Rate And Regulatory Matters (Narrative-Missouri) (Detail) (USD $)							1 Months Ended	9 Months Ended		1 Months Ended
	Sep. 30, 2014	Dec. 31, 2013	Sep. 30, 2014 Union Electric Company	Dec. 31, 2013 Union Electric Company	Sep. 30, 2014 Ameren Illinois Company	Dec. 31, 2013 Ameren Illinois Company	Jul. 31, 2014 Electric Distribution [Member] Union Electric Company Pending Rate Case [Member]	Sep. 30, 2014 Electric Distribution [Member] Union Electric Company Pending Rate Case [Member]	Sep. 30, 2014 Electric Distribution [Member] Union Electric Company Accounting Authority Order Request [Member]	Feb. 28, 2014 Customer [Domain] Electric Distribution [Member] Union Electric Company	Feb. 13, 2014 Customer [Domain] Electric Distribution [Member] Union Electric Company
Rate And Regulatory Matters [Line Items]
Public Utilities, Requested Rate Increase (Decrease), Amount							$ 264,000,000
Plant Additions to Rate Base							1,400,000,000
Requested Rate Increase Related to Net Energy Costs							127,000,000
Sharing Level For Fac							95.00%
Public Utilities, Requested Return on Equity, Percentage							10.40%
Public Utilities, Requested Equity Capital Structure, Percentage							51.60%
Rate Base							7,300,000,000
Number of Months to complete MoPSC Electric Service Proceeding							11 months
Long-term Contract for Purchase of Electric Power, Date of Contract Expiration								15 years
Regulatory assets	$ 1,259,000,000	$ 1,240,000,000	$ 539,000,000	$ 534,000,000	$ 712,000,000	$ 701,000,000			$ 36,000,000
Number of Customers Filed Complaint Case											37
Contested return on equity										9.80%