Retirement Benefits (Components Of Net Periodic Benefit Cost) (Detail) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Total non-service cost components	$ (23)	$ (22)
Pension Plan
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Service cost	27	22
Interest cost	43	47
Expected return on plan assets	(73)	(69)
Prior service cost (benefit)	0	0
Actuarial loss (gain)	14	6
Total non-service cost components	(16)	(16)
Net periodic benefit cost	11	6
Other Postretirement Benefit Plan, Defined Benefit
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Service cost	4	4
Interest cost	10	11
Expected return on plan assets	(20)	(19)
Prior service cost (benefit)	(1)	(1)
Actuarial loss (gain)	(2)	(4)
Total non-service cost components	(13)	(13)
Net periodic benefit cost	(9)	(9)
Union Electric Company
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Total non-service cost components	(5)	(4)
Union Electric Company | Pension Plan
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Net periodic benefit cost	4	1
Union Electric Company | Other Postretirement Benefit Plan, Defined Benefit
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Net periodic benefit cost	(1)	(2)
Ameren Illinois Company
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Total non-service cost components	(13)	(12)
Ameren Illinois Company | Pension Plan
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Net periodic benefit cost	7	5
Ameren Illinois Company | Other Postretirement Benefit Plan, Defined Benefit
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Net periodic benefit cost	$ (8)	$ (7)