Level 2)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets	0	1
Derivative liabilities	1	1
Power | Commodity Contract | Significant Other Unobservable Inputs (Level 3)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets	11	3
Derivative liabilities	207	186
Natural Gas | Commodity Contract
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets	3	3
Derivative liabilities	23	19
Natural Gas | 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	0	0
Derivative liabilities	2	0
Natural Gas | Commodity Contract | Significant Other Observable Inputs (Level 2)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets	0	2
Derivative liabilities	17	15
Natural Gas | Commodity Contract | Significant Other Unobservable Inputs (Level 3)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets	3	1
Derivative liabilities	4	4
Equity Securities | U.S. large capitalization
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund	518	427
Equity Securities | Quoted Prices In Active Markets For Identical Assets or Liabilities (Level 1) | U.S. large capitalization
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund	518	427
Equity Securities | Significant Other Observable Inputs (Level 2) | U.S. large capitalization
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund	0	0
Equity Securities | Significant Other Unobservable Inputs (Level 3) | U.S. large capitalization
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	86	72
Debt Securities | US treasury and government securities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund	136	148
Debt Securities | Other Debt Securities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund	50	32
Debt Securities | Quoted Prices In Active Markets For Identical Assets or Liabilities (Level 1) | Corporate bonds
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund	0	0
Debt Securities | Quoted Prices In Active Markets For Identical Assets or Liabilities (Level 1) | US treasury and government securities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund	0	0
Debt Securities | Quoted Prices In Active Markets For Identical Assets or Liabilities (Level 1) | Other Debt Securities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund	0	0
Debt Securities | Significant Other Observable Inputs (Level 2) | Corporate bonds
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund	86	72
Debt Securities | Significant Other Observable Inputs (Level 2) | US treasury and government securities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund	136	148
Debt Securities | Significant Other Observable Inputs (Level 2) | Other Debt Securities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund	50	32
Debt Securities | Significant Other Unobservable Inputs (Level 3) | Corporate bonds
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund	0	0
Debt Securities | Significant Other Unobservable Inputs (Level 3) | US treasury and government securities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund	0	0
Debt Securities | Significant Other Unobservable Inputs (Level 3) | Other Debt Securities
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	790	679
Assets fair value	808	691
Excluded receivables, payables, and accrued income, net	8	5
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	518	427
Assets fair value	519	428
Union Electric Company | Significant Other Observable Inputs (Level 2)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund	272	252
Assets fair value	272	253
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 fair value	17	10
Union Electric Company | Commodity Contract
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets	18	12
Derivative liabilities	20	22
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	1	1
Derivative liabilities	4	2
Union Electric Company | Commodity Contract | Significant Other Observable Inputs (Level 2)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets	0	1
Derivative liabilities	4	6
Union Electric Company | Commodity Contract | Significant Other Unobservable Inputs (Level 3)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets	17	10
Derivative liabilities	12	14
Union Electric Company | Fuel Oils | Commodity Contract
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets	6	8
Derivative liabilities	12	13
Union Electric Company | Fuel Oils | 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	0	1
Derivative liabilities	4	2
Union Electric Company | Fuel Oils | Commodity Contract | Significant Other Observable Inputs (Level 2)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets	0	0
Derivative liabilities	0	0
Union Electric Company | Fuel Oils | Commodity Contract | Significant Other Unobservable Inputs (Level 3)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets	6	7
Derivative liabilities	8	11
Union Electric Company | Power | Commodity Contract
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets	12	4
Derivative liabilities	4	4
Union Electric Company | Power | 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	1	0
Derivative liabilities	0	0
Union Electric Company | Power | Commodity Contract | Significant Other Observable Inputs (Level 2)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets	0	1
Derivative liabilities	1	1
Union Electric Company | Power | Commodity Contract | Significant Other Unobservable Inputs (Level 3)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets	11	3
Derivative liabilities	3	3
Union Electric Company | Natural Gas | Commodity Contract
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative liabilities	4	5
Union Electric Company | Natural Gas | 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	0	0
Union Electric Company | Natural Gas | Commodity Contract | Significant Other Observable Inputs (Level 2)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative liabilities	3	5
Union Electric Company | Natural Gas | Commodity Contract | Significant Other Unobservable Inputs (Level 3)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative liabilities	1	0
Union Electric Company | Equity Securities | U.S. large capitalization
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund	518	427
Union Electric Company | Equity Securities | Quoted Prices In Active Markets For Identical Assets or Liabilities (Level 1) | U.S. large capitalization
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund	518	427
Union Electric Company | Equity Securities | Significant Other Observable Inputs (Level 2) | U.S. large capitalization
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund	0	0
Union Electric Company | Equity Securities | Significant Other Unobservable Inputs (Level 3) | U.S. large capitalization
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	86	72
Union Electric Company | Debt Securities | US treasury and government securities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund	136	148
Union Electric Company | Debt Securities | Other Debt Securities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund	50	32
Union Electric Company | Debt Securities | Quoted Prices In Active Markets For Identical Assets or Liabilities (Level 1) | Corporate bonds
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund	0	0
Union Electric Company | Debt Securities | Quoted Prices In Active Markets For Identical Assets or Liabilities (Level 1) | US treasury and government securities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund	0	0
Union Electric Company | Debt Securities | Quoted Prices In Active Markets For Identical Assets or Liabilities (Level 1) | Other Debt Securities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund	0	0
Union Electric Company | Debt Securities | Significant Other Observable Inputs (Level 2) | Corporate bonds
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund	86	72
Union Electric Company | Debt Securities | Significant Other Observable Inputs (Level 2) | US treasury and government securities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund	136	148
Union Electric Company | Debt Securities | Significant Other Observable Inputs (Level 2) | Other Debt Securities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund	50	32
Union Electric Company | Debt Securities | Significant Other Unobservable Inputs (Level 3) | Corporate bonds
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund	0	0
Union Electric Company | Debt Securities | Significant Other Unobservable Inputs (Level 3) | US treasury and government securities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund	0	0
Union Electric Company | Debt Securities | Significant Other Unobservable Inputs (Level 3) | Other Debt Securities
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	223	197
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	2	0
Ameren Illinois Company | Commodity Contract | Significant Other Observable Inputs (Level 2)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative liabilities	14	10
Ameren Illinois Company | Commodity Contract | Significant Other Unobservable Inputs (Level 3)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative liabilities	207	187
Ameren Illinois Company | Power | Commodity Contract
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative liabilities	204	183
Ameren Illinois Company | Power | 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	0	0
Ameren Illinois Company | Power | Commodity Contract | Significant Other Observable Inputs (Level 2)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative liabilities	0	0
Ameren Illinois Company | Power | Commodity Contract | Significant Other Unobservable Inputs (Level 3)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative liabilities	204	183
Ameren Illinois Company | Natural Gas | Commodity Contract
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets	3	3
Derivative liabilities	19	14
Ameren Illinois Company | Natural Gas | 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	0	0
Derivative liabilities	2	0
Ameren Illinois Company | Natural Gas | Commodity Contract | Significant Other Observable Inputs (Level 2)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets	0	2
Derivative liabilities	14	10
Ameren Illinois Company | Natural Gas | Commodity Contract | Significant Other Unobservable Inputs (Level 3)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets	3	1
Derivative liabilities	$ 3	$ 4