Retirement Benefits (Schedule Of Matching Contributions) (Details) (401 (K) [Member], USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2011		Dec. 31, 2010		Dec. 31, 2009
401 (K) [Member]
Employer contributions	$ 28	[1]	$ 27	[1]	$ 24	[1]

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