Rate And Regulatory Matters (Narrative-Federal) (Detail) (USD $) In Millions, unless otherwise specified													1 Months Ended			12 Months Ended		3 Months Ended
	Mar. 31, 2014	Dec. 31, 2013	Mar. 31, 2014 Ameren Illinois Company	Dec. 31, 2013 Ameren Illinois Company	Mar. 31, 2014 Ameren Illinois Company Pending Ferc Case [Member] Maximum	Mar. 31, 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	Mar. 31, 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	Mar. 31, 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	Mar. 31, 2014 Entergy Refund [Member] Union Electric Company	Mar. 31, 2014 Midwest Independent Transmission System Operator, Inc [Member] Pending Ferc Case [Member]
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	$ 248	$ 216	$ 201	$ 159		$ 47	$ 57				$ 14	$ 13		$ 14	$ 13
Customer Requested Rate on Equity																		9.15%
Rate Of Return On Common Equity																		12.38%
Proceeds from Legal Settlements																31
Range of possible loss, minimum					19												25
Investments in Power and Distribution Projects								$ 69	$ 80	$ 100