Segment Information (Schedule Of Segment Reporting Information By Segment) (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended			9 Months Ended
	Sep. 30, 2013		Sep. 30, 2012	Sep. 30, 2013		Sep. 30, 2012	Dec. 31, 2012
Segment Reporting Information [Line Items]
External revenues	$ 1,638		$ 1,709	$ 4,516		$ 4,523
Continuing Operations	305		309	464		507
Total assets	22,385			22,385			22,219
Union Electric Company
Segment Reporting Information [Line Items]
External revenues	1,088		1,059	2,760		2,583
Intersegment revenues	5		5	18		16
Continuing Operations	238		236	362		400
Total assets	13,231			13,231			13,043
Ameren Illinois Company
Segment Reporting Information [Line Items]
External revenues	547		647	1,744		1,935
Intersegment revenues			1	3		1
Continuing Operations	77		71	139		130
Total assets	7,439			7,439			7,282
Other Segment [Member]
Segment Reporting Information [Line Items]
External revenues	3		3	12		5
Intersegment revenues	1			2		2
Continuing Operations	(10)		2	(37)		(23)
Total assets	1,375			1,375			1,228
Intersegment Elimination [Member]
Segment Reporting Information [Line Items]
External revenues
Intersegment revenues	(6)		(6)	(23)		(19)
Continuing Operations
Total assets	(1,055)			(1,055)			(934)
Segment, Continuing Operations [Member]
Segment Reporting Information [Line Items]
External revenues	1,638		1,709	4,516		4,523
Continuing Operations	305		309	464		507
Total assets	$ 20,990	[1]		$ 20,990	[1]		$ 20,619	[1]

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