Rate And Regulatory Matters (Narrative-Federal) (Detail) (USD $) In Millions, unless otherwise specified																12 Months Ended		9 Months Ended			9 Months Ended					1 Months Ended
	Sep. 30, 2014	Dec. 31, 2013	Sep. 30, 2014 Ameren Illinois Company	Dec. 31, 2013 Ameren Illinois Company	Sep. 30, 2014 Ameren Illinois Company Pending Ferc Case [Member] Minimum	Sep. 30, 2014 Ameren Illinois Company Pending Ferc Case [Member] Maximum	Sep. 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	Sep. 30, 2014 Wholesale Distribution Rate Case [Member]	Dec. 31, 2013 Wholesale Distribution Rate Case [Member]	Sep. 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	Sep. 30, 2014 Entergy Refund [Member] Union Electric Company	Sep. 30, 2014 Midwest Independent Transmission System Operator, Inc [Member] Pending Ferc Case [Member]	Sep. 30, 2014 Midwest Independent Transmission System Operator, Inc [Member] Pending Ferc Case [Member] Minimum	Sep. 30, 2014 Midwest Independent Transmission System Operator, Inc [Member] Pending Ferc Case [Member] Ameren Illinois Company	Sep. 30, 2014 New England Transmission Owners [Member] Pending Ferc Case [Member]	Sep. 30, 2014 New England Transmission Owners [Member] Pending Ferc Case [Member] Minimum	Sep. 30, 2014 New England Transmission Owners [Member] Pending Ferc Case [Member] Maximum	Sep. 30, 2014 Estimated Annual Reduction to Revenues From MISO ROE [Member] Midwest Independent Transmission System Operator, Inc [Member] Pending Ferc Case [Member]	Sep. 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	Nov. 30, 2014 Subsequent Event [Member]
Rate And Regulatory Matters [Line Items]
Current regulatory liabilities	$ 132	$ 216	$ 121	$ 159			$ 11	$ 57				$ 24	$ 13	$ 24	$ 13
Loss Contingency, Estimate of Possible Loss					4	19											25	14		11				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%
Incentive adder to FERC allowed base return on common equity																										50
Proceeds from Legal Settlements																31
Investments in Power and Distribution Projects									$ 69	$ 80	$ 120