Retirement Benefits (Summary Of Benefit Liability Recorded) (Details) - USD ($) $ in Millions	Dec. 31, 2020	Dec. 31, 2019
Defined Benefit Plan Disclosure [Line Items]
Benefit liability recorded on the balance sheet	$ (249)	$ (216)
Union Electric Company
Defined Benefit Plan Disclosure [Line Items]
Benefit liability recorded on the balance sheet	(25)	(142)
Ameren Illinois Company
Defined Benefit Plan Disclosure [Line Items]
Benefit liability recorded on the balance sheet	$ (210)	$ (16)