Retirement Benefits (Schedule Of Matching Contributions) (Details) (401 (K) [Member], USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2012		Dec. 31, 2011		Dec. 31, 2010
Defined Benefit Plan Disclosure [Line Items]
Employer contributions	$ 29	[1]	$ 28	[1]	$ 27	[1]
Union Electric Company [Member]
Defined Benefit Plan Disclosure [Line Items]
Employer contributions	16		16		16
Ameren Illinois Company [Member]
Defined Benefit Plan Disclosure [Line Items]
Employer contributions	9		8		8
Other [Member]
Defined Benefit Plan Disclosure [Line Items]
Employer contributions	$ 4		$ 4		$ 3

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