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, 2017		Dec. 31, 2016
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund		$ 633	[1]	$ 605	[2]
Assets fair value		645	[3]	632	[4]
Excluded receivables, payables, and accrued income, net		2		2
Union Electric Company
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund		633	[1]	605	[2]
Assets fair value		640	[3]	619	[4]
Excluded receivables, payables, and accrued income, net		2		(1)
Commodity Contract
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets		12	[3]	27	[4]
Derivative liabilities		223	[3]	211	[4]
Commodity Contract | Union Electric Company
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets		7	[3]	14	[4]
Derivative liabilities		14	[3]	18	[4]
Commodity Contract | Ameren Illinois Company
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative liabilities		209	[3]	193	[4]
Cash and cash equivalents
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund		1		1
Cash and cash equivalents | Union Electric Company
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund		1		1
Equity Securities | U.S. large capitalization
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund		420		408
Equity Securities | U.S. large capitalization | Union Electric Company
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund		420		408
Debt Securities | Corporate bonds
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund		70		67
Debt Securities | Corporate bonds | Union Electric Company
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund		70		67
Debt Securities | US treasury and government securities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund		118		112
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		112
Debt Securities | Other Debt Securities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund		24		17
Debt Securities | Other Debt Securities | Union Electric Company
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund		24		17
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		421		409
Assets fair value		423	[3]	413	[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		421		409
Assets fair value		422	[3]	411	[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		2	[3]	4	[4]
Derivative liabilities		4	[3]	5	[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		1	[3]	2	[4]
Derivative liabilities		4	[3]	5	[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]	0	[4]
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		1
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		1
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		420		408
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		420		408
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		212		196
Assets fair value		216	[3]	208	[4]
Significant Other Observable Inputs (Level 2) | Union Electric Company
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund		212		196
Assets fair value		212	[3]	197	[4]
Significant Other Observable Inputs (Level 2) | Commodity Contract
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets		4	[3]	12	[4]
Derivative liabilities		21	[3]	14	[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		8	[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		13	[3]	7	[4]
Significant Other Observable Inputs (Level 2) | Cash and cash equivalents
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund		0		0
Significant Other Observable Inputs (Level 2) | Cash and cash equivalents | 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) | 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		70		67
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		70		67
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		112
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		112
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		24		17
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		24		17
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		6	[3]	11	[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		6	[3]	11	[4]
Significant Other Unobservable Inputs (Level 3) | Commodity Contract
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets		6	[3]	11	[4]
Derivative liabilities		198	[3]	192	[4]
Significant Other Unobservable Inputs (Level 3) | Commodity Contract | Union Electric Company
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets		6	[3]	11	[4]
Derivative liabilities		2	[3]	6	[4]
Significant Other Unobservable Inputs (Level 3) | Commodity Contract | Ameren Illinois Company
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative liabilities		196	[3]	186	[4]
Significant Other Unobservable Inputs (Level 3) | Cash and cash equivalents
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund		0		0
Significant Other Unobservable Inputs (Level 3) | Cash and cash equivalents | 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) | 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
Uranium | Commodity Contract
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative liabilities	[4]			4
Uranium | Commodity Contract | Union Electric Company
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative liabilities	[4]			4
Uranium | 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 liabilities	[4]			0
Uranium | 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 liabilities	[4]			0
Uranium | Significant Other Observable Inputs (Level 2) | Commodity Contract
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative liabilities	[4]			0
Uranium | Significant Other Observable Inputs (Level 2) | Commodity Contract | Union Electric Company
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative liabilities	[4]			0
Uranium | Significant Other Unobservable Inputs (Level 3) | Commodity Contract
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative liabilities	[4]			4
Uranium | Significant Other Unobservable Inputs (Level 3) | Commodity Contract | Union Electric Company
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative liabilities	[4]			4
Fuel Oils | Commodity Contract
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets		2	[3]	3	[4]
Derivative liabilities		5	[3]	5	[4]
Fuel Oils | Commodity Contract | Union Electric Company
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets		2	[3]	3	[4]
Derivative liabilities		5	[3]	5	[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		1	[3]	2	[4]
Derivative liabilities		4	[3]	5	[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		1	[3]	2	[4]
Derivative liabilities		4	[3]	5	[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]
Derivative liabilities		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]
Derivative liabilities		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		1	[3]	1	[4]
Derivative liabilities		1	[3]	0	[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		1	[3]	1	[4]
Derivative liabilities		1	[3]	0	[4]
Power | Commodity Contract
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets		5	[3]	9	[4]
Derivative liabilities		195	[3]	188	[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]	3	[4]
Power | Commodity Contract | Ameren Illinois Company
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative liabilities		194	[3]	185	[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]	0	[4]
Derivative liabilities		0	[3]	1	[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]	0	[4]
Derivative liabilities		0	[3]	1	[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
Power | Significant Other Unobservable Inputs (Level 3) | Commodity Contract
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets		5	[3]	9	[4]
Derivative liabilities		195	[3]	187	[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]	9	[4]
Derivative liabilities		1	[3]	2	[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		194	[3]	185	[4]
Natural Gas | Commodity Contract
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets		5	[3]	15	[4]
Derivative liabilities		23	[3]	14	[4]
Natural Gas | Commodity Contract | Union Electric Company
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets	[4]			2
Derivative liabilities		8	[3]	6	[4]
Natural Gas | Commodity Contract | Ameren Illinois Company
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets		5	[3]	13	[4]
Derivative liabilities		15	[3]	8	[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		1	[3]	2	[4]
Derivative liabilities		0	[3]	0	[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		1	[3]	2	[4]
Derivative liabilities		0	[3]	0	[4]
Natural Gas | Significant Other Observable Inputs (Level 2) | Commodity Contract
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets		4	[3]	12	[4]
Derivative liabilities		21	[3]	13	[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]			1
Derivative liabilities		8	[3]	6	[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		4	[3]	11	[4]
Derivative liabilities		13	[3]	7	[4]
Natural Gas | Significant Other Unobservable Inputs (Level 3) | Commodity Contract
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets	[4]			1
Derivative liabilities		2	[3]	1	[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	[3]	0
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		0	[3]	0	[4]
Derivative liabilities		$ 2	[3]	$ 1	[4]

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

[2] Balance excludes $2 million of 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.