Other Income And Expenses (Other Income And Expenses) (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2011		Dec. 31, 2010		Dec. 31, 2009
Other Income And Expenses [Abstract]
Interest and dividend income	$ 4	[1]	$ 5	[1]	$ 2	[1]
Interest income on industrial development revenue bonds	28	[1]	28	[1]	28	[1]
Allowance for equity funds used during construction	34	[1]	52	[1]	36	[1]
Other	3	[1]	5	[1]	5	[1]
Total miscellaneous income	69	[1]	90	[1]	71	[1]
Donations	8	[1]	19	[1]	12	[1]
Other	15	[1]	14	[1]	11	[1]
Total miscellaneous expense	$ 23	[1]	$ 33	[1]	$ 23	[1]

[1] Includes amounts for Ameren registrant and nonregistrant subsidiaries and intercompany eliminations