Commitments And Contingencies (Asbestos-Related Litigation And Tax Exemptions And Credits) (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2012 defendant
Asbestos-Related [Member]
Loss Contingencies [Line Items]
Number of defendants	79
Number of pending asbestos lawsuits as of the balance sheet date	121,000,000	[1]
Percent of allowed cash expenditures in excess of base rates to be recovered through charges assessed to customers	90.00%
Asbestos trust fund balance	$ 23
Percent of difference to be contributed to the asbestos trust fund if cash expenditures are less than amount included in base electric rates.	90.00%
Loss contingency range of possible loss, minimum	23.0
Claimed Manufacturing Exemptions And Credits [Member]
Loss Contingencies [Line Items]
Loss contingency range of possible loss, maximum	27.0
Minimum [Member] | Asbestos-Related [Member]
Loss Contingencies [Line Items]
Number of defendants	2
Maximum [Member] | Asbestos-Related [Member]
Loss Contingencies [Line Items]
Number of defendants	272
Parent Company [Member] | Asbestos-Related [Member]
Loss Contingencies [Line Items]
Number of pending asbestos lawsuits as of the balance sheet date	4,000,000
Union Electric Company [Member] | Asbestos-Related [Member]
Loss Contingencies [Line Items]
Number of pending asbestos lawsuits as of the balance sheet date	74,000,000
Loss contingency range of possible loss, minimum	9.0
Ameren Illinois Company [Member] | Asbestos-Related [Member]
Loss Contingencies [Line Items]
Number of pending asbestos lawsuits as of the balance sheet date	96,000,000
Loss contingency range of possible loss, minimum	14.0
Ameren Illinois Company [Member] | Collectibility of Taxes [Member]
Loss Contingencies [Line Items]
Number of customers affected	2,100
Loss contingency range of possible loss, minimum	2.0
Loss contingency range of possible loss, maximum	$ 4.0

[1] (a)Total does not equal the sum of the subsidiary unit lawsuits because some of the lawsuits name multiple Ameren entities as defendants.