Rate And Regulatory Matters (Narrative-Illinois) (Detail) (USD $) In Millions, unless otherwise specified	9 Months Ended											9 Months Ended
	Sep. 30, 2012	Sep. 30, 2011	Sep. 30, 2012 Ameren Illinois Company [Member]	Sep. 30, 2011 Ameren Illinois Company [Member]	Sep. 30, 2012 Ameren Illinois Company [Member] Electric Delivery Service [Member]	Sep. 30, 2012 Ieima [Member] Ameren Illinois Company [Member] Electric Delivery Service [Member]	Jan. 03, 2012 Ieima [Member] Ameren Illinois Company [Member] Electric Delivery Service [Member] Final Rate Order [Member]	Apr. 20, 2012 Ieima [Member] Ameren Illinois Company [Member] Electric Delivery Service [Member] Update Filing [Member]	Apr. 20, 2012 Ieima [Member] Ameren Illinois Company [Member] Electric Delivery Service [Member] Update Filing [Member] Icc Staff Recommendation [Member]	Nov. 07, 2012 Ieima [Member] Ameren Illinois Company [Member] Electric Delivery Service [Member] Update Filing [Member] Administrative Law Judges Recommendation [Member] Subsequent Event [Member]	Jan. 03, 2012 Ieima [Member] Ameren Illinois Company [Member] Electric Delivery Service Calculated [Member] Final Rate Order [Member]	Sep. 30, 2012 Ieima [Member] Ameren Illinois Company [Member] Smart Grid [Member] Y
Rate And Regulatory Matters [Line Items]
Revenue requirement							$ 779	$ 796	$ 765	$ 764
Utility revenue decrease									14	15	55
Reduction in planned capital expenditures					30
Utility revenue increase								17
Maximum Basis Points above or below allowed return						0.50%
Regulatory liability						35
Required smart grid spending over the IEIMA required time period	$ 905	$ 758	$ 309	$ 261								$ 360
Smart grid assets in years												10