Summary Of Significant Accounting Policies (Narrative) (Detail) (USD $) In Millions, except Per Share data, unless otherwise specified	3 Months Ended
	Mar. 31, 2014		Dec. 31, 2013
Basis Of Presentation And Significant Accounting Policies [Line Items]
Maximum shares available for grants	8
Closing common share price			$ 36.16
Book value of renewable energy credits	$ 17		$ 22
Unrecognized tax benefit recorded in Accumulated deferred income taxes, net	84		84
Estimated unrecognized tax decreases resulting from settlements with taxing authorities	20
Union Electric Company
Basis Of Presentation And Significant Accounting Policies [Line Items]
Book value of renewable energy credits	17		22
Unrecognized tax benefit recorded in Accumulated deferred income taxes, net	14		15
Estimated unrecognized tax decreases resulting from settlements with taxing authorities	13
Ameren Illinois Company
Basis Of Presentation And Significant Accounting Policies [Line Items]
Unrecognized tax benefit recorded in Accumulated deferred income taxes, net	$ 0		$ 0
Electric Energy, Inc
Basis Of Presentation And Significant Accounting Policies [Line Items]
Percentage of EEI not owned by Ameren			20.00%
Performance Shares
Basis Of Presentation And Significant Accounting Policies [Line Items]
Fair value of each share unit, per share	$ 38.90	[1]
Performance period	3 years
Risk free interest rate period	3 years
Three-year risk-free rate	0.78%
Volatility rate, minimum	12.00%
Volatility rate, maximum	18.00%
Power | Union Electric Company
Basis Of Presentation And Significant Accounting Policies [Line Items]
Number of customers			1,200,000
Power | Ameren Illinois Company
Basis Of Presentation And Significant Accounting Policies [Line Items]
Number of customers			1,200,000
Natural Gas | Union Electric Company
Basis Of Presentation And Significant Accounting Policies [Line Items]
Number of customers			127,000
Natural Gas | Ameren Illinois Company
Basis Of Presentation And Significant Accounting Policies [Line Items]
Number of customers			807,000

[1] Includes performance share units (share units) granted to certain executive and nonexecutive officers and other eligible employees in 2014 under the 2006 Plan.