Summary Of Significant Accounting Policies (Equity Changes Attributable To Noncontrolling Interest) (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended				9 Months Ended						3 Months Ended		9 Months Ended
	Sep. 30, 2013		Sep. 30, 2012		Sep. 30, 2013		Sep. 30, 2012		Sep. 30, 2013 Electric Energy, Inc	Dec. 31, 2012 Electric Energy, Inc	Sep. 30, 2013 Noncontrolling Interest	Sep. 30, 2012 Noncontrolling Interest	Sep. 30, 2013 Noncontrolling Interest	Sep. 30, 2012 Noncontrolling Interest
Stockholders' Equity Attributable to Noncontrolling Interest [Roll Forward]
Noncontrolling interest, beginning of period	$ 151	[1]	$ 145	[1]	$ 151	[1]	$ 149	[1]
Net income from continuing operations attributable to noncontrolling interests	2		2		5		5
Net loss from discontinued operations attributable to noncontrolling interests			(2)				(6)
Dividends paid to noncontrolling interest holders	(2)		(2)		(5)		(5)
Other comprehensive Income attributable to noncontrolling interest		[2]	(9)	[2]		[2]	(9)	[2]
Noncontrolling interest, end of period	151	[1]	152	[1]	151	[1]	152	[1]
Percentage of EEI not owned by Ameren									20.00%	20.00%
Pension and other postretirement benefit plan activity, tax expense (benefit)	$ (5)		$ 0		$ 3		$ 0				$ 0	$ 6	$ 0	$ 6

[1] Includes the 20% EEI ownership interest not owned by Ameren. The assets and liabilities of EEI were consolidated in Ameren’s balance sheet at a 100% ownership level and were included in “Current assets of discontinued operations” and “Current liabilities of discontinued operations.” The 20% ownership interest not owned by Ameren was included in “Noncontrolling interests” on Ameren’s September 30, 2013, and December 31, 2012 balance sheets. See Note 2 - Divestiture Transactions and Discontinued Operations for additional information.

[2] Represents the noncontrolling interest of EEI’s pension and other postretirement benefit plan activity, net of income taxes of $-, $6, $-, and $6, respectively.