Summary Of Significant Accounting Policies (Schedule Of Asset Retirement Obligations) (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2011		Dec. 31, 2010
Summary Of Significant Accounting Policies [Abstract]
Balance	$ 475	[1]	$ 434	[1]
Liabilities incurred			8	[1]
Liabilities settled	(3)	[1]	(4)	[1]
Accretion in period	27	[1],[2]	26	[1],[2]
Change in estimates	(66)	[1],[3]	11	[1],[4]
Balance	433	[1],[5]	475	[1]
Nuclear decommissioning trust fund	357		337
Asset retirement obligation included in other current liabilities	$ 5

[1] The nuclear decommissioning trust fund assets of $357 million and $337 million as of December 31, 2011, and 2010, respectively, were restricted for decommissioning of the Callaway energy center.

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

[3] 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. Additionally, Ameren Missouri, Genco and AERG changed estimates related to retirement costs for asbestos removal, river structures and their coal combustion byproduct storage areas.

[4] Ameren Missouri and Genco changed their estimates for asbestos removal. Additionally, Genco changed the estimates related to retirement costs for its coal combustion byproduct storage areas.

[5] Balance included $5 million in "Other current liabilities" on the balance sheet as of December 31, 2011.