Income Taxes (Schedule Of Net Operating Loss Carryforwards And Tax Credit Carryforwards) (Details) - USD ($) $ in Millions	Dec. 31, 2020	Dec. 31, 2019
Operating Loss Carryforwards [Line Items]
Net operating loss carryforwards	$ 7
Tax credit carryforwards	90	$ 98
Deferred Tax Assets, Charitable Contribution Carryforwards	3	3
Deferred Tax Assets, Valuation Allowance, Noncurrent	(3)	(3)
Deferred Tax Assets, Charitable Contribution Carryforwards, Net Of Valuation Allowance	0	0
Federal
Operating Loss Carryforwards [Line Items]
Tax credit carryforwards	83	95
State
Operating Loss Carryforwards [Line Items]
Net operating loss carryforwards	7
Tax credit carryforwards	7	3
Union Electric Company
Operating Loss Carryforwards [Line Items]
Net operating loss carryforwards	0
Tax credit carryforwards	26	25
Deferred Tax Assets, Charitable Contribution Carryforwards	0	0
Deferred Tax Assets, Valuation Allowance, Noncurrent	0	0
Deferred Tax Assets, Charitable Contribution Carryforwards, Net Of Valuation Allowance	0	0
Union Electric Company | Federal
Operating Loss Carryforwards [Line Items]
Tax credit carryforwards	26	25
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	3
Tax credit carryforwards	3	3
Deferred Tax Assets, Charitable Contribution Carryforwards	0	0
Deferred Tax Assets, Valuation Allowance, Noncurrent	0	0
Deferred Tax Assets, Charitable Contribution Carryforwards, Net Of Valuation Allowance	0	0
Ameren Illinois Company | Federal
Operating Loss Carryforwards [Line Items]
Tax credit carryforwards	3	3
Ameren Illinois Company | State
Operating Loss Carryforwards [Line Items]
Net operating loss carryforwards	3
Tax credit carryforwards	0	0
Other
Operating Loss Carryforwards [Line Items]
Net operating loss carryforwards	4
Tax credit carryforwards	61	70
Deferred Tax Assets, Charitable Contribution Carryforwards	3	3
Deferred Tax Assets, Valuation Allowance, Noncurrent	(3)	(3)
Deferred Tax Assets, Charitable Contribution Carryforwards, Net Of Valuation Allowance	0	0
Other | Federal
Operating Loss Carryforwards [Line Items]
Tax credit carryforwards	54	67
Other | State
Operating Loss Carryforwards [Line Items]
Net operating loss carryforwards	4
Tax credit carryforwards	$ 7	$ 3