Retirement Benefits (Schedule Of Matching Contributions) (Details) - 401 (K) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Defined Benefit Plan Disclosure [Line Items]
Employer contributions	$ 29	$ 29	$ 28
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	12	12	11
Other
Defined Benefit Plan Disclosure [Line Items]
Employer contributions	$ 1	$ 1	$ 1