Income Taxes (Schedule Of Net Operating Loss Carryforwards And Tax Credit Carryforwards) (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2012
Operating Loss Carryforwards [Line Items]
Net operating loss carryforwards	$ 241
Tax credit carryforwards	118
Change in valuation allowance	2
Federal [Member]
Operating Loss Carryforwards [Line Items]
Net operating loss carryforwards	212	[1]
Tax credit carryforwards	87	[2]
Net operating loss carryforward, expiration period start	2028
Tax credit carryforward, expiration period start	Jan. 01, 2029
State [Member]
Operating Loss Carryforwards [Line Items]
Net operating loss carryforwards	29	[3]
Tax credit carryforwards	35	[4]
Tax Credit Carryforward, Valuation Allowance	(4)	[5]
Net operating loss carryforward, expiration period start	2017
Tax credit carryforward, expiration period start	Jan. 01, 2013
Union Electric Company [Member]
Operating Loss Carryforwards [Line Items]
Net operating loss carryforwards	64
Tax credit carryforwards	11
Union Electric Company [Member] | Federal [Member]
Operating Loss Carryforwards [Line Items]
Net operating loss carryforwards	61	[1]
Tax credit carryforwards	11	[2]
Union Electric Company [Member] | State [Member]
Operating Loss Carryforwards [Line Items]
Net operating loss carryforwards	3	[3]
Tax credit carryforwards	1	[4]
Tax Credit Carryforward, Valuation Allowance	(1)	[5]
Ameren Illinois Company [Member]
Operating Loss Carryforwards [Line Items]
Net operating loss carryforwards	72
Tax credit carryforwards
Change in valuation allowance	1
Ameren Illinois Company [Member] | Federal [Member]
Operating Loss Carryforwards [Line Items]
Net operating loss carryforwards	61	[1]
Tax credit carryforwards
Ameren Illinois Company [Member] | State [Member]
Operating Loss Carryforwards [Line Items]
Net operating loss carryforwards	11	[3]
Tax credit carryforwards	1	[4]
Tax Credit Carryforward, Valuation Allowance	$ (1)	[5]

[1] These will begin to expire in 2028.

[2] These will begin to expire in 2029.

[3] These will begin to expire in 2017.

[4] These will begin to expire in 2013.

[5] This balance increased by $2 million, $- million and $1 million for Ameren, Ameren Missouri and Ameren Illinois respectively during 2012.