Rate And Regulatory Matters (Narrative-Illinois) (Detail) (USD $) In Millions, unless otherwise specified	6 Months Ended			6 Months Ended				6 Months Ended
	Jun. 30, 2012	Jun. 30, 2011	Dec. 31, 2011	Jun. 30, 2012 Ameren Illinois Company [Member]	Jun. 30, 2011 Ameren Illinois Company [Member]	Dec. 31, 2011 Ameren Illinois Company [Member]	Jun. 30, 2012 Ieima [Member] Ameren Illinois Company [Member]	Jun. 30, 2012 Ieima [Member] Ameren Illinois Company [Member] Smart Grid [Member] Year	Jan. 03, 2012 Ieima [Member] Ameren Illinois Company [Member] Electric Delivery Service Current [Member]	Jan. 03, 2012 Ieima [Member] Ameren Illinois Company [Member] Electric Delivery Service Calculated [Member] Initial Filing [Member]	Jan. 03, 2012 Ieima [Member] Ameren Illinois Company [Member] Electric Delivery Service [Member] Initial Filing [Member] Icc Staff Recommendation [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]
Rate And Regulatory Matters [Line Items]
Revenue requirement									$ 834	$ 814	$ 785	$ 798	$ 776
Utility revenue decrease										20	49	16	9
Regulatory assets	1,551		1,603	775		748	12
Smart grid assets	$ 565	$ 507		$ 184	$ 174			$ 360
Smart grid assets in years								10