Retirement Benefits (Schedule Of Matching Contributions) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Defined Benefit Plan Disclosure [Line Items]
Employer contributions	$ 38	$ 35	$ 33
Union Electric Company
Defined Benefit Plan Disclosure [Line Items]
Employer contributions	20	19	17
Ameren Illinois Company
Defined Benefit Plan Disclosure [Line Items]
Employer contributions	17	16	15
Other
Defined Benefit Plan Disclosure [Line Items]
Employer contributions	$ 1	$ 0	$ 1