Fair Value Measurements (Schedule Of Fair Value Hierarchy Of Assets And Liabilities Measured At Fair Value On Recurring Basis) (Detail) (USD $) In Millions, unless otherwise specified	Sep. 30, 2014		Dec. 31, 2013
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund	$ 527	[1]	$ 494
Assets fair value	543	[2]	528	[2]
Excluded receivables, payables, and accrued income, net	2
Union Electric Company
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund	527	[1]	494
Assets fair value	541	[2]	527	[2]
Commodity Contract
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets	16	[2]	34	[2]
Derivative liabilities	167	[2]	178	[2]
Commodity Contract | Union Electric Company
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets	14	[2]	33	[2]
Derivative liabilities	21	[2]	24	[2]
Commodity Contract | Ameren Illinois Company
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets	2		1
Derivative liabilities	146	[2]	154	[2]
Commodity Contract | Fuel Oils
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets	3	[2]	9	[2]
Derivative liabilities	6	[2]	3	[2]
Commodity Contract | Fuel Oils | Union Electric Company
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets	3	[2]	9	[2]
Derivative liabilities	6	[2]	3	[2]
Commodity Contract | Natural Gas
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets	2	[2]	2	[2]
Derivative liabilities	28	[2]	57	[2]
Commodity Contract | Natural Gas | Union Electric Company
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets			1	[2]
Derivative liabilities	6	[2]	11	[2]
Commodity Contract | Natural Gas | Ameren Illinois Company
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets	2	[2]	1	[2]
Derivative liabilities	22	[2]	46	[2]
Commodity Contract | Power
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets	11	[2]	23	[2]
Derivative liabilities	130	[2]	112	[2]
Commodity Contract | Power | Union Electric Company
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets	11	[2]	23	[2]
Derivative liabilities	6	[2]	4	[2]
Commodity Contract | Power | Ameren Illinois Company
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative liabilities	124	[2]	108	[2]
Commodity Contract | Uranium
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative liabilities	3	[2]	6	[2]
Commodity Contract | Uranium | Union Electric Company
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative liabilities	3	[2]	6	[2]
Cash and cash equivalents
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund	1		3
Cash and cash equivalents | Union Electric Company
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund	1		3
Equity Securities | U.S. large capitalization
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund	348		332
Equity Securities | U.S. large capitalization | Union Electric Company
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund	348		332
Debt Securities | Corporate bonds
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund	60		52
Debt Securities | Corporate bonds | Union Electric Company
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund	60		52
Debt Securities | Municipal bonds
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund	2		2
Debt Securities | Municipal bonds | Union Electric Company
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund	2		2
Debt Securities | US treasury and government securities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund	100		94
Debt Securities | US treasury and government securities | Union Electric Company
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund	100		94
Debt Securities | Asset-backed Securities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund	11		10
Debt Securities | Asset-backed Securities | Union Electric Company
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund	11		10
Debt Securities | Other Debt Securities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund	5		1
Debt Securities | Other Debt Securities | Union Electric Company
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund	5		1
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	349		335
Assets fair value	349	[2]	336	[2]
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	349		335
Assets fair value	349	[2]	336	[2]
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		[2]	1	[2]
Derivative liabilities	5	[2]	3	[2]
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		[2]	1	[2]
Derivative liabilities	5	[2]	3	[2]
Quoted Prices In Active Markets For Identical Assets or Liabilities (Level 1) | Commodity Contract | Fuel Oils
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets		[2]	1	[2]
Derivative liabilities	3	[2]
Quoted Prices In Active Markets For Identical Assets or Liabilities (Level 1) | Commodity Contract | Fuel Oils | Union Electric Company
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets		[2]	1	[2]
Derivative liabilities	3	[2]
Quoted Prices In Active Markets For Identical Assets or Liabilities (Level 1) | Commodity Contract | Natural Gas
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative liabilities	2	[2]	3	[2]
Quoted Prices In Active Markets For Identical Assets or Liabilities (Level 1) | Commodity Contract | Natural Gas | Union Electric Company
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets				[2]
Derivative liabilities	2	[2]	3	[2]
Quoted Prices In Active Markets For Identical Assets or Liabilities (Level 1) | Commodity Contract | Natural Gas | Ameren Illinois Company
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative liabilities				[2]
Quoted Prices In Active Markets For Identical Assets or Liabilities (Level 1) | Cash and cash equivalents
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund	1		3
Quoted Prices In Active Markets For Identical Assets or Liabilities (Level 1) | Cash and cash equivalents | Union Electric Company
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund	1		3
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	348		332
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	348		332
Significant Other Observable Inputs (Level 2)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund	178		159
Assets fair value	180	[2]	163	[2]
Significant Other Observable Inputs (Level 2) | Union Electric Company
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund	178		159
Assets fair value	179	[2]	162	[2]
Significant Other Observable Inputs (Level 2) | Commodity Contract
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets	2	[2]	4	[2]
Derivative liabilities	27	[2]	56	[2]
Significant Other Observable Inputs (Level 2) | Commodity Contract | Union Electric Company
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets	1	[2]	3	[2]
Derivative liabilities	5	[2]	10	[2]
Significant Other Observable Inputs (Level 2) | Commodity Contract | Ameren Illinois Company
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative liabilities	22	[2]	46	[2]
Significant Other Observable Inputs (Level 2) | Commodity Contract | Natural Gas
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets	1	[2]	2	[2]
Derivative liabilities	26	[2]	54	[2]
Significant Other Observable Inputs (Level 2) | Commodity Contract | Natural Gas | Union Electric Company
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets			1	[2]
Derivative liabilities	4	[2]	8	[2]
Significant Other Observable Inputs (Level 2) | Commodity Contract | Natural Gas | Ameren Illinois Company
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets	1	[2]	1	[2]
Derivative liabilities	22	[2]	46	[2]
Significant Other Observable Inputs (Level 2) | Commodity Contract | Power
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets	1	[2]	2	[2]
Derivative liabilities	1	[2]	2	[2]
Significant Other Observable Inputs (Level 2) | Commodity Contract | Power | Union Electric Company
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets	1	[2]	2	[2]
Derivative liabilities	1	[2]	2	[2]
Significant Other Observable Inputs (Level 2) | Debt Securities | Corporate bonds
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund	60		52
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	60		52
Significant Other Observable Inputs (Level 2) | Debt Securities | Municipal bonds
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund	2		2
Significant Other Observable Inputs (Level 2) | Debt Securities | Municipal bonds | Union Electric Company
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund	2		2
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	100		94
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	100		94
Significant Other Observable Inputs (Level 2) | Debt Securities | Asset-backed Securities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund	11		10
Significant Other Observable Inputs (Level 2) | Debt Securities | Asset-backed Securities | Union Electric Company
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund	11		10
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	5		1
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	5		1
Significant Other Unobservable Inputs (Level 3)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Assets fair value	14	[2]	29	[2]
Significant Other Unobservable Inputs (Level 3) | Union Electric Company
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Assets fair value	13	[2]	29	[2]
Significant Other Unobservable Inputs (Level 3) | Commodity Contract
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets	14	[2]	29	[2]
Derivative liabilities	135	[2]	119	[2]
Significant Other Unobservable Inputs (Level 3) | Commodity Contract | Union Electric Company
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets	13	[2]	29	[2]
Derivative liabilities	11	[2]	11	[2]
Significant Other Unobservable Inputs (Level 3) | Commodity Contract | Ameren Illinois Company
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative liabilities	124	[2]	108	[2]
Significant Other Unobservable Inputs (Level 3) | Commodity Contract | Fuel Oils
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets	3	[2]	8	[2]
Derivative liabilities	3	[2]	3	[2]
Significant Other Unobservable Inputs (Level 3) | Commodity Contract | Fuel Oils | Union Electric Company
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets	3	[2]	8	[2]
Derivative liabilities	3	[2]	3	[2]
Significant Other Unobservable Inputs (Level 3) | Commodity Contract | Natural Gas
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets	1	[2]
Derivative liabilities		[2]
Significant Other Unobservable Inputs (Level 3) | Commodity Contract | Natural Gas | Ameren Illinois Company
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets	1	[2]		[2]
Significant Other Unobservable Inputs (Level 3) | Commodity Contract | Power
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets	10	[2]	21	[2]
Derivative liabilities	129	[2]	110	[2]
Significant Other Unobservable Inputs (Level 3) | Commodity Contract | Power | Union Electric Company
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets	10	[2]	21	[2]
Derivative liabilities	5	[2]	2	[2]
Significant Other Unobservable Inputs (Level 3) | Commodity Contract | Power | Ameren Illinois Company
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative liabilities	124	[2]	108	[2]
Significant Other Unobservable Inputs (Level 3) | Commodity Contract | Uranium
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative liabilities	3	[2]	6	[2]
Significant Other Unobservable Inputs (Level 3) | Commodity Contract | Uranium | Union Electric Company
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative liabilities	$ 3	[2]	$ 6	[2]

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

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