Retirement Benefits (Schedule Of Matching Contributions) (Details) (401 (K), USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2014		Dec. 31, 2013		Dec. 31, 2012
Defined Benefit Plan Disclosure [Line Items]
Employer contributions	$ 28	[1]	$ 27	[1]	$ 26	[1]
Union Electric Company
Defined Benefit Plan Disclosure [Line Items]
Employer contributions	16		16		16
Ameren Illinois Company
Defined Benefit Plan Disclosure [Line Items]
Employer contributions	11		10		9
Other
Defined Benefit Plan Disclosure [Line Items]
Employer contributions	$ 1		$ 1		$ 1

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