Income Taxes (Schedule Of Net Operating Loss Carryforwards And Tax Credit Carryforwards) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Operating Loss Carryforwards [Line Items]
Net operating loss carryforwards	$ 235	$ 539
Tax credit carryforwards	120	130
Deferred Tax Assets, Charitable Contribution Carryforwards	11	18
Deferred Tax Assets, Valuation Allowance, Noncurrent	(5)	(11)
Deferred Tax Assets, Net of Valuation Allowance, Noncurrent	6	7
Federal
Operating Loss Carryforwards [Line Items]
Net operating loss carryforwards	203	494
Tax credit carryforwards	113	109
State
Operating Loss Carryforwards [Line Items]
Net operating loss carryforwards	32	45
Tax credit carryforwards	7	21
Union Electric Company
Operating Loss Carryforwards [Line Items]
Net operating loss carryforwards	0	37
Tax credit carryforwards	31	29
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	33
Tax credit carryforwards	31	29
Union Electric Company | State
Operating Loss Carryforwards [Line Items]
Net operating loss carryforwards	0	4
Tax credit carryforwards	0	0
Ameren Illinois Company
Operating Loss Carryforwards [Line Items]
Net operating loss carryforwards	41	137
Tax credit carryforwards	2	1
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	41	137
Tax credit carryforwards	2	1
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	194	365
Tax credit carryforwards	87	100
Deferred Tax Assets, Charitable Contribution Carryforwards	11	18
Deferred Tax Assets, Valuation Allowance, Noncurrent	(5)	(11)
Deferred Tax Assets, Net of Valuation Allowance, Noncurrent	6	7
Other | Federal
Operating Loss Carryforwards [Line Items]
Net operating loss carryforwards	162	324
Tax credit carryforwards	80	79
Other | State
Operating Loss Carryforwards [Line Items]
Net operating loss carryforwards	32	41
Tax credit carryforwards	$ 7	$ 21
Minimum | Federal
Operating Loss Carryforwards [Line Items]
Net operating loss carryforward, expiration period start	Jan. 01, 2033
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, 2033
Tax credit carryforward, expiration period start	Jan. 01, 2019
Minimum | Federal Contribution
Operating Loss Carryforwards [Line Items]
Net operating loss carryforward, expiration period start	Jan. 01, 2018
Maximum | Federal
Operating Loss Carryforwards [Line Items]
Net operating loss carryforward, expiration period start	Jan. 01, 2036
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, 2036
Tax credit carryforward, expiration period start	Jan. 01, 2022
Maximum | Federal Contribution
Operating Loss Carryforwards [Line Items]
Net operating loss carryforward, expiration period start	Jan. 01, 2021