Segment Information (Schedule Of Segment Reporting Information By Segment) (Detail) $ in Millions		3 Months Ended		9 Months Ended
		Sep. 30, 2015 USD ($)	Sep. 30, 2014 USD ($)	Sep. 30, 2015 USD ($)	Sep. 30, 2014 USD ($)	Dec. 31, 2014 USD ($)
Segment Reporting Information [Line Items]
Number of Reportable Segments				2
External revenues		$ 1,833	$ 1,670	$ 4,790	$ 4,683
Net income (loss) attributable to Ameren Common Stockholders from continuing operations		343	294	549	541
Total assets		23,459		23,459		$ 22,676
Assets of discontinued operations (Note 12)		17		$ 17		15
Union Electric Company
Segment Reporting Information [Line Items]
Number of Reportable Segments				1
External revenues		1,160	1,089	$ 2,825	2,793
Intersegment revenues		11	8	30	21
Net income (loss) attributable to Ameren Common Stockholders from continuing operations		239	222	341	395
Total assets		13,941		$ 13,941		13,541
Ameren Illinois Company
Segment Reporting Information [Line Items]
Number of Reportable Segments				1
External revenues		654	572	$ 1,910	1,864
Intersegment revenues		1		3	1
Net income (loss) attributable to Ameren Common Stockholders from continuing operations		98	75	182	156
Total assets		8,656		8,656		8,381
Other
Segment Reporting Information [Line Items]
External revenues		19	9	55	26
Intersegment revenues		1	2	2	3
Net income (loss) attributable to Ameren Common Stockholders from continuing operations		6	(3)	26	(10)
Total assets		1,358		1,358		942
Intersegment Eliminations
Segment Reporting Information [Line Items]
Intersegment revenues		(13)	(10)	(35)	(25)
Total assets		(513)		(513)		(203)
Consolidated, Continuing Operations
Segment Reporting Information [Line Items]
External revenues		1,833	1,670	4,790	4,683
Net income (loss) attributable to Ameren Common Stockholders from continuing operations		343	$ 294	549	$ 541
Total assets	[1]	$ 23,442		$ 23,442		$ 22,661

[1] Excludes total assets from discontinued operations of $17 million and $15 million as of September 30, 2015 and December 31, 2014, respectively.