Income Taxes (Schedule Of Net Operating Loss Carryforwards And Tax Credit Carryforwards) (Details) - USD ($) $ in Millions	Dec. 31, 2019	Dec. 31, 2018
Operating Loss Carryforwards [Line Items]
Net operating loss carryforwards		$ 91
Tax credit carryforwards	$ 98	127
Deferred Tax Assets, Charitable Contribution Carryforwards	3	14
Deferred Tax Assets, Valuation Allowance, Noncurrent	(3)	(5)
Deferred Tax Assets, Net of Valuation Allowance, Noncurrent	0	9
Federal
Operating Loss Carryforwards [Line Items]
Net operating loss carryforwards		78
Tax credit carryforwards	95	117
State
Operating Loss Carryforwards [Line Items]
Net operating loss carryforwards		13
Tax credit carryforwards	3	10
Union Electric Company
Operating Loss Carryforwards [Line Items]
Net operating loss carryforwards		0
Tax credit carryforwards	25	35
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
Tax credit carryforwards	25	35
Union Electric Company \| State
Operating Loss Carryforwards [Line Items]
Net operating loss carryforwards		0
Tax credit carryforwards	0	0
Ameren Illinois Company
Operating Loss Carryforwards [Line Items]
Net operating loss carryforwards		23
Tax credit carryforwards	3	3
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
Tax credit carryforwards	3	3
Ameren Illinois Company \| State
Operating Loss Carryforwards [Line Items]
Net operating loss carryforwards		0
Tax credit carryforwards	0	0
Other
Operating Loss Carryforwards [Line Items]
Net operating loss carryforwards		68
Tax credit carryforwards	70	89
Deferred Tax Assets, Charitable Contribution Carryforwards	3	14
Deferred Tax Assets, Valuation Allowance, Noncurrent	(3)	(5)
Deferred Tax Assets, Net of Valuation Allowance, Noncurrent	0	9
Other \| Federal
Operating Loss Carryforwards [Line Items]
Net operating loss carryforwards		55
Tax credit carryforwards	67	79
Other \| State
Operating Loss Carryforwards [Line Items]
Net operating loss carryforwards		13
Tax credit carryforwards	$ 3	$ 10

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

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