Summary Of Significant Accounting Policies (Equity Changes Attributable To Noncontrolling Interest) (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended				9 Months Ended
	Sep. 30, 2014		Sep. 30, 2013		Sep. 30, 2014		Sep. 30, 2013		Dec. 31, 2013 Electric Energy, Inc
Stockholders' Equity Attributable to Noncontrolling Interest [Roll Forward]
Noncontrolling interest, beginning of period	$ 142	[1]	$ 151	[1]	$ 142		$ 151	[1]
Net income from continuing operations attributable to noncontrolling interests	2		2		5		5
Dividends paid to noncontrolling interest holders	(2)		(2)		(5)		(5)
Noncontrolling interest, end of period	$ 142	[1]	$ 151	[1]	$ 142	[1]	$ 151	[1]
Percentage of EEI not owned by Ameren									20.00%

[1] Included the 20% EEI ownership interest not owned by Ameren prior to the divestiture of New AER to IPH. Prior to the divestiture of New AER, the assets and liabilities of EEI were consolidated in Ameren’s balance sheet at a 100% ownership level and were included in “Assets of discontinued operations” and “Liabilities of discontinued operations,” respectively. The divestiture of New AER, which included EEI, was completed in the fourth quarter of 2013. See Note 12 - Divestiture Transactions and Discontinued Operations for additional information.