Income Taxes (Schedule Of Deferred Tax Assets And Liabilities Resulting From Temporary Differences) (Details) - USD ($) $ in Millions	Dec. 31, 2016	Dec. 31, 2015
Income Taxes [Line Items]
Plant related	$ 5,019	$ 4,555
Regulatory assets, net	74	80
Deferred benefit costs	(211)	(207)
Revenue Requirement Reconciliation Adjustment	34	66
Tax carryforwards	(676)	(603)
Other	24
Other		(6)
Total net accumulated deferred income tax liabilities	4,264	3,885
Union Electric Company
Income Taxes [Line Items]
Plant related	3,103	2,931
Regulatory assets, net	75	81
Deferred benefit costs	(76)	(76)
Revenue Requirement Reconciliation Adjustment	0	0
Tax carryforwards	(66)	(65)
Other	(23)	(27)
Total net accumulated deferred income tax liabilities	3,013	2,844
Ameren Illinois Company
Income Taxes [Line Items]
Plant related	1,769	1,587
Regulatory assets, net	(1)	(1)
Deferred benefit costs	(38)	(40)
Revenue Requirement Reconciliation Adjustment	34	66
Tax carryforwards	(138)	(133)
Other	5	1
Total net accumulated deferred income tax liabilities	1,631	1,480
Other
Income Taxes [Line Items]
Plant related	147	37
Regulatory assets, net	0	0
Deferred benefit costs	(97)	(91)
Revenue Requirement Reconciliation Adjustment	0	0
Tax carryforwards	(472)	(405)
Other	42	20
Total net accumulated deferred income tax liabilities	$ (380)	$ (439)