Commitments And Contingencies (Asbestos-Related Litigation And Tax Exemptions And Credits) (Detail) (USD $) In Millions, unless otherwise specified	6 Months Ended						1 Months Ended	3 Months Ended
	Jun. 30, 2013 Asbestos-Related [Member] defendant lawsuit		Jun. 30, 2013 Asbestos-Related [Member] Ameren Missouri [Member] lawsuit	Jun. 30, 2013 Asbestos-Related [Member] Ameren Illinois Company [Member] lawsuit	Jun. 30, 2013 Asbestos-Related [Member] Ameren Corporation [Member] lawsuit	Jun. 30, 2013 Collectibility of Taxes [Member] Ameren Illinois Company [Member] customer	Jul. 31, 2013 Collectibility of Taxes [Member] Ameren Illinois Company [Member] Subsequent Event [Member]	Jun. 30, 2013 Claimed Manufacturing Exemptions And Credits [Member] Ameren Illinois Company [Member]
Loss Contingencies [Line Items]
Average number of total defendants named	80
Asbestos-related lawsuits were pending	90	[1]	58	68	2
Range of possible loss, minimum	$ 16.0		$ 7.0	$ 9.0		$ 0.5
Range of possible loss maximum						4.0
Loss contingency accrual						0.5
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							4
Gain (Loss) Related to Litigation Settlement								$ 7

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