Summary Of Significant Accounting Policies (Schedule Of Asset Retirement Obligations) (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2012		Dec. 31, 2011
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]
Balance	$ 433	[1],[2]	$ 475	[1]
Liabilities incurred	2	[1]	1	[1],[3]
Liabilities settled	(6)	[1]	(3)	[1]
Accretion in period	24	[1],[4]	27	[1],[4]
Change in estimates	1	[1],[3],[5]	(66)	[1],[6]
Balance	453	[1],[7]	433	[1],[2]
Nuclear decommissioning trust fund	408		357
Asset retirement obligation included in other current liabilities	8		5
Union Electric Company [Member]
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]
Balance	328	[1]	363	[1]
Liabilities incurred		[1]		[1]
Liabilities settled	(1)	[1]	(1)	[1]
Accretion in period	18	[1],[4]	20	[1],[4]
Change in estimates	1	[1],[5]	(54)	[1],[6]
Balance	346	[1]	328	[1]
Nuclear decommissioning trust fund	408		357
Ameren Illinois Company [Member]
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]
Balance	3	[8]	3	[8]
Liabilities incurred		[8]		[8]
Liabilities settled	1	[3],[8]	1	[3],[8]
Accretion in period	1	[3],[4],[8]	1	[3],[4],[8]
Change in estimates	1	[3],[5],[8]	1	[3],[6],[8]
Balance	3	[8]	3	[8]
Ameren Energy Generating Company [Member]
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]
Balance	71		74
Liabilities incurred	2		1	[3]
Liabilities settled	(5)		(2)
Accretion in period	4	[4]	5	[4]
Change in estimates	(3)	[5]	(6)	[6]
Balance	69		71
Ameren Energy Resources Generating Company [Member]
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]
Balance	31		35
Liabilities incurred
Liabilities settled	1	[3]	1	[3]
Accretion in period	2	[4]	2	[4]
Change in estimates	2	[5]	(6)	[6]
Balance	$ 35		$ 31

[1] The nuclear decommissioning trust fund assets of $408 million and $357 million as of December 31, 2012, and 2011, respectively, were restricted for decommissioning of the Callaway energy center.

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

[3] (c)Less than $1 million.

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

[5] Ameren Missouri and Genco changed their fair value estimates for asbestos removal. The estimates for asbestos removal costs at Genco's Hutsonville and Meredosia energy centers decreased because less asbestos than anticipated was found in the energy centers' structures during reviews made after the closure of these energy centers, and because removal was more cost efficient than anticipated due to the closure. Additionally, Genco and AERG changed their fair value estimates related to updated retirement dates for certain CCR storage facilities.

[6] Ameren Missouri changed its fair value estimate 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 their fair value estimates related to retirement costs for asbestos removal, river structures and their CCR storage facilities.

[7] Balance included $8 million in "Other current liabilities" on the balance sheet as of December 31, 2012.

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