Rate And Regulatory Matters (Narrative-Illinois) (Detail) (USD $)					12 Months Ended		9 Months Ended			9 Months Ended					9 Months Ended		9 Months Ended
	Sep. 30, 2014	Dec. 31, 2013	Sep. 30, 2014 Ameren Illinois Company	Dec. 31, 2013 Ameren Illinois Company	Dec. 31, 2013 Ameren Illinois Company Gas Distribution [Member] Final Rate Order [Member]	Dec. 31, 2013 Ameren Illinois Company Gas Distribution [Member] Rate order appeal [Member]	Sep. 30, 2014 Ieima [Member] Ameren Illinois Company Electric Distribution [Member] IEMA Revenue Requirement Reconciliation [Member]	Sep. 30, 2014 Ieima [Member] Ameren Illinois Company Electric Distribution [Member] 2013 IEMA Revenue Requirement Reconciliation [Member]	Sep. 30, 2014 Ieima [Member] Ameren Illinois Company Electric Distribution [Member] 2012 IEMA Revenue Requirement Reconciliation [Member]	Sep. 30, 2014 Icc Staff Recommendation [Member] Ieima [Member] Ameren Illinois Company Electric Distribution [Member] IEMA Revenue Requirement Reconciliation [Member]	Sep. 30, 2014 Other Intervenors [Member] Ieima [Member] Ameren Illinois Company Electric Distribution [Member] IEMA Revenue Requirement Reconciliation [Member]	Sep. 30, 2014 Administrative Law Judge [Member] Ieima [Member] Ameren Illinois Company Electric Distribution [Member] IEMA Revenue Requirement Reconciliation [Member]	Sep. 30, 2014 Minimum Ameren Illinois Company Pending Ferc Case [Member]	Sep. 30, 2014 Maximum Ameren Illinois Company Pending Ferc Case [Member]	Sep. 30, 2014 Pending Ferc Case [Member] Midwest Independent Transmission System Operator, Inc [Member]	Sep. 30, 2014 Pending Ferc Case [Member] Midwest Independent Transmission System Operator, Inc [Member] Ameren Illinois Company	Sep. 30, 2014 Pending Ferc Case [Member] Minimum Midwest Independent Transmission System Operator, Inc [Member]
Rate And Regulatory Matters [Line Items]
Regulatory assets	$ 1,259,000,000	$ 1,240,000,000	$ 712,000,000	$ 701,000,000			$ 76,000,000	$ 64,000,000
Current regulatory liabilities	132,000,000	216,000,000	121,000,000	159,000,000					13,000,000
Public Utilities, Requested Rate Increase (Decrease), Amount							205,000,000			205,000,000	7,000,000	204,000,000
Authorized Increase in Revenue from Utility Service					32,000,000
Public Utilities, Approved Return on Equity, Percentage					9.10%	10.40%									12.38%		10.57%
Percent Of Capital Structure Composed Of Equity					51.70%
Rate Base					1,100,000,000
Loss Contingency, Estimate of Possible Loss													$ 4,000,000	$ 19,000,000	$ 14,000,000	$ 11,000,000