Summary Of Significant Accounting Policies (Schedule Of Asset Retirement Obligations) (Detail) (USD $) In Millions, unless otherwise specified	9 Months Ended
	Sep. 30, 2012
Balance at December 31, 2011	$ 433	[1],[2]
Liabilities incurred	1	[1]
Liabilities settled	(6)	[1]
Accretion in 2012	19	[1],[3]
Change in estimates	(10)	[1],[4]
Balance at September 30, 2012	437	[1],[5]
Ameren Missouri [Member]
Balance at December 31, 2011	328	[1]
Liabilities settled	(1)	[1]
Accretion in 2012	14	[1],[3]
Change in estimates	1	[1],[4]
Balance at September 30, 2012	342	[1]
Ameren Illinois Company [Member]
Balance at December 31, 2011	3	[6]
Liabilities settled		[6],[7]
Accretion in 2012		[3],[6],[7]
Change in estimates		[4],[6],[7]
Balance at September 30, 2012	3	[6]
Genco [Member]
Balance at December 31, 2011	71	[2]
Liabilities incurred	1
Liabilities settled	(5)
Accretion in 2012	3	[3]
Change in estimates	(11)	[4]
Balance at September 30, 2012	59
AERG [Member]
Balance at December 31, 2011	31
Accretion in 2012	2	[3]
Balance at September 30, 2012	$ 33

[1] The nuclear decommissioning trust fund assets of $407 million and $357 million as of September 30, 2012, and December 31, 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] Accretion expense was recorded as an increase to regulatory assets at Ameren Missouri and Ameren Illinois.

[4] Ameren Missouri and Genco changed their estimates for asbestos removal. The estimates for asbestos removal costs at Genco's Hutsonville and Meredosia energy centers decreased due to less asbestos than anticipated in the energy centers' structures discovered during reviews made after the closure of these energy centers, and more cost efficient removal than anticipated being made possible due to the closure of the energy centers. Additionally, Genco changed the estimates related to retirement costs for its coal combustion byproduct storage areas.

[5] Balance included $8 million in "Other current liabilities" on the balance sheet as of September 30, 2012.

[6] Balance included in "Other deferred credits and liabilities" on the balance sheet.

[7] Less than $1 million.