Summary Of Significant Accounting Policies Summary of Significant Accounting Policies (Schedule of Asset Retirement Obligations) (Details) (USD $) In Millions, unless otherwise specified	9 Months Ended
	Sep. 30, 2013		Dec. 31, 2012
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]
Balance at December 31, 2012	$ 375	[1]
Liabilities incurred	0	[1]
Liabilities settled	1	[1],[2]
Accretion in 2013	15	[1],[3]
Change in estimates	2	[1],[4]
Balance at September 30, 2013	392	[1]
Nuclear decommissioning trust fund	459		408
Union Electric Company
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]
Balance at December 31, 2012	346	[1]
Liabilities incurred	0	[1]
Liabilities settled	1	[1],[2]
Accretion in 2013	14	[1],[3]
Change in estimates	2	[1],[4]
Balance at September 30, 2013	362	[1]
Nuclear decommissioning trust fund	459		408
Ameren Illinois Company
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]
Balance at December 31, 2012	3	[5]
Liabilities incurred	0	[5]
Liabilities settled	1	[2],[5]
Accretion in 2013	1	[2],[3],[5]
Change in estimates	1	[2],[4],[5]
Balance at September 30, 2013	3	[5]
Other
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]
Balance at December 31, 2012	26	[6]
Liabilities incurred	0	[6]
Liabilities settled	1	[2],[6]
Accretion in 2013	1	[3],[6]
Change in estimates	1	[2],[4],[6]
Balance at September 30, 2013	$ 27	[6]

[1] The nuclear decommissioning trust fund assets of $459 million and $408 million as of September 30, 2013, and December 31, 2012, respectively, were restricted for decommissioning of the Callaway energy center.

[2] Less than $1 million.

[3] Accretion was recorded as an increase to regulatory assets at Ameren Missouri and Ameren Illinois.

[4] Ameren Missouri changed its fair value estimates for asbestos removal and certain CCR storage facilities.

[5] Balance included in “Other deferred credits and liabilities” on the balance sheet.

[6] Represents amounts for the Meredosia and Hutsonville energy centers. Pursuant to the transaction agreement to divest New AER to IPH, Ameren will retain the AROs associated with the Meredosia and Hutsonville energy centers. See Note 2 - Divestiture Transactions and Discontinued Operations for additional information.