Rate And Regulatory Matters (Narrative-Missouri) (Detail) (USD $)								1 Months Ended		1 Months Ended
	Jun. 30, 2014	Dec. 31, 2013	Jun. 30, 2014 Union Electric Company	Dec. 31, 2013 Union Electric Company	Jun. 30, 2014 Ameren Illinois Company	Dec. 31, 2013 Ameren Illinois Company	Jun. 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 customer	Jul. 31, 2014 Subsequent Event [Member] Electric Distribution [Member] Union Electric Company Pending Rate Case [Member]
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
Regulatory assets	1,213,000,000	1,240,000,000	529,000,000	534,000,000	677,000,000	701,000,000	36,000,000
Number of Customers Filed Complaint Case									37
Contested return on equity								9.80%
Customer Requested Rate Decrease								$ 49,000,000
Customer Requested Rate on Equity								9.40%