Rate And Regulatory Matters (Narrative-Missouri) (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended					1 Months Ended							1 Months Ended
	Mar. 31, 2012	Mar. 31, 2011	Dec. 31, 2011	Dec. 31, 2010	Feb. 29, 2012 MoPSC Staff Report [Member] FAC Prudence Review [Member] Subsequent Periods After September 30, 2009 [Member]	Apr. 30, 2011 Ameren Missouri [Member] FAC Prudence Review [Member]	Sep. 30, 2011 Ameren Missouri [Member] FAC Prudence Review [Member] M	Mar. 31, 2012 Ameren Missouri [Member] 2009 Final Rate Order [Member] Electric Distribution [Member]	May 31, 2012 Ameren Missouri [Member] Final Rate Order [Member] Electric Distribution [Member]	Jul. 31, 2011 Ameren Missouri [Member] Final Rate Order [Member] Electric Distribution [Member]	May 31, 2010 Ameren Missouri [Member] Final Rate Order [Member] Electric Distribution [Member] customer	Feb. 03, 2012 Ameren Missouri [Member] Pending Rate Order [Member] Electric Distribution [Member]	Jan. 31, 2012 Ameren Missouri [Member] Pending Rate Order [Member] MEEIA Filing [Member] Electric Distribution [Member]	Jul. 31, 2011 Ameren Missouri [Member] Accounting Authority Order Request [Member] FAC Prudence Review [Member]
Rate And Regulatory Matters [Line Items]
Number of industrial customers who received a stay from Circuit Court											4
Amount held by Circuit Court based on appeal of electric rate order									$ 2		$ 16
Cash received	208	573	255	545				21	14
Authorized increase in revenue from utility service										173
Utility revenue increase requested												376	81
Energy Efficiency program spending													145
Number of years energy efficiency spending will occur													3
Time required to complete FAC prudence reviews, in months							18
Contested amounts under the FAC					26	18
Interest charges	113	119				1
Request to defer fixed costs not recovered from Noranda, amount														$ 36