Retirement Benefits (Summary Of Benefit Plan Costs Incurred) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Pension Benefits
Defined Benefit Plan Disclosure [Line Items]
Net periodic benefit cost	$ 52	$ 23	$ 60
Postretirement Benefits
Defined Benefit Plan Disclosure [Line Items]
Net periodic benefit cost	(35)	(36)	(26)
Union Electric Company | Pension Benefits
Defined Benefit Plan Disclosure [Line Items]
Net periodic benefit cost	22	5	22
Union Electric Company | Postretirement Benefits
Defined Benefit Plan Disclosure [Line Items]
Net periodic benefit cost	(5)	(6)	(1)
Ameren Illinois Company | Pension Benefits
Defined Benefit Plan Disclosure [Line Items]
Net periodic benefit cost	32	20	39
Ameren Illinois Company | Postretirement Benefits
Defined Benefit Plan Disclosure [Line Items]
Net periodic benefit cost	(31)	(30)	(25)
Other | Pension Benefits
Defined Benefit Plan Disclosure [Line Items]
Net periodic benefit cost	(2)	(2)	(1)
Other | Postretirement Benefits
Defined Benefit Plan Disclosure [Line Items]
Net periodic benefit cost	$ 1	$ 0	$ 0