Segment Information (Schedule Of Segment Reporting Information By Segment) (Detail) $ in Millions		3 Months Ended
		Mar. 31, 2016 USD ($)	Mar. 31, 2015 USD ($)	Dec. 31, 2015 USD ($)
Segment Reporting Information [Line Items]
Number of Reportable Segments		2
External revenues		$ 1,434	$ 1,556
Net income (loss) attributable to Ameren Common Shareholders from continuing operations		105	108
Total assets		23,385		$ 23,640
Assets of discontinued operations		$ 14		14
Union Electric Company
Segment Reporting Information [Line Items]
Number of Reportable Segments		1
External revenues		$ 726	793
Intersegment revenues		15	7
Net income (loss) attributable to Ameren Common Shareholders from continuing operations		14	41
Total assets		$ 13,443		13,851
Ameren Illinois Company
Segment Reporting Information [Line Items]
Number of Reportable Segments		1
External revenues		$ 676	744
Intersegment revenues		1	1
Net income (loss) attributable to Ameren Common Shareholders from continuing operations		59	53
Total assets		8,944		8,903
Other
Segment Reporting Information [Line Items]
External revenues		32	19
Intersegment revenues		0	1
Net income (loss) attributable to Ameren Common Shareholders from continuing operations		32	14
Total assets		1,214		1,139
Intersegment Eliminations
Segment Reporting Information [Line Items]
Intersegment revenues		(16)	(9)
Total assets		(230)		(267)
Consolidated, Continuing Operations
Segment Reporting Information [Line Items]
External revenues		1,434	1,556
Net income (loss) attributable to Ameren Common Shareholders from continuing operations		105	$ 108
Total assets	[1]	$ 23,371		$ 23,626

[1] Excludes total assets from discontinued operations of $14 million as of March 31, 2016, and December 31, 2015.