Retirement Benefits (Components Of Net Periodic Benefit Cost) (Detail) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Total non-service cost components	$ (17)	$ (11)	$ (52)	$ (33)
Pension Plan
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Service cost	25	24	75	70
Interest cost	42	44	126	134
Expected return on plan assets	(68)	(65)	(206)	(196)
Prior service cost (benefit)	0	(1)	0	(1)
Actuarial loss	17	14	51	41
Total non-service cost components	(9)	(8)	(29)	(22)
Net periodic benefit cost	16	16	46	48
Other Postretirement Benefit Plan, Defined Benefit
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Service cost	6	6	16	16
Interest cost	10	12	30	35
Expected return on plan assets	(20)	(19)	(58)	(56)
Prior service cost (benefit)	(1)	(2)	(3)	(4)
Actuarial loss	(2)	(2)	(5)	(5)
Total non-service cost components	(13)	(11)	(36)	(30)
Net periodic benefit cost	(7)	(5)	(20)	(14)
Union Electric Company
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Total non-service cost components	(4)	(5)	(13)	(17)
Union Electric Company | Pension Plan
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Net periodic benefit cost	6	6	17	18
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)	(1)	(1)	(3)
Ameren Illinois Company
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Total non-service cost components	(8)	(4)	(25)	(8)
Ameren Illinois Company | Pension Plan
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Net periodic benefit cost	11	10	30	30
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	$ (6)	$ (3)	$ (19)	$ (10)