Commitments And Contingencies (Asbestos-Related Litigation And Tax Exemptions And Credits) (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended		3 Months Ended
	Dec. 31, 2014 defendant		Dec. 31, 2014
Asbestos-Related
Loss Contingencies [Line Items]
Number of defendants	78
Number of pending asbestos lawsuits as of the balance sheet date	70	[1]	70	[1]
Loss contingency range of possible loss, minimum	$ 12.0		$ 12.0
Percent of allowed cash expenditures in excess of base rates to be recovered through charges assessed to customers	90.00%		90.00%
Asbestos trust fund balance	22
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%		90.00%
Parent Company | Asbestos-Related
Loss Contingencies [Line Items]
Number of pending asbestos lawsuits as of the balance sheet date	1		1
Union Electric Company | Asbestos-Related
Loss Contingencies [Line Items]
Number of pending asbestos lawsuits as of the balance sheet date	45		45
Loss contingency range of possible loss, minimum	5.0		5.0
Ameren Illinois Company | Asbestos-Related
Loss Contingencies [Line Items]
Number of pending asbestos lawsuits as of the balance sheet date	57		57
Loss contingency range of possible loss, minimum	7.0		7.0
State | Ameren Illinois Company
Loss Contingencies [Line Items]
Loss Contingency, Damages Awarded, Value			$ 1

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