Income Taxes (Schedule Of Deferred Tax Assets And Liabilities Resulting From Temporary Differences) (Details) - USD ($) $ in Millions	Dec. 31, 2020	Dec. 31, 2019
Income Taxes [Line Items]
Plant related	$ 3,876	$ 3,616
Regulatory assets and liabilities, net	(515)	(548)
Deferred employee benefit costs - asset	(104)	(111)
Tax carryforwards	(97)	(98)
Other - liabilities	17	21
Total net accumulated deferred income tax liabilities	3,177	2,880
Accumulated Deferred Investment Tax Credit	34	39
Accumulated deferred income taxes and investment tax credits, net	3,211	2,919
Union Electric Company
Income Taxes [Line Items]
Plant related	2,112	2,000
Regulatory assets and liabilities, net	(285)	(310)
Deferred employee benefit costs - asset	(58)	(59)
Tax carryforwards	(26)	(25)
Other - assets	(35)	(33)
Total net accumulated deferred income tax liabilities	1,708	1,573
Accumulated Deferred Investment Tax Credit	34	39
Accumulated deferred income taxes and investment tax credits, net	1,742	1,612
Ameren Illinois Company
Income Taxes [Line Items]
Plant related	1,559	1,423
Regulatory assets and liabilities, net	(207)	(214)
Deferred employee benefit costs - liability	8	7
Tax carryforwards	(6)	(3)
Other - liabilities	13	11
Total net accumulated deferred income tax liabilities	1,367	1,224
Accumulated Deferred Investment Tax Credit	0	0
Accumulated deferred income taxes and investment tax credits, net	1,367	1,224
Other
Income Taxes [Line Items]
Plant related	205	193
Regulatory assets and liabilities, net	(23)	(24)
Deferred employee benefit costs - asset	(54)	(59)
Tax carryforwards	(65)	(70)
Other - liabilities	39	43
Total net accumulated deferred income tax liabilities	102	83
Accumulated Deferred Investment Tax Credit	0	0
Accumulated deferred income taxes and investment tax credits, net	$ 102	$ 83