Retirement Benefits (Schedule Of Matching Contributions) (Details) - United States Postretirement Benefit Plan of US Entity [Member] - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Defined Benefit Plan Disclosure [Line Items]
Employer contributions	$ 30	$ 29	$ 29
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	13	12	12
Other
Defined Benefit Plan Disclosure [Line Items]
Employer contributions	$ 1	$ 1	$ 1