Retirement Benefits (Components Of Net Periodic Benefit Cost) (Detail) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Total non-service cost components	$ (19)	$ (10)	$ (35)	$ (22)
Pension Plan
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Service cost	25	23	50	46
Interest cost	42	45	84	90
Expected return on plan assets	(69)	(65)	(138)	(131)
Prior service cost (benefit)	0	0	0	0
Actuarial loss	18	13	34	27
Total non-service cost components	(9)	(7)	(20)	(14)
Net periodic benefit cost	16	16	30	32
Other Postretirement Benefit Plan, Defined Benefit
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Service cost	5	5	10	10
Interest cost	9	11	20	23
Expected return on plan assets	(19)	(18)	(38)	(37)
Prior service cost (benefit)	(1)	(1)	(2)	(2)
Actuarial loss	(3)	(1)	(3)	(3)
Total non-service cost components	(14)	(9)	(23)	(19)
Net periodic benefit cost	(9)	(4)	(13)	(9)
Union Electric Company
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Total non-service cost components	(4)	(6)	(9)	(12)
Union Electric Company | Pension Plan
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Net periodic benefit cost	6	6	11	12
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	(2)
Ameren Illinois Company
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Total non-service cost components	(10)	(1)	(17)	(4)
Ameren Illinois Company | Pension Plan
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Net periodic benefit cost	10	10	19	20
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	$ (9)	$ (3)	$ (13)	$ (7)