Summary Of Significant Accounting Policies (Narrative) (Details) (USD $) In Millions, except Per Share data, unless otherwise specified	1 Months Ended	3 Months Ended				1 Months Ended
	Jan. 31, 2012	Mar. 31, 2012 M		Mar. 31, 2011	Dec. 31, 2011	Feb. 29, 2012 Medina Valley Energy Center [Member]
Summary Of Significant Accounting Policies [Line Items]
Fair value of each share unit, per share	$ 35.68	$ 35.68	[1]
Closing common share price					$ 33.13
Three-year risk-free rate		0.41%
Volatility rate, minimum		17.00%
Volatility rate, maximum		31.00%
Share-based compensation expense		$ 6		$ 6
Employee service share-based compensation, tax benefit from compensation expense		2		2
Compensation cost not yet recognized		32
Expected weighted average recognition period for share-based compensation expense, in months		26
Book value		9			7
Amortization expense		1		2
Unrecognized tax benefits		150
Unrecognized tax benefits that would impact effective tax rate		1
ARO Incurred		1
Percentage of EEI not owned by Ameren		20.00%
Proceeds from sales of properties						16
Additional contingent proceeds from sale of properties						1
Pretax gain recognized on sale						$ 10

[1] Includes performance share units (share units) granted to certain executive and nonexecutive officers and other eligible employees in January 2012 under the 2006 Plan.