Summary Of Significant Accounting Policies (Schedule Of Asset Retirement Obligations) (Details) (USD $) In Millions	9 Months Ended
	Sep. 30, 2011		Dec. 31, 2010
Regulatory Assets [Line Items]
Balance at December 31, 2010	$ 475	[1],[2]
Liabilities incurred		[1],[2],[3]
Liabilities settled	(2)	[1],[2]
Accretion in 2011	21	[1],[2],[4]
Change in estimates	(49)	[1],[2],[5]
Balance at September 30, 2011	445	[1],[2],[6]
Nuclear decommissioning trust fund	330		337
Other current liabilities	251		283
Asset Retirement Obligation [Member]
Regulatory Assets [Line Items]
Other current liabilities	$ 6

[1] Includes amounts for Ameren registrant and nonregistrant subsidiaries.

[2] The nuclear decommissioning trust fund assets of $330 million and $337 million as of September 30, 2011, and December 31, 2010, respectively, were restricted for decommissioning of the Callaway energy center.

[3] Less than $1 million.

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

[5] Ameren Missouri changed estimates related to its Callaway energy center decommissioning costs because of a cost study performed in 2011 and a decline in the cost escalation factor assumptions. Ameren Missouri and Genco changed estimates related to retirement costs for asbestos removal and river structures. Additionally, Genco and AERG changed estimates related to retirement costs for their coal combustion byproduct storage areas.

[6] Balance included $6 million in "Other current liabilities" on the balance sheet as of September 30, 2011.