Rate And Regulatory Matters (Narrative-Federal) (Detail) (USD $) In Millions, unless otherwise specified														1 Months Ended			12 Months Ended		1 Months Ended	6 Months Ended			6 Months Ended
	Jun. 30, 2014	Dec. 31, 2013	Jun. 30, 2014 Ameren Illinois Company	Dec. 31, 2013 Ameren Illinois Company	Jun. 30, 2014 Ameren Illinois Company Pending Ferc Case [Member] Minimum	Jun. 30, 2014 Ameren Illinois Company Pending Ferc Case [Member] Maximum	Jun. 30, 2014 Union Electric Company	Dec. 31, 2013 Union Electric Company	Dec. 31, 2013 Union Electric Company New Nuclear Energy Center COL [Member]	Dec. 31, 2013 Union Electric Company New Nuclear Energy Center COL [Member] Minimum	Dec. 31, 2013 Union Electric Company New Nuclear Energy Center COL [Member] Maximum	Jun. 30, 2014 Wholesale Distribution Rate Case [Member]	Dec. 31, 2013 Wholesale Distribution Rate Case [Member]	Jan. 31, 2011 Wholesale Distribution Rate Case [Member] Ameren Illinois Company appraiser	Jun. 30, 2014 Wholesale Distribution Rate Case [Member] Ameren Illinois Company	Dec. 31, 2013 Wholesale Distribution Rate Case [Member] Ameren Illinois Company	Dec. 31, 2012 Entergy Refund [Member] Union Electric Company	Jun. 30, 2014 Entergy Refund [Member] Union Electric Company	Jun. 30, 2008 Ferc Relicensing [Member] Taum Sauk Energy Center [Member]	Jun. 30, 2014 Midwest Independent Transmission System Operator, Inc [Member] Pending Ferc Case [Member]	Jun. 30, 2014 Midwest Independent Transmission System Operator, Inc [Member] Pending Ferc Case [Member] Minimum	Jun. 30, 2014 Midwest Independent Transmission System Operator, Inc [Member] Pending Ferc Case [Member] Ameren Illinois Company	Jun. 30, 2014 New England Transmission Owners [Member] Pending Ferc Case [Member]	Jun. 30, 2014 New England Transmission Owners [Member] Pending Ferc Case [Member] Minimum	Jun. 30, 2014 New England Transmission Owners [Member] Pending Ferc Case [Member] Maximum	Jun. 30, 2014 Estimated Annual Reduction to Revenues From MISO ROE [Member] Midwest Independent Transmission System Operator, Inc [Member] Pending Ferc Case [Member]	Jun. 30, 2014 Estimated Annual Reduction to Revenues From MISO ROE [Member] Midwest Independent Transmission System Operator, Inc [Member] Pending Ferc Case [Member] Ameren Illinois Company
Rate And Regulatory Matters [Line Items]
Number of Customers, Reached Agreements														4
Number of Wholesale Customers														9
Number of Customers, Remaining on Agreements														5
Current regulatory liabilities	$ 218	$ 216	$ 179	$ 159			$ 39	$ 57				$ 16	$ 13		$ 14	$ 13
Loss Contingency, Estimate of Possible Loss					4	19												25		9		7				16	12
Customer Requested Rate on Equity																				9.15%
Public Utilities, Approved Return on Equity, Percentage																				12.38%	10.57%		11.14%	10.57%	11.74%
Proceeds from Legal Settlements																	31
Investments in Power and Distribution Projects									$ 69	$ 80	$ 120
Number Of Years For Proposed Relicensing Application Filed With Ferc																			30 years