Document And Entity Information (USD $)	12 Months Ended			12 Months Ended		12 Months Ended		12 Months Ended
	Dec. 31, 2011	Jun. 30, 2011	Jan. 31, 2012 Ameren Corporation [Member]	Dec. 31, 2011 Union Electric Company [Member]	Jan. 31, 2012 Union Electric Company [Member]	Dec. 31, 2011 Ameren Illinois Company [Member]	Jan. 31, 2012 Ameren Illinois Company [Member]	Dec. 31, 2011 Ameren Energy Generating Company [Member]	Jan. 31, 2012 Ameren Energy Generating Company [Member]
Document Type	10-K			10-K		10-K		10-K
Amendment Flag	false			false		false		false
Document Period End Date	Dec. 31, 2011			Dec. 31, 2011		Dec. 31, 2011		Dec. 31, 2011
Document Fiscal Year Focus	2011			2011		2011		2011
Document Fiscal Period Focus	FY			FY		FY		FY
Trading Symbol	AEE
Entity Registrant Name	AMEREN CORP			UNION ELECTRIC CO		AMEREN ILLINOIS CO		AMEREN ENERGY GENERATING CO
Entity Central Index Key	0001002910			0000100826		0000018654		0001135361
Current Fiscal Year End Date	--12-31			--12-31		--12-31		--12-31
Entity Filer Category	Large Accelerated Filer			Non-accelerated Filer		Non-accelerated Filer		Non-accelerated Filer
Entity Common Stock, Shares Outstanding			242,634,742		102,123,834		25,452,373		2,000
Entity Voluntary Filers	No			No		No		Yes
Entity Current Reporting Status	Yes			Yes		Yes		Yes
Entity Public Float		$ 6,967,355,641
Entity Well-known Seasoned Issuer	Yes			No		No		No