Summary Of Significant Accounting Policies (Equity Changes Attributable To Noncontrolling Interest) (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended
	Mar. 31, 2013		Mar. 31, 2012		Mar. 31, 2013 Electric Energy, Inc [Member]	Dec. 31, 2012 Electric Energy, Inc [Member]
Stockholders' Equity Attributable to Noncontrolling Interest [Roll Forward]
Noncontrolling interest, beginning of period	$ 151	[1]	$ 149	[1]
Net income from continuing operations attributable to noncontrolling interests	2		2
Net income (loss) from discontinued operations attributable to noncontrolling interests			(2)
Dividends paid to noncontrolling interest holders	(2)		(2)
Noncontrolling interest, end of period	$ 151	[1]	$ 147	[1]
Percentage of EEI not owned by Ameren					20.00%	20.00%

[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,” however, the 20% ownership interest not owned by Ameren was included in “Noncontrolling interests” on Ameren’s March 31, 2013, and December 31, 2012 balance sheet. See Note 2 - Divestiture Transactions and Discontinued Operations for additional information.