Retirement Benefits (Summary of Benefit Plan Costs Incurred) (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2026	Sep. 30, 2025	Sep. 30, 2026	Sep. 30, 2025
Pension Plan
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Net periodic benefit cost	$ 18	$ (6)	$ 42	$ (16)
Pension Plan \| Union Electric Company
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Net periodic benefit cost	7	(4)	18	(11)
Pension Plan \| Ameren Illinois Company
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Net periodic benefit cost	10	(1)	22	(3)
Pension Plan \| Other Affiliated Entities And Intercompany Eliminations
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Net periodic benefit cost	1	(1)	2	(2)
Other Postretirement Benefit Plan, Defined Benefit
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Net periodic benefit cost	(20)	(20)	(57)	(60)
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	(6)	(7)	(17)	(21)
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	(14)	(13)	(40)	(39)
Other Postretirement Benefit Plan, Defined Benefit \| Other Affiliated Entities And Intercompany Eliminations
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Net periodic benefit cost	$ 1	$ 1	$ 1	$ 1