Retirement Benefits (Summary Of Benefit Plan Costs Incurred) (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2012		Dec. 31, 2011		Dec. 31, 2010		Dec. 31, 2009
Pension Benefits [Member]
Net periodic benefit cost	$ 91	[1]	$ 80	[1]	$ 65	[1]	$ 81	[1]
Postretirement Benefits [Member]
Net periodic benefit cost	$ 23	[1]	$ 25	[1]	$ 21	[1]	$ 34	[1]

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