Retirement Benefits (Summary Of Benefit Liability Recorded) (Details) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
Defined Benefit Plan Disclosure [Line Items]
Benefit liability recorded on the balance sheet	$ 551	$ 774
Union Electric Company
Defined Benefit Plan Disclosure [Line Items]
Benefit liability recorded on the balance sheet	215	293
Ameren Illinois Company
Defined Benefit Plan Disclosure [Line Items]
Benefit liability recorded on the balance sheet	$ 213	$ 315