Segment Information (Schedule Of Segment Reporting Information By Segment) (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended
	Mar. 31, 2015		Mar. 31, 2014	Dec. 31, 2014
Segment Reporting Information [Line Items]
Number of Reportable Segments	2
External revenues	$ 1,556		$ 1,594
Net income (loss) attributable to Ameren Corporation from continuing operations	108		97
Total assets	22,884			22,676
Assets of discontinued operations (Note 12)	15			15
Union Electric Company
Segment Reporting Information [Line Items]
External revenues	793		811
Intersegment revenues	7		6
Net income (loss) attributable to Ameren Corporation from continuing operations	41		47
Total assets	13,555			13,541
Ameren Illinois Company
Segment Reporting Information [Line Items]
External revenues	744		774
Intersegment revenues	1
Net income (loss) attributable to Ameren Corporation from continuing operations	53		53
Total assets	8,456			8,381
Other
Segment Reporting Information [Line Items]
External revenues	19		9
Intersegment revenues	1		1
Net income (loss) attributable to Ameren Corporation from continuing operations	14		(3)
Total assets	1,082			942
Intersegment Eliminations
Segment Reporting Information [Line Items]
Intersegment revenues	(9)		(7)
Total assets	(224)			(203)
Consolidated, Continuing Operations
Segment Reporting Information [Line Items]
External revenues	1,556		1,594
Net income (loss) attributable to Ameren Corporation from continuing operations	108		97
Total assets	$ 22,869	[1]		$ 22,661	[1]

[1] Excludes total assets from discontinued operations of $15 million as of March 31, 2015, and December 31, 2014.