Rate And Regulatory Matters (Narrative-Federal) (Detail) (USD $)	1 Months Ended	9 Months Ended									12 Months Ended					1 Months Ended
	Nov. 30, 2012 design	Sep. 30, 2013 design	Dec. 31, 2012	Sep. 30, 2013 Ameren Illinois Company	Dec. 31, 2012 Ameren Illinois Company	Sep. 30, 2013 Ameren Illinois Company Pending Ferc Case [Member] Minimum	Sep. 30, 2013 Ameren Illinois Company Pending Ferc Case [Member] Maximum	Sep. 30, 2013 Union Electric Company	Dec. 31, 2012 Union Electric Company	Sep. 30, 2013 Union Electric Company New Nuclear Energy Center COL [Member]	Dec. 31, 2012 Union Electric Company New Nuclear Energy Center COL [Member] Minimum	Sep. 30, 2013 Union Electric Company New Nuclear Energy Center COL [Member] Minimum	Sep. 30, 2013 Union Electric Company New Nuclear Energy Center COL [Member] Maximum	Sep. 30, 2013 Wholesale Distribution Rate Case [Member]	Dec. 31, 2012 Wholesale Distribution Rate Case [Member]	Jan. 31, 2011 Wholesale Distribution Rate Case [Member] Ameren Illinois Company appraiser	Sep. 30, 2013 Wholesale Distribution Rate Case [Member] Ameren Illinois Company
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		$ 149,000,000	$ 100,000,000	$ 88,000,000	$ 82,000,000			$ 61,000,000	$ 18,000,000					$ 12,000,000	$ 8,000,000		$ 9,000,000
Range of possible loss, minimum						10,000,000	15,000,000
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										$ 69,000,000		$ 80,000,000	$ 100,000,000