Retirement Benefits (Summary Of Benefit Plan Costs Incurred) (Details) - USD ($) $ in Millions		12 Months Ended
		Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Pension Benefits
Defined Benefit Plan Disclosure [Line Items]
Net periodic benefit cost		$ 44	$ 92	$ 81
Postretirement Benefits
Defined Benefit Plan Disclosure [Line Items]
Net periodic benefit cost		(19)	4	(8)
Union Electric Company | Pension Benefits
Defined Benefit Plan Disclosure [Line Items]
Net periodic benefit cost	[1]	26	54	50
Union Electric Company | Postretirement Benefits
Defined Benefit Plan Disclosure [Line Items]
Net periodic benefit cost	[1]	(5)	8	3
Ameren Illinois Company | Pension Benefits
Defined Benefit Plan Disclosure [Line Items]
Net periodic benefit cost		22	38	30
Ameren Illinois Company | Postretirement Benefits
Defined Benefit Plan Disclosure [Line Items]
Net periodic benefit cost		(13)	(3)	(9)
Other | Pension Benefits
Defined Benefit Plan Disclosure [Line Items]
Net periodic benefit cost		(4)	0	1
Other | Postretirement Benefits
Defined Benefit Plan Disclosure [Line Items]
Net periodic benefit cost		(1)	(1)	(2)
Parent Company | Pension Benefits
Defined Benefit Plan Disclosure [Line Items]
Net periodic benefit cost		44	92	81
Parent Company | Postretirement Benefits
Defined Benefit Plan Disclosure [Line Items]
Net periodic benefit cost		$ (19)	$ 4	$ (8)

[1] (a)Does not include the impact of the regulatory tracking mechanism for the difference between the level of pension and postretirement benefit costs incurred by Ameren Missouri under GAAP and the level of such costs included in rates.