Fair Value Measurements (Schedule Of Fair Value Hierarchy Of Assets And Liabilities Measured At Fair Value On Recurring Basis) (Detail) - USD ($) $ in Millions		Mar. 31, 2018		Dec. 31, 2017
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund		$ 694	[1]	$ 700	[2]
Assets fair value		708	[3]	717	[4]
Excluded receivables, payables, and accrued income, net		4		4
Union Electric Company
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund		694	[1]	700	[2]
Assets fair value		707	[3]	717	[4]
Excluded receivables, payables, and accrued income, net		4		4
Commodity Contract
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets		14	[3]	17	[4]
Derivative liabilities		230	[3]	226	[4]
Commodity Contract | Union Electric Company
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets		13	[3]	17	[4]
Derivative liabilities		10	[3]	9	[4]
Commodity Contract | Ameren Illinois Company
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative liabilities		220	[3]	217	[4]
Equity Securities | U.S. large capitalization
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund		464		468
Equity Securities | U.S. large capitalization | Union Electric Company
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund		464		468
Debt Securities | Corporate bonds
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund		80		82
Debt Securities | Corporate bonds | Union Electric Company
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund		80		82
Debt Securities | US treasury and government securities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund		118		125
Debt Securities | US treasury and government securities | Union Electric Company
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund		118		125
Debt Securities | Other Debt Securities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund		32		25
Debt Securities | Other Debt Securities | Union Electric Company
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund		32		25
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		464		468
Assets fair value		470	[3]	472	[4]
Quoted Prices In Active Markets For Identical Assets or Liabilities (Level 1) | Union Electric Company
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund		464		468
Assets fair value		470	[3]	472	[4]
Quoted Prices In Active Markets For Identical Assets or Liabilities (Level 1) | Commodity Contract
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets		6	[3]	4	[4]
Derivative liabilities		0	[3]	1	[4]
Quoted Prices In Active Markets For Identical Assets or Liabilities (Level 1) | Commodity Contract | Union Electric Company
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets		6	[3]	4	[4]
Derivative liabilities		0	[3]	0	[4]
Quoted Prices In Active Markets For Identical Assets or Liabilities (Level 1) | Commodity Contract | Ameren Illinois Company
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative liabilities		0	[3]	1	[4]
Quoted Prices In Active Markets For Identical Assets or Liabilities (Level 1) | Equity Securities | U.S. large capitalization
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund		464		468
Quoted Prices In Active Markets For Identical Assets or Liabilities (Level 1) | Equity Securities | U.S. large capitalization | Union Electric Company
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund		464		468
Quoted Prices In Active Markets For Identical Assets or Liabilities (Level 1) | Debt Securities | Corporate bonds
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund		0		0
Quoted Prices In Active Markets For Identical Assets or Liabilities (Level 1) | Debt Securities | Corporate bonds | Union Electric Company
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund		0		0
Quoted Prices In Active Markets For Identical Assets or Liabilities (Level 1) | Debt Securities | US treasury and government securities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund		0		0
Quoted Prices In Active Markets For Identical Assets or Liabilities (Level 1) | Debt Securities | US treasury and government securities | Union Electric Company
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund		0		0
Quoted Prices In Active Markets For Identical Assets or Liabilities (Level 1) | Debt Securities | Other Debt Securities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund		0		0
Quoted Prices In Active Markets For Identical Assets or Liabilities (Level 1) | Debt Securities | Other Debt Securities | Union Electric Company
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund		0		0
Significant Other Observable Inputs (Level 2)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund		230		232
Assets fair value		231	[3]	233	[4]
Significant Other Observable Inputs (Level 2) | Union Electric Company
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund		230		232
Assets fair value		230	[3]	233	[4]
Significant Other Observable Inputs (Level 2) | Commodity Contract
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets		1	[3]	1	[4]
Derivative liabilities		32	[3]	25	[4]
Significant Other Observable Inputs (Level 2) | Commodity Contract | Union Electric Company
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets		0	[3]	1	[4]
Derivative liabilities		9	[3]	7	[4]
Significant Other Observable Inputs (Level 2) | Commodity Contract | Ameren Illinois Company
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative liabilities		23	[3]	18	[4]
Significant Other Observable Inputs (Level 2) | Equity Securities | U.S. large capitalization
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund		0		0
Significant Other Observable Inputs (Level 2) | Equity Securities | U.S. large capitalization | Union Electric Company
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund		0		0
Significant Other Observable Inputs (Level 2) | Debt Securities | Corporate bonds
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund		80		82
Significant Other Observable Inputs (Level 2) | Debt Securities | Corporate bonds | Union Electric Company
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund		80		82
Significant Other Observable Inputs (Level 2) | Debt Securities | US treasury and government securities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund		118		125
Significant Other Observable Inputs (Level 2) | Debt Securities | US treasury and government securities | Union Electric Company
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund		118		125
Significant Other Observable Inputs (Level 2) | Debt Securities | Other Debt Securities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund		32		25
Significant Other Observable Inputs (Level 2) | Debt Securities | Other Debt Securities | Union Electric Company
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund		32		25
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		7	[3]	12	[4]
Significant Other Unobservable Inputs (Level 3) | Union Electric Company
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund		0		0
Assets fair value		7	[3]	12	[4]
Significant Other Unobservable Inputs (Level 3) | Commodity Contract
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets		7	[3]	12	[4]
Derivative liabilities		198	[3]	200	[4]
Significant Other Unobservable Inputs (Level 3) | Commodity Contract | Union Electric Company
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets		7	[3]	12	[4]
Derivative liabilities		1	[3]	2	[4]
Significant Other Unobservable Inputs (Level 3) | Commodity Contract | Ameren Illinois Company
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative liabilities		197	[3]	198	[4]
Significant Other Unobservable Inputs (Level 3) | Equity Securities | U.S. large capitalization
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund		0		0
Significant Other Unobservable Inputs (Level 3) | Equity Securities | U.S. large capitalization | Union Electric Company
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund		0		0
Significant Other Unobservable Inputs (Level 3) | Debt Securities | Corporate bonds
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund		0		0
Significant Other Unobservable Inputs (Level 3) | Debt Securities | Corporate bonds | Union Electric Company
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund		0		0
Significant Other Unobservable Inputs (Level 3) | Debt Securities | US treasury and government securities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund		0		0
Significant Other Unobservable Inputs (Level 3) | Debt Securities | US treasury and government securities | Union Electric Company
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund		0		0
Significant Other Unobservable Inputs (Level 3) | Debt Securities | Other Debt Securities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund		0		0
Significant Other Unobservable Inputs (Level 3) | Debt Securities | Other Debt Securities | Union Electric Company
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund		0		0
Fuel Oils | Commodity Contract
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets		8	[3]	7	[4]
Fuel Oils | Commodity Contract | Union Electric Company
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets		8	[3]	7	[4]
Fuel Oils | Quoted Prices In Active Markets For Identical Assets or Liabilities (Level 1) | Commodity Contract
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets		6	[3]	4	[4]
Fuel Oils | Quoted Prices In Active Markets For Identical Assets or Liabilities (Level 1) | Commodity Contract | Union Electric Company
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets		6	[3]	4	[4]
Fuel Oils | Significant Other Observable Inputs (Level 2) | Commodity Contract
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets		0	[3]	0	[4]
Fuel Oils | Significant Other Observable Inputs (Level 2) | Commodity Contract | Union Electric Company
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets		0	[3]	0	[4]
Fuel Oils | Significant Other Unobservable Inputs (Level 3) | Commodity Contract
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets		2	[3]	3	[4]
Fuel Oils | Significant Other Unobservable Inputs (Level 3) | Commodity Contract | Union Electric Company
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets		2	[3]	3	[4]
Power | Commodity Contract
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets		5	[3]	9	[4]
Derivative liabilities		192	[3]	196	[4]
Power | Commodity Contract | Union Electric Company
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets		5	[3]	9	[4]
Derivative liabilities		1	[3]	1	[4]
Power | Commodity Contract | Ameren Illinois Company
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative liabilities		191	[3]	195	[4]
Power | Quoted Prices In Active Markets For Identical Assets or Liabilities (Level 1) | Commodity Contract
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets		0	[3]	0	[4]
Derivative liabilities		0	[3]	0	[4]
Power | Quoted Prices In Active Markets For Identical Assets or Liabilities (Level 1) | Commodity Contract | Union Electric Company
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets		0	[3]	0	[4]
Derivative liabilities		0	[3]	0	[4]
Power | Quoted Prices In Active Markets For Identical Assets or Liabilities (Level 1) | Commodity Contract | Ameren Illinois Company
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative liabilities		0	[3]	0	[4]
Power | Significant Other Observable Inputs (Level 2) | Commodity Contract
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets		0	[3]	1	[4]
Derivative liabilities		0	[3]	0	[4]
Power | Significant Other Observable Inputs (Level 2) | Commodity Contract | Union Electric Company
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets		0	[3]	1	[4]
Derivative liabilities		0	[3]	0	[4]
Power | Significant Other Observable Inputs (Level 2) | Commodity Contract | Ameren Illinois Company
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative liabilities		0	[3]	0	[4]
Power | Significant Other Unobservable Inputs (Level 3) | Commodity Contract
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets		5	[3]	8	[4]
Derivative liabilities		192	[3]	196	[4]
Power | Significant Other Unobservable Inputs (Level 3) | Commodity Contract | Union Electric Company
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets		5	[3]	8	[4]
Derivative liabilities		1	[3]	1	[4]
Power | Significant Other Unobservable Inputs (Level 3) | Commodity Contract | Ameren Illinois Company
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative liabilities		191	[3]	195	[4]
Natural Gas | Commodity Contract
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets		1	[3]	1	[4]
Derivative liabilities		38	[3]	30	[4]
Natural Gas | Commodity Contract | Union Electric Company
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets	[4]			1
Derivative liabilities		9	[3]	8	[4]
Natural Gas | Commodity Contract | Ameren Illinois Company
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets	[3]	1
Derivative liabilities		29	[3]	22	[4]
Natural Gas | Quoted Prices In Active Markets For Identical Assets or Liabilities (Level 1) | Commodity Contract
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets		0	[3]	0	[4]
Derivative liabilities		0	[3]	1	[4]
Natural Gas | Quoted Prices In Active Markets For Identical Assets or Liabilities (Level 1) | Commodity Contract | Union Electric Company
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets	[4]			0
Derivative liabilities		0	[3]	0	[4]
Natural Gas | Quoted Prices In Active Markets For Identical Assets or Liabilities (Level 1) | Commodity Contract | Ameren Illinois Company
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets	[3]	0
Derivative liabilities		0	[3]	1	[4]
Natural Gas | Significant Other Observable Inputs (Level 2) | Commodity Contract
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets		1	[3]	0	[4]
Derivative liabilities		32	[3]	25	[4]
Natural Gas | Significant Other Observable Inputs (Level 2) | Commodity Contract | Union Electric Company
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets	[4]			0
Derivative liabilities		9	[3]	7	[4]
Natural Gas | Significant Other Observable Inputs (Level 2) | Commodity Contract | Ameren Illinois Company
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets	[3]	1
Derivative liabilities		23	[3]	18	[4]
Natural Gas | Significant Other Unobservable Inputs (Level 3) | Commodity Contract
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets		0	[3]	1	[4]
Derivative liabilities		6	[3]	4	[4]
Natural Gas | Significant Other Unobservable Inputs (Level 3) | Commodity Contract | Union Electric Company
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets	[4]			1
Derivative liabilities		0	[3]	1	[4]
Natural Gas | Significant Other Unobservable Inputs (Level 3) | Commodity Contract | Ameren Illinois Company
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets	[3]	0
Derivative liabilities		$ 6	[3]	$ 3	[4]

[1] Balance excludes $4 million of cash and cash equivalents, receivables, payables, and accrued income, net.

[2] Balance excludes $4 million of cash and cash equivalents, receivables, payables, and accrued income, net.

[3] The derivative asset and liability balances are presented net of counterparty credit considerations.

[4] The derivative asset and liability balances are presented net of counterparty credit considerations.