Retirement Benefits (Cash Contributions Made To Benefit Plans) (Details) (USD $)	12 Months Ended
	Dec. 31, 2011		Dec. 31, 2010		Dec. 31, 2009
Pension Benefits [Member]
Cash contributions to benefit plans	$ 103,000,000	[1]	$ 81,000,000	[1]	$ 99,000,000	[1]
Postretirement Benefits [Member]
Cash contributions to benefit plans	$ 129,000,000	[1]	$ 36,000,000	[1]	$ 49,000,000	[1]

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