Summary Of Significant Accounting Policies (Narrative) (Detail) (USD $) In Millions, except Per Share data, unless otherwise specified	3 Months Ended					3 Months Ended					3 Months Ended			1 Months Ended	3 Months Ended		0 Months Ended
	Mar. 31, 2013	Sep. 30, 2012	Jun. 30, 2012	Mar. 31, 2012	Dec. 31, 2012	Mar. 31, 2013 Union Electric Company [Member]	Sep. 30, 2012 Union Electric Company [Member]	Jun. 30, 2012 Union Electric Company [Member]	Mar. 31, 2012 Union Electric Company [Member]	Dec. 31, 2012 Union Electric Company [Member]	Mar. 31, 2013 Ameren Illinois Company [Member]	Mar. 31, 2013 Electric Energy, Inc [Member]	Dec. 31, 2012 Electric Energy, Inc [Member]	Jan. 31, 2013 Performance Shares [Member]	Mar. 31, 2013 Performance Shares [Member]		Mar. 14, 2013 Elgin, Gibson City and Grand Tower Energy Centers [Member] Ameren Energy Generating Company [Member]
Basis Of Presentation And Significant Accounting Policies [Line Items]
Ownership interest												80.00%
Initial payment received																	$ 100
Identified immaterial errors		49	26	14			49	26	14
Fair value of each share unit, per share														$ 31.19	$ 31.19	[1]
Closing common share price					$ 30.72
Performance period															3 years
Three-year risk-free rate															0.36%
Volatility rate, minimum															12.00%
Volatility rate, maximum															21.00%
Book value	16				14	16				14
Unrecognized tax benefits	190					126					4
Unrecognized tax benefits (detriments) that would impact effective tax rate	48					1					(1)
Increase in Unrecognized Tax benefits	47
Estimated unrecognized tax decreases resulting from settlements with taxing authorities	$ 126					$ 110					$ 5
Percentage of EEI not owned by Ameren												20.00%	20.00%

[1] Includes performance share units (share units) granted to certain executive and nonexecutive officers and other eligible employees in January 2013 under the 2006 Plan