Income Taxes (Schedule Of Deferred Tax Assets And Liabilities Resulting From Temporary Differences) (Details) - USD ($) $ in Millions	Dec. 31, 2019	Dec. 31, 2018
Income Taxes [Line Items]
Plant related	$ 3,616	$ 3,534
Regulatory assets and liabilities, net	(548)	(589)
Deferred employee benefit costs - asset	(111)	(126)
Tax carryforwards	(98)	(227)
Other - liabilities	21	31
Deferred Income Tax Liabilities, Net	2,880	2,623
Accumulated Deferred Investment Tax Credit	39	43
Total net accumulated deferred income tax liabilities	2,919	2,666
Union Electric Company
Income Taxes [Line Items]
Plant related	2,000	2,010
Regulatory assets and liabilities, net	(310)	(343)
Deferred employee benefit costs - asset	(59)	(58)
Tax carryforwards	(25)	(35)
Other - assets	(33)	(40)
Deferred Income Tax Liabilities, Net	1,573	1,534
Accumulated Deferred Investment Tax Credit	39	42
Total net accumulated deferred income tax liabilities	1,612	1,576
Ameren Illinois Company
Income Taxes [Line Items]
Plant related	1,423	1,345
Regulatory assets and liabilities, net	(214)	(221)
Deferred employee benefit costs - asset		(4)
Deferred employee benefit costs - liability	7
Tax carryforwards	(3)	(26)
Other - liabilities	11	24
Deferred Income Tax Liabilities, Net	1,224	1,118
Accumulated Deferred Investment Tax Credit	0	1
Total net accumulated deferred income tax liabilities	1,224	1,119
Other
Income Taxes [Line Items]
Plant related	193	179
Regulatory assets and liabilities, net	(24)	(25)
Deferred employee benefit costs - asset	(59)	(64)
Tax carryforwards	(70)	(166)
Other - liabilities	43	47
Deferred Income Tax Liabilities, Net	83	(29)
Accumulated Deferred Investment Tax Credit	0	0
Total net accumulated deferred income tax liabilities	$ 83	$ (29)