Segment Information (Schedule Of Segment Reporting Information By Segment) (Detail) $ in Millions		3 Months Ended		6 Months Ended
		Jun. 30, 2015 USD ($)	Jun. 30, 2014 USD ($)	Jun. 30, 2015 USD ($)	Jun. 30, 2014 USD ($)	Dec. 31, 2014 USD ($)
Segment Reporting Information [Line Items]
Number of Reportable Segments				2
External revenues		$ 1,401	$ 1,419	$ 2,957	$ 3,013
Net income (loss) attributable to Ameren Corporation from continuing operations		98	150	206	247
Total assets		23,152		23,152		$ 22,676
Assets of discontinued operations (Note 12)		15		$ 15		15
Union Electric Company
Segment Reporting Information [Line Items]
Number of Reportable Segments				1
External revenues		872	893	$ 1,665	1,704
Intersegment revenues		12	7	19	13
Net income (loss) attributable to Ameren Corporation from continuing operations		61	126	102	173
Total assets		13,721		$ 13,721		13,541
Ameren Illinois Company
Segment Reporting Information [Line Items]
Number of Reportable Segments				1
External revenues		512	518	$ 1,256	1,292
Intersegment revenues		1	1	2	1
Net income (loss) attributable to Ameren Corporation from continuing operations		31	28	84	81
Total assets		8,500		8,500		8,381
Other
Segment Reporting Information [Line Items]
External revenues		17	8	36	17
Intersegment revenues				1	1
Net income (loss) attributable to Ameren Corporation from continuing operations		6	(4)	20	(7)
Total assets		1,036		1,036		942
Intersegment Eliminations
Segment Reporting Information [Line Items]
Intersegment revenues		(13)	(8)	(22)	(15)
Total assets		(120)		(120)		(203)
Consolidated, Continuing Operations
Segment Reporting Information [Line Items]
External revenues		1,401	1,419	2,957	3,013
Net income (loss) attributable to Ameren Corporation from continuing operations		98	$ 150	206	$ 247
Total assets	[1]	$ 23,137		$ 23,137		$ 22,661

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