Segment Information (Schedule Of Segment Reporting Information By Segment) (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended
	Mar. 31, 2014		Mar. 31, 2013	Dec. 31, 2013
Segment Reporting Information [Line Items]
External revenues	$ 1,594		$ 1,475
Net income (loss) attributable to Ameren Corporation from continuing operations	97		54
Total assets	21,166			21,042
Assets of discontinued operations (Note 12)	15			165
Union Electric Company
Segment Reporting Information [Line Items]
External revenues	811		789
Intersegment revenues	6		7
Net income (loss) attributable to Ameren Corporation from continuing operations	47		40
Total assets	12,948			12,904
Ameren Illinois Company
Segment Reporting Information [Line Items]
External revenues	774		684
Intersegment revenues
Net income (loss) attributable to Ameren Corporation from continuing operations	53		31
Total assets	7,640			7,454
Other
Segment Reporting Information [Line Items]
External revenues	9		2
Intersegment revenues	1		2
Net income (loss) attributable to Ameren Corporation from continuing operations	(3)		(17)
Total assets	898			752
Intersegment Eliminations
Segment Reporting Information [Line Items]
External revenues
Intersegment revenues	(7)		(9)
Net income (loss) attributable to Ameren Corporation from continuing operations
Total assets	(335)			(233)
Consolidated, Continuing Operations
Segment Reporting Information [Line Items]
External revenues	1,594		1,475
Intersegment revenues
Net income (loss) attributable to Ameren Corporation from continuing operations	97		54
Total assets	$ 21,151	[1]		$ 20,877	[1]

[1] Excludes total assets from discontinued operations of $15 million and $165 million as of March 31, 2014, and December 31, 2013, respectively.