Retirement Benefits (Components Of Net Periodic Benefit Cost) (Detail) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Total non-service cost components	$ (16)	$ (12)
Pension Plan
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Service cost	25	23
Interest cost	42	45
Expected return on plan assets	(69)	(66)
Prior service cost (benefit)	0	0
Actuarial loss	16	14
Total non-service cost components	(11)	(7)
Net periodic benefit cost	14	16
Other Postretirement Benefit Plan, Defined Benefit
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Service cost	5	5
Interest cost	11	12
Expected return on plan assets	(19)	(19)
Prior service cost (benefit)	(1)	(1)
Actuarial loss	0	(2)
Total non-service cost components	(9)	(10)
Net periodic benefit cost	(4)	(5)
Union Electric Company
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Total non-service cost components	(5)	(6)
Union Electric Company | Pension Plan
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Net periodic benefit cost	5	6
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	0	(1)
Ameren Illinois Company
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Total non-service cost components	(7)	(3)
Ameren Illinois Company | Pension Plan
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Net periodic benefit cost	9	10
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	$ (4)	$ (4)