Rate And Regulatory Matters (Narrative-Missouri) (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended				1 Months Ended
	Mar. 31, 2013	Mar. 31, 2012	Mar. 31, 2013 Union Electric Company [Member]	Mar. 31, 2012 Union Electric Company [Member]	Apr. 30, 2011 Union Electric Company [Member] Fac Prudence Review [Member]	Jul. 31, 2011 Union Electric Company [Member] Accounting Authority Order Request [Member] Fac Prudence Review [Member]	Feb. 29, 2012 Subsequent Periods After September 30, 2009 [Member] Mopsc Staff Report [Member] Fac Prudence Review [Member]
Rate And Regulatory Matters [Line Items]
Contested amounts under the FAC					$ 18		$ 26
Interest Charges	101	98	60	56	1
Request to defer fixed costs not recovered from Noranda, amount						$ 36