Rate And Regulatory Matters (Narrative-Federal) (Detail) (USD $)	1 Months Ended	6 Months Ended									12 Months Ended					1 Months Ended
	Nov. 30, 2012 design	Jun. 30, 2013 design	Dec. 31, 2012	Jun. 30, 2013 Ameren Illinois Company [Member]	Dec. 31, 2012 Ameren Illinois Company [Member]	Jun. 30, 2013 Ameren Illinois Company [Member] Pending Ferc Case [Member] Minimum [Member]	Jun. 30, 2013 Ameren Illinois Company [Member] Pending Ferc Case [Member] Maximum [Member]	Jun. 30, 2013 Ameren Missouri [Member]	Dec. 31, 2012 Ameren Missouri [Member]	Jun. 30, 2013 Ameren Missouri [Member] New Nuclear Energy Center COL [Member]	Dec. 31, 2012 Ameren Missouri [Member] New Nuclear Energy Center COL [Member] Minimum [Member]	Jun. 30, 2013 Ameren Missouri [Member] New Nuclear Energy Center COL [Member] Minimum [Member]	Jun. 30, 2013 Ameren Missouri [Member] New Nuclear Energy Center COL [Member] Maximum [Member]	Jun. 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 [Member] appraiser	Jun. 30, 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		$ 180,000,000	$ 100,000,000	$ 110,000,000	$ 82,000,000			$ 71,000,000	$ 18,000,000					$ 11,000,000	$ 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