Retirement Benefits (Components Of Net Periodic Benefit Cost) (Detail) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Total non-service cost components	$ (22)	$ (19)	$ (44)	$ (35)
Pension Plan
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Service cost	22	25	44	50
Interest cost	46	42	93	84
Expected return on plan assets	(69)	(69)	(138)	(138)
Prior service cost (benefit)	0	0	0	0
Actuarial loss	7	18	13	34
Total non-service cost components	(16)	(9)	(32)	(20)
Net periodic benefit cost	6	16	12	30
Other Postretirement Benefit Plan, Defined Benefit
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Service cost	5	5	9	10
Interest cost	10	9	21	20
Expected return on plan assets	(19)	(19)	(38)	(38)
Prior service cost (benefit)	(2)	(1)	(3)	(2)
Actuarial loss	(3)	(3)	(7)	(3)
Total non-service cost components	(14)	(14)	(27)	(23)
Net periodic benefit cost	(9)	(9)	(18)	(13)
Union Electric Company
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Total non-service cost components	(4)	(4)	(9)	(9)
Union Electric Company | Pension Plan
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Net periodic benefit cost	1	6	2	11
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)	0	(3)	0
Ameren Illinois Company
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Total non-service cost components	(12)	(10)	(24)	(17)
Ameren Illinois Company | Pension Plan
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Net periodic benefit cost	5	10	10	19
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)	$ (9)	$ (15)	$ (13)