Commitments And Contingencies (Asbestos-Related Litigation And Tax Exemptions And Credits) (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended						1 Months Ended
	Mar. 31, 2014 Asbestos-Related defendant lawsuit		Mar. 31, 2014 Asbestos-Related Union Electric Company lawsuit	Mar. 31, 2014 Asbestos-Related Ameren Illinois Company lawsuit	Mar. 31, 2014 Asbestos-Related Ameren Corporation lawsuit	Mar. 31, 2014 Collectibility of Taxes Ameren Illinois Company customer	Sep. 30, 2013 State and Local Jurisdiction [Member] Collectibility of Taxes Ameren Illinois Company	Jul. 31, 2013 State and Local Jurisdiction [Member] Collectibility of Taxes Ameren Illinois Company
Loss Contingencies [Line Items]
Average number of total defendants named	85
Asbestos-related lawsuits were pending	71	[1]	47	56	1
Range of possible loss, minimum	$ 12.0		$ 5.0	$ 7.0		$ 1.0
Range of possible loss maximum						5.0
Loss contingency accrual						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%
Public Utilities, Number of Customers						2,400
Loss Contingency, Damages Awarded, Value							$ 0.5	$ 4.0

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