Segment Information (Schedule Of Segment Reporting Information By Segment) (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended
	Mar. 31, 2013		Mar. 31, 2012	Dec. 31, 2012
Segment Reporting Information [Line Items]
External revenues	$ 1,475		$ 1,412
Continuing Operations	54		37
Net income (loss) attributable to Ameren Corporation	(145)		(403)
Total assets	21,952			22,209
Union Electric Company [Member]
Segment Reporting Information [Line Items]
External revenues	789		686
Intersegment revenues	7		5
Continuing Operations	40		21
Total assets	12,867			13,043
Ameren Illinois Company [Member]
Segment Reporting Information [Line Items]
External revenues	684		724
Intersegment revenues	0		0
Continuing Operations	31		27
Total assets	7,288			7,282
Other Segment [Member]
Segment Reporting Information [Line Items]
External revenues	2		2
Intersegment revenues	2		1
Continuing Operations	(17)		(11)
Total assets	1,289			1,228
Intersegment Elimination [Member]
Segment Reporting Information [Line Items]
External revenues	0		0
Intersegment revenues	(9)		(6)
Total assets	(992)			(944)
Segment, Continuing Operations [Member]
Segment Reporting Information [Line Items]
External revenues	1,475		1,412
Continuing Operations	54		37
Total assets	$ 20,452	[1]		$ 20,609	[1]

[1] Excludes “Current assets of discontinued operations.” See Note 2 - Divestiture Transactions and Discontinued Operations for additional information.