Rate And Regulatory Matters (Narrative-Federal) (Detail) (USD $) In Millions, unless otherwise specified	1 Months Ended	3 Months Ended							12 Months Ended				1 Months Ended
	Nov. 30, 2012 design	Mar. 31, 2013 design	Dec. 31, 2012	Mar. 31, 2013 Ameren Illinois Company [Member]	Dec. 31, 2012 Ameren Illinois Company [Member]	Mar. 31, 2013 Union Electric Company [Member]	Dec. 31, 2012 Union Electric Company [Member]	Mar. 31, 2013 Union Electric Company [Member] New Nuclear Energy Center COL [Member]	Dec. 31, 2012 Union Electric Company [Member] New Nuclear Energy Center COL [Member] Minimum [Member]	Mar. 31, 2013 Union Electric Company [Member] New Nuclear Energy Center COL [Member] Minimum [Member]	Mar. 31, 2013 Union Electric Company [Member] New Nuclear Energy Center COL [Member] Maximum [Member]	Mar. 31, 2013 Wholesale Distribution Rate Case [Member]	Jan. 31, 2011 Wholesale Distribution Rate Case [Member] Ameren Illinois Company [Member] appraiser	Mar. 31, 2013 Wholesale Distribution Rate Case [Member] Ameren Illinois Company [Member]
Rate And Regulatory Matters [Line Items]
Number of Customers, Reached Agreements													4
Number of Wholesale Customers													9
Number of Customers, Remaining on Agreements													5
Current regulatory liabilities		$ 135	$ 100	$ 113	$ 82	$ 23	$ 18					$ 9		$ 9
Department of Energy, Investing Funding Support, Number of Small Modular Reactor Designs		2
Department of Energy, Investing Funding Support, Period		5 years
Department of Energy, Investing Funding Support, Number of Small Modular Reactor Designs Awarded	1
Number of years an COL is valid for									40 years
Investments in Power and Distribution Projects								$ 69		$ 80	$ 100