Retirement Benefits Retirement Benefits (Summary of Benefit Plan Costs Incurred) (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Pension Plan
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Net periodic benefit cost	$ 6	$ 14
Pension Plan | Union Electric Company
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Net periodic benefit cost	1	5
Pension Plan | Ameren Illinois Company
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Net periodic benefit cost	5	9
Other Postretirement Benefit Plan, Defined Benefit
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Net periodic benefit cost	(9)	(4)
Other Postretirement Benefit Plan, Defined Benefit | Union Electric Company
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Net periodic benefit cost	(2)	0
Other Postretirement Benefit Plan, Defined Benefit | Ameren Illinois Company
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Net periodic benefit cost	$ (7)	$ (4)