Segment Information (Schedule Of Segment Reporting Information By Segment) (Detail) $ in Millions		3 Months Ended		6 Months Ended
		Jun. 30, 2016 USD ($)	Jun. 30, 2015 USD ($)	Jun. 30, 2016 USD ($) segment	Jun. 30, 2015 USD ($)	Dec. 31, 2015 USD ($)
Segment Reporting Information [Line Items]
Number of Reportable Segments | segment				2
External revenues		$ 1,427	$ 1,401	$ 2,861	$ 2,957
Net income (loss) attributable to Ameren Common Shareholders from continuing operations		147	98	252	206
Total assets		23,793		23,793		$ 23,640
Assets of discontinued operations		14		$ 14		14
Union Electric Company
Segment Reporting Information [Line Items]
Number of Reportable Segments | segment				1
External revenues		857	872	$ 1,583	1,665
Intersegment revenues		10	12	25	19
Net income (loss) attributable to Ameren Common Shareholders from continuing operations		92	61	106	102
Total assets		13,649		$ 13,649		13,851
Ameren Illinois Company
Segment Reporting Information [Line Items]
Number of Reportable Segments				1
External revenues		541	512	$ 1,217	1,256
Intersegment revenues		1	1	2	2
Net income (loss) attributable to Ameren Common Shareholders from continuing operations		45	31	104	84
Total assets		8,999		8,999		8,903
Other
Segment Reporting Information [Line Items]
External revenues		29	17	61	36
Intersegment revenues		1	0	1	1
Net income (loss) attributable to Ameren Common Shareholders from continuing operations		10	6	42	20
Total assets		1,276		1,276		1,139
Intersegment Eliminations
Segment Reporting Information [Line Items]
Intersegment revenues		(12)	(13)	(28)	(22)
Total assets		(145)		(145)		(267)
Consolidated, Continuing Operations
Segment Reporting Information [Line Items]
External revenues		1,427	1,401	2,861	2,957
Net income (loss) attributable to Ameren Common Shareholders from continuing operations		147	$ 98	252	$ 206
Total assets	[1]	$ 23,779		$ 23,779		$ 23,626

[1] Excludes total assets from discontinued operations of $14 million as of June 30, 2016, and December 31, 2015.