Segment Information (Schedule Of Segment Reporting Information By Segment) (Detail) (USD $)	3 Months Ended			9 Months Ended
	Sep. 30, 2014		Sep. 30, 2013	Sep. 30, 2014		Sep. 30, 2013	Dec. 31, 2013
Segment Reporting Information [Line Items]
Number of Reportable Segments				2
External revenues	$ 1,670,000,000		$ 1,638,000,000	$ 4,683,000,000		$ 4,516,000,000
Net income (loss) attributable to Ameren Corporation from continuing operations	294,000,000		305,000,000	541,000,000		464,000,000
Total assets	21,876,000,000			21,876,000,000			21,042,000,000
Assets of discontinued operations (Note 12)	15,000,000			15,000,000			165,000,000
Union Electric Company
Segment Reporting Information [Line Items]
External revenues	1,089,000,000		1,088,000,000	2,793,000,000		2,760,000,000
Intersegment revenues	8,000,000		5,000,000	21,000,000		18,000,000
Net income (loss) attributable to Ameren Corporation from continuing operations	222,000,000		238,000,000	395,000,000		362,000,000
Total assets	13,179,000,000			13,179,000,000			12,904,000,000
Ameren Illinois Company
Segment Reporting Information [Line Items]
External revenues	572,000,000		547,000,000	1,864,000,000		1,744,000,000
Intersegment revenues				1,000,000		3,000,000
Net income (loss) attributable to Ameren Corporation from continuing operations	75,000,000		77,000,000	156,000,000		139,000,000
Total assets	7,983,000,000			7,983,000,000			7,454,000,000
Other
Segment Reporting Information [Line Items]
External revenues	9,000,000		3,000,000	26,000,000		12,000,000
Intersegment revenues	2,000,000		1,000,000	3,000,000		2,000,000
Net income (loss) attributable to Ameren Corporation from continuing operations	(3,000,000)		(10,000,000)	(10,000,000)		(37,000,000)
Total assets	810,000,000			810,000,000			752,000,000
Intersegment Eliminations
Segment Reporting Information [Line Items]
External revenues
Intersegment revenues	(10,000,000)		(6,000,000)	(25,000,000)		(23,000,000)
Net income (loss) attributable to Ameren Corporation from continuing operations
Total assets	(111,000,000)			(111,000,000)			(233,000,000)
Consolidated, Continuing Operations
Segment Reporting Information [Line Items]
External revenues	1,670,000,000		1,638,000,000	4,683,000,000		4,516,000,000
Intersegment revenues
Net income (loss) attributable to Ameren Corporation from continuing operations	294,000,000		305,000,000	541,000,000		464,000,000
Total assets	$ 21,861,000,000	[1]		$ 21,861,000,000	[1]		$ 20,877,000,000	[1]

[1] Excludes total assets from discontinued operations of $15 million and $165 million as of September 30, 2014, and December 31, 2013, respectively.