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, 2016		Dec. 31, 2015
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund		$ 605	[1]	$ 557	[2]
Assets		632	[3]	575	[4]
Excluded receivables, payables, and accrued income, net		2		(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		409		368
Assets		413	[3]	368	[4]
Significant Other Observable Inputs (Level 2)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund		196		189
Assets		208	[3]	190	[4]
Significant Other Unobservable Inputs (Level 3)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Assets		11	[3]	17	[4]
Cash And Cash Equivalents
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund		1		4
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		1		4
Commodity Contract
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets		27	[3]	18	[4]
Derivative liabilities		211	[3]	264	[4]
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	[3]	4
Derivative liabilities		5	[3]	30	[4]
Commodity Contract | Significant Other Observable Inputs (Level 2)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets		12	[3]	1	[4]
Derivative liabilities		14	[3]	62	[4]
Commodity Contract | Significant Other Unobservable Inputs (Level 3)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets		11	[3]	17	[4]
Derivative liabilities		192	[3]	172	[4]
Fuel Oils
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets	[3]	3
Derivative liabilities		5	[3]	29	[4]
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	[3]	2
Derivative liabilities		5	[3]	29	[4]
Fuel Oils | Significant Other Unobservable Inputs (Level 3)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets	[3]	1
Derivative liabilities	[3]
Natural Gas
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets		15	[3]	2	[4]
Derivative liabilities		14	[3]	64	[4]
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	[3]	2
Derivative liabilities		0	[3]	1	[4]
Natural Gas | Significant Other Observable Inputs (Level 2)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets		12	[3]	1	[4]
Derivative liabilities		13	[3]	62	[4]
Natural Gas | Significant Other Unobservable Inputs (Level 3)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets	[3]	1
Derivative liabilities		1	[3]	1	[4]
Power
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets		9	[3]	16	[4]
Derivative liabilities		188	[3]	170	[4]
Power | Significant Other Observable Inputs (Level 2)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets	[4]			0
Derivative liabilities		1	[3]	0	[4]
Power | Significant Other Unobservable Inputs (Level 3)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets	[3]	9
Derivative liabilities		187	[3]	170	[4]
Uranium
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative liabilities		4	[3]	1	[4]
Uranium | Significant Other Unobservable Inputs (Level 3)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative liabilities		4	[3]	1	[4]
Equity Securities | U.S. Large Capitalization
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund		408		364
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		408		364
Debt Securities | Corporate Bonds
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund		67		58
Debt Securities | Corporate Bonds | Significant Other Observable Inputs (Level 2)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund		67		58
Debt Securities | U.S. treasury and agency securities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund		112		109
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		112		109
Debt Securities | Other debt securities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund		17		22
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		17		22
Union Electric Company
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund		605	[1]	557	[2]
Assets		619	[3]	574	[4]
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		409		368
Assets		411	[3]	368	[4]
Union Electric Company | Significant Other Observable Inputs (Level 2)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund		196		189
Assets		197	[3]	189	[4]
Union Electric Company | Significant Other Unobservable Inputs (Level 3)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Assets		11	[3]	17	[4]
Union Electric Company | Cash And Cash Equivalents
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund		1		4
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		1		4
Union Electric Company | Commodity Contract
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets		14	[3]	17	[4]
Derivative liabilities		18	[3]	44	[4]
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	[3]	2
Derivative liabilities		5	[3]	29	[4]
Union Electric Company | Commodity Contract | Significant Other Observable Inputs (Level 2)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets		1	[3]	0	[4]
Derivative liabilities		7	[3]	13	[4]
Union Electric Company | Commodity Contract | Significant Other Unobservable Inputs (Level 3)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets		11	[3]	17	[4]
Derivative liabilities		6	[3]	2	[4]
Union Electric Company | Fuel Oils
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets	[3]	3
Derivative liabilities		5	[3]	29	[4]
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	[3]	2
Derivative liabilities		5	[3]	29	[4]
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
Union Electric Company | Natural Gas
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets		2	[3]	1	[4]
Derivative liabilities		6	[3]	14	[4]
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 liabilities	[4]			0
Union Electric Company | Natural Gas | Significant Other Observable Inputs (Level 2)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets	[3]	1
Derivative liabilities		6	[3]	13	[4]
Union Electric Company | Natural Gas | Significant Other Unobservable Inputs (Level 3)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets		1	[3]	1	[4]
Derivative liabilities	[4]			1
Union Electric Company | Power
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets		9	[3]	16	[4]
Derivative liabilities	[3]	3
Union Electric Company | Power | Significant Other Observable Inputs (Level 2)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets	[4]			0
Derivative liabilities	[3]	1
Union Electric Company | Power | Significant Other Unobservable Inputs (Level 3)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets		9	[3]	16	[4]
Derivative liabilities	[3]	2
Union Electric Company | Uranium
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative liabilities		4	[3]	1	[4]
Union Electric Company | Uranium | Significant Other Unobservable Inputs (Level 3)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative liabilities		4	[3]	1	[4]
Union Electric Company | Equity Securities | U.S. Large Capitalization
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund		408		364
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		408		364
Union Electric Company | Debt Securities | Corporate Bonds
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund		67		58
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		67		58
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		112		109
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		112		109
Union Electric Company | Debt Securities | Other debt securities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund		17		22
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		17		22
Ameren Illinois Company | Commodity Contract
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative liabilities		193	[3]	220	[4]
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	[4]			1
Ameren Illinois Company | Commodity Contract | Significant Other Observable Inputs (Level 2)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative liabilities		7	[3]	49	[4]
Ameren Illinois Company | Commodity Contract | Significant Other Unobservable Inputs (Level 3)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative liabilities		186	[3]	170	[4]
Ameren Illinois Company | Natural Gas
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets		13	[3]	1	[4]
Derivative liabilities		8	[3]	50	[4]
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	[3]	2
Derivative liabilities	[4]			1
Ameren Illinois Company | Natural Gas | Significant Other Observable Inputs (Level 2)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets		11	[3]	1	[4]
Derivative liabilities		7	[3]	49	[4]
Ameren Illinois Company | Natural Gas | Significant Other Unobservable Inputs (Level 3)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative liabilities	[3]	1
Ameren Illinois Company | Power
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative liabilities		185	[3]	170	[4]
Ameren Illinois Company | Power | Significant Other Unobservable Inputs (Level 3)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative liabilities		$ 185	[3]	$ 170	[4]

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

[2] Balance excludes $(1) 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.