Income Taxes (Schedule Of Deferred Tax Assets And Liabilities Resulting From Temporary Differences) (Details) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
Income Taxes [Line Items]
Plant related	$ 3,474	$ 5,019
Regulatory assets, net		74
Regulatory assets, net	(547)
Deferred benefit costs	(131)	(211)
Revenue Requirement Reconciliation Adjustment	20	34
Tax carryforwards	(361)	(676)
Other	51	24
Total net accumulated deferred income tax liabilities	2,506	4,264
Union Electric Company
Income Taxes [Line Items]
Plant related	2,064	3,103
Regulatory assets, net		75
Regulatory assets, net	(317)
Deferred benefit costs	(53)	(76)
Revenue Requirement Reconciliation Adjustment	0	0
Tax carryforwards	(31)	(66)
Other	(13)	(23)
Total net accumulated deferred income tax liabilities	1,650	3,013
Ameren Illinois Company
Income Taxes [Line Items]
Plant related	1,264	1,769
Regulatory assets, net	(206)	(1)
Deferred benefit costs	(17)	(38)
Revenue Requirement Reconciliation Adjustment	20	34
Tax carryforwards	(43)	(138)
Other	3	5
Total net accumulated deferred income tax liabilities	1,021	1,631
Other
Income Taxes [Line Items]
Plant related	146	147
Regulatory assets, net		0
Regulatory assets, net	(24)
Deferred benefit costs	(61)	(97)
Revenue Requirement Reconciliation Adjustment	0	0
Tax carryforwards	(287)	(472)
Other	61	42
Total net accumulated deferred income tax liabilities	$ (165)	$ (380)