Retirement Benefits (Components Of Net Periodic Benefit Cost) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Non-service Cost or Income Components	$ 70	$ 44	$ 56
Pension Benefits
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Non-service Cost or Income Components	(40)	(29)	(37)
Service cost	100	93	81
Interest cost	169	179	185
Expected return on plan assets	(276)	(262)	(253)
Prior service credit	(1)	(1)	(1)
Actuarial (gain) loss	68	55	32
Net periodic benefit cost (income)	60	64	44
Postretirement Benefits
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Non-service Cost or Income Components	(47)	(39)	(38)
Service cost	21	21	19
Interest cost	40	47	50
Expected return on plan assets	(77)	(75)	(72)
Prior service credit	(4)	(5)	(5)
Actuarial (gain) loss	(6)	(6)	(11)
Net periodic benefit cost (income)	(26)	(18)	(19)
Ameren Illinois Company
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Non-service Cost or Income Components	34	10	24
Ameren Illinois Company | Pension Benefits
Defined Benefit Plan Disclosure [Line Items]
Net periodic benefit cost (income)	39	41	22
Ameren Illinois Company | Postretirement Benefits
Defined Benefit Plan Disclosure [Line Items]
Net periodic benefit cost (income)	(25)	(14)	(13)
Union Electric Company
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Non-service Cost or Income Components	17	22	18
Union Electric Company | Pension Benefits
Defined Benefit Plan Disclosure [Line Items]
Net periodic benefit cost (income)	22	24	26
Union Electric Company | Postretirement Benefits
Defined Benefit Plan Disclosure [Line Items]
Net periodic benefit cost (income)	$ (1)	$ (4)	$ (5)