Summary Of Significant Accounting Policies (Narrative) (Detail) (USD $) In Millions, except Per Share data, unless otherwise specified	9 Months Ended			9 Months Ended			9 Months Ended			1 Months Ended	9 Months Ended		0 Months Ended
	Sep. 30, 2013	Sep. 30, 2012	Dec. 31, 2012	Sep. 30, 2013 Union Electric Company	Sep. 30, 2012 Union Electric Company	Dec. 31, 2012 Union Electric Company	Sep. 30, 2013 Ameren Illinois Company	Sep. 30, 2013 Electric Energy, Inc	Dec. 31, 2012 Electric Energy, Inc	Jan. 31, 2013 Performance Shares	Sep. 30, 2013 Performance Shares		Mar. 14, 2013 Elgin, Gibson City and Grand Tower Energy Centers Ameren Energy Generating Company	Oct. 11, 2013 Subsequent Event [Member] Elgin, Gibson City and Grand Tower Energy Centers Ameren Energy Generating Company
Basis Of Presentation And Significant Accounting Policies [Line Items]
Ownership interest								80.00%
Initial payment received													$ 100
Additional payment received														37.5
Identified immaterial error		49			49
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%
Risk free interest rate period											3 years
Volatility rate, minimum											12.00%
Volatility rate, maximum											21.00%
Book value or renewable energy credits	19		14	19		14
Unrecognized tax benefits (detriments)	86			27			(2)
Unrecognized tax benefits (detriments) that would impact effective tax rate	51			2			(1)
Increase in unrecognized tax benefits that would impact effective tax rate	50
Reduction to unrecognized tax benefits due to IRS guidance	113			103			5
Estimated unrecognized tax decreases resulting from settlements with taxing authorities	$ 23			$ 15			$ 0
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 2013 under the 2006 Plan.