Segment Information (Schedule Of Segment Reporting Information By Segment) (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended			6 Months Ended
	Jun. 30, 2013		Jun. 30, 2012	Jun. 30, 2013		Jun. 30, 2012	Dec. 31, 2012
Segment Reporting Information [Line Items]
External revenues	$ 1,403		$ 1,402	$ 2,878		$ 2,814
Continuing Operations	105		161	159		198
Total assets	22,276			22,276			22,209
Ameren Missouri [Member]
Segment Reporting Information [Line Items]
External revenues	883		838	1,672		1,524
Intersegment revenues	6		6	13		11
Continuing Operations	84		143	124		164
Total assets	13,131			13,131			13,043
Ameren Illinois Company [Member]
Segment Reporting Information [Line Items]
External revenues	514		564	1,197		1,288
Intersegment revenues	2			3
Continuing Operations	31		32	62		59
Total assets	7,366			7,366			7,282
Other Segment [Member]
Segment Reporting Information [Line Items]
External revenues	6			9		2
Intersegment revenues			1	1		2
Continuing Operations	(10)		(14)	(27)		(25)
Total assets	1,354			1,354			1,228
Intersegment Elimination [Member]
Segment Reporting Information [Line Items]
External revenues
Intersegment revenues	(8)		(7)	(17)		(13)
Continuing Operations
Total assets	(1,061)			(1,061)			(944)
Segment, Continuing Operations [Member]
Segment Reporting Information [Line Items]
External revenues	1,403		1,402	2,878		2,814
Continuing Operations	105		161	159		198
Total assets	$ 20,790	[1]		$ 20,790	[1]		$ 20,609	[1]

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