Income Taxes (Schedule Of Net Operating Loss Carryforwards And Tax Credit Carryforwards) (Details) - USD ($) $ in Millions	12 Months Ended
Dec. 31, 2018	Dec. 31, 2017
Operating Loss Carryforwards [Line Items]
Net operating loss carryforwards	$ 91	$ 235
Tax credit carryforwards	127	120
Deferred Tax Assets, Charitable Contribution Carryforwards	14	11
Deferred Tax Assets, Valuation Allowance, Noncurrent	(5)	(5)
Deferred Tax Assets, Net of Valuation Allowance, Noncurrent	9	6
Federal
Operating Loss Carryforwards [Line Items]
Net operating loss carryforwards	78	203
Tax credit carryforwards	117	113
State
Operating Loss Carryforwards [Line Items]
Net operating loss carryforwards	13	32
Tax credit carryforwards	10	7
Union Electric Company
Operating Loss Carryforwards [Line Items]
Net operating loss carryforwards	0	0
Tax credit carryforwards	35	31
Deferred Tax Assets, Charitable Contribution Carryforwards	0	0
Deferred Tax Assets, Valuation Allowance, Noncurrent	0	0
Deferred Tax Assets, Net of Valuation Allowance, Noncurrent	0	0
Union Electric Company \| Federal
Operating Loss Carryforwards [Line Items]
Net operating loss carryforwards	0	0
Tax credit carryforwards	35	31
Union Electric Company \| State
Operating Loss Carryforwards [Line Items]
Net operating loss carryforwards	0	0
Tax credit carryforwards	0	0
Ameren Illinois Company
Operating Loss Carryforwards [Line Items]
Net operating loss carryforwards	23	41
Tax credit carryforwards	3	2
Deferred Tax Assets, Charitable Contribution Carryforwards	0	0
Deferred Tax Assets, Valuation Allowance, Noncurrent	0	0
Deferred Tax Assets, Net of Valuation Allowance, Noncurrent	0	0
Ameren Illinois Company \| Federal
Operating Loss Carryforwards [Line Items]
Net operating loss carryforwards	23	41
Tax credit carryforwards	3	2
Ameren Illinois Company \| State
Operating Loss Carryforwards [Line Items]
Net operating loss carryforwards	0	0
Tax credit carryforwards	0	0
Other
Operating Loss Carryforwards [Line Items]
Net operating loss carryforwards	68	194
Tax credit carryforwards	89	87
Deferred Tax Assets, Charitable Contribution Carryforwards	14	11
Deferred Tax Assets, Valuation Allowance, Noncurrent	(5)	(5)
Deferred Tax Assets, Net of Valuation Allowance, Noncurrent	9	6
Other \| Federal
Operating Loss Carryforwards [Line Items]
Net operating loss carryforwards	55	162
Tax credit carryforwards	79	80
Other \| State
Operating Loss Carryforwards [Line Items]
Net operating loss carryforwards	13	32
Tax credit carryforwards	$ 10	$ 7
Minimum \| Federal
Operating Loss Carryforwards [Line Items]
Net operating loss carryforward, expiration period start	Jan. 01, 2034
Tax credit carryforward, expiration period start	Jan. 01, 2029
Minimum \| State
Operating Loss Carryforwards [Line Items]
Net operating loss carryforward, expiration period start	Jan. 01, 2034
Tax credit carryforward, expiration period start	Jan. 01, 2019
Minimum \| Federal Contribution
Operating Loss Carryforwards [Line Items]
Tax credit carryforward, expiration period start	Jan. 01, 2019
Maximum \| Federal
Operating Loss Carryforwards [Line Items]
Net operating loss carryforward, expiration period start	Jan. 01, 2037
Tax credit carryforward, expiration period start	Jan. 01, 2037
Maximum \| State
Operating Loss Carryforwards [Line Items]
Net operating loss carryforward, expiration period start	Jan. 01, 2037
Tax credit carryforward, expiration period start	Jan. 01, 2022
Maximum \| Federal Contribution
Operating Loss Carryforwards [Line Items]
Tax credit carryforward, expiration period start	Jan. 01, 2023

Amount before allocation of valuation allowances of deferred tax asset attributable to deductible charitable contribution carryforwards.

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