Retirement Benefits (Schedule Of Matching Contributions) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Defined Benefit Plan Disclosure [Line Items]
Employer contributions	$ 35	$ 33	$ 30
Union Electric Company
Defined Benefit Plan Disclosure [Line Items]
Employer contributions	19	17	16
Ameren Illinois Company
Defined Benefit Plan Disclosure [Line Items]
Employer contributions	16	15	13
Other
Defined Benefit Plan Disclosure [Line Items]
Employer contributions	$ 0	$ 1	$ 1