Retirement Benefits (Components Of Net Periodic Benefit Cost) (Detail) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Total non-service cost components	$ (23)	$ (17)	$ (67)	$ (52)
Pension Plan
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Service cost	22	25	66	75
Interest cost	46	42	139	126
Expected return on plan assets	(69)	(68)	(207)	(206)
Prior service cost (benefit)	0	0	0	0
Actuarial loss	6	17	19	51
Total non-service cost components	(17)	(9)	(49)	(29)
Net periodic benefit cost	5	16	17	46
Other Postretirement Benefit Plan, Defined Benefit
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Service cost	4	6	13	16
Interest cost	11	10	32	30
Expected return on plan assets	(19)	(20)	(57)	(58)
Prior service cost (benefit)	(1)	(1)	(4)	(3)
Actuarial loss	(4)	(2)	(11)	(5)
Total non-service cost components	(13)	(13)	(40)	(36)
Net periodic benefit cost	(9)	(7)	(27)	(20)
Union Electric Company
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Total non-service cost components	(4)	(4)	(13)	(13)
Union Electric Company | Pension Plan
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Net periodic benefit cost	1	6	3	17
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)	(4)	(1)
Ameren Illinois Company
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Total non-service cost components	(12)	(8)	(36)	(25)
Ameren Illinois Company | Pension Plan
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Net periodic benefit cost	5	11	15	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	$ (8)	$ (6)	$ (23)	$ (19)