Summary Of Significant Accounting Policies (Narrative) (Detail) (USD $) In Millions, except Per Share data, unless otherwise specified	3 Months Ended		9 Months Ended			9 Months Ended			1 Months Ended		9 Months Ended
	Sep. 30, 2014	Sep. 30, 2013	Sep. 30, 2014	Sep. 30, 2013	Dec. 31, 2013	Sep. 30, 2014 Union Electric Company	Dec. 31, 2013 Union Electric Company	Dec. 31, 2013 Electric Energy, Inc	Apr. 30, 2014 Performance Shares		Sep. 30, 2014 Performance Shares		Dec. 31, 2013 Power Union Electric Company customer	Dec. 31, 2013 Power Ameren Illinois Company customer	Dec. 31, 2013 Natural Gas Union Electric Company customer	Dec. 31, 2013 Natural Gas Ameren Illinois Company customer
Basis Of Presentation And Significant Accounting Policies [Line Items]
Antidilutive Securities Excluded from Computation of Earnings Per Share, Amount	0	0	0	0
Asset Retirement Obligation, Liabilities Incurred			$ 2			$ 2
Number of customers													1,200,000	1,200,000	127,000	807,000
Maximum shares available for grants	8		8
Fair value of each share unit, per share									$ 50.34	[1]	$ 38.90	[2]
Closing common share price					$ 36.16
Performance period											3 years
Risk free interest rate period											3 years
Three-year risk-free rate											0.78%
Volatility rate, minimum									12.00%		12.00%
Volatility rate, maximum									18.00%		18.00%
Book value of renewable energy credits	$ 20		$ 20		$ 22	$ 20	$ 22
Percentage of EEI not owned by Ameren								20.00%

[1] In April 2014, certain executive officers were granted additional share units under the 2006 Incentive Plan and the 2014 Incentive Plan. The significant assumptions used to calculate fair value included a prorated three-year risk-free rate ranging from 0.76% to 0.79%, volatility of 12% to 18% for the peer group, and Ameren’s attainment of a three-year average earnings per share threshold during the performance period.

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