Rate And Regulatory Matters (Narrative-Missouri) (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended		9 Months Ended		3 Months Ended		9 Months Ended				1 Months Ended	3 Months Ended
	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013 Union Electric Company	Sep. 30, 2012 Union Electric Company	Sep. 30, 2013 Union Electric Company	Sep. 30, 2012 Union Electric Company	Jul. 31, 2011 Union Electric Company Accounting Authority Order Request [Member]	Dec. 30, 2012 Electric Distribution [Member] Union Electric Company Final Rate Order [Member]	Jul. 31, 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] Union Electric Company	Sep. 30, 2013 Fac Prudence Review [Member] Subsequent Periods After September 30, 2009 [Member] Union Electric Company
Rate And Regulatory Matters [Line Items]
Contested amounts under the FAC											$ 26	$ 23	$ 3
Interest Charges	88	99	289	295	43	55	159	167			1	1
Request to defer fixed costs not recovered from Noranda, amount									36
Authorized Increase in Revenue from Utility Service										$ 260