Rate And Regulatory Matters (Narrative-Missouri) (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended		6 Months Ended		3 Months Ended		6 Months Ended		1 Months Ended				6 Months Ended	3 Months Ended
	Jun. 30, 2013	Jun. 30, 2012	Jun. 30, 2013	Jun. 30, 2012	Jun. 30, 2013 Ameren Missouri [Member]	Jun. 30, 2012 Ameren Missouri [Member]	Jun. 30, 2013 Ameren Missouri [Member]	Jun. 30, 2012 Ameren Missouri [Member]	Apr. 30, 2011 Ameren Missouri [Member] Fac Prudence Review [Member]	Jul. 31, 2011 Ameren Missouri [Member] Accounting Authority Order Request [Member]	Dec. 30, 2012 Electric Distribution [Member] Ameren Missouri [Member] Final Rate Order [Member]	Jun. 30, 2013 Fac Prudence Review [Member] Subsequent Periods After September 30, 2009 [Member]	Jun. 30, 2013 Fac Prudence Review [Member] Subsequent Periods After September 30, 2009 [Member] MoPSC order [Member]	Jun. 30, 2013 Fac Prudence Review [Member] Subsequent Periods After September 30, 2009 [Member] Ameren Missouri [Member]
Rate And Regulatory Matters [Line Items]
Contested amounts under the FAC									$ 18			$ 3	$ 26	$ 23
Interest Charges	100	98	201	196	56	56	116	112	1				1	1
Request to defer fixed costs not recovered from Noranda, amount										36
Authorized Increase in Revenue from Utility Service											$ 260