Segment Information (Schedule Of Segment Reporting Information By Segment) (Detail) (USD $)	3 Months Ended			6 Months Ended
	Jun. 30, 2014		Jun. 30, 2013	Jun. 30, 2014		Jun. 30, 2013	Dec. 31, 2013
Segment Reporting Information [Line Items]
Number of Reportable Segments				2
External revenues	$ 1,419,000,000		$ 1,403,000,000	$ 3,013,000,000		$ 2,878,000,000
Net income (loss) attributable to Ameren Corporation from continuing operations	150,000,000		105,000,000	247,000,000		159,000,000
Total assets	21,588,000,000			21,588,000,000			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	893,000,000		883,000,000	1,704,000,000		1,672,000,000
Intersegment revenues	7,000,000		6,000,000	13,000,000		13,000,000
Net income (loss) attributable to Ameren Corporation from continuing operations	126,000,000		84,000,000	173,000,000		124,000,000
Total assets	13,203,000,000			13,203,000,000			12,904,000,000
Ameren Illinois Company
Segment Reporting Information [Line Items]
External revenues	518,000,000		514,000,000	1,292,000,000		1,197,000,000
Intersegment revenues	1,000,000		2,000,000	1,000,000		3,000,000
Net income (loss) attributable to Ameren Corporation from continuing operations	28,000,000		31,000,000	81,000,000		62,000,000
Total assets	7,719,000,000			7,719,000,000			7,454,000,000
Other
Segment Reporting Information [Line Items]
External revenues	8,000,000		6,000,000	17,000,000		9,000,000
Intersegment revenues				1,000,000		1,000,000
Net income (loss) attributable to Ameren Corporation from continuing operations	(4,000,000)		(10,000,000)	(7,000,000)		(27,000,000)
Total assets	773,000,000			773,000,000			752,000,000
Intersegment Eliminations
Segment Reporting Information [Line Items]
External revenues
Intersegment revenues	(8,000,000)		(8,000,000)	(15,000,000)		(17,000,000)
Net income (loss) attributable to Ameren Corporation from continuing operations
Total assets	(122,000,000)			(122,000,000)			(233,000,000)
Consolidated, Continuing Operations
Segment Reporting Information [Line Items]
External revenues	1,419,000,000		1,403,000,000	3,013,000,000		2,878,000,000
Intersegment revenues
Net income (loss) attributable to Ameren Corporation from continuing operations	150,000,000		105,000,000	247,000,000		159,000,000
Total assets	$ 21,573,000,000	[1]		$ 21,573,000,000	[1]		$ 20,877,000,000	[1]

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