Segment Information (Schedule Of Segment Reporting Information By Segment) (Detail) $ in Millions		3 Months Ended
		Mar. 31, 2017 USD ($) segment	Mar. 31, 2016 USD ($)
Segment Reporting Information [Line Items]
Number of Reportable Segments | segment		4
External revenues		$ 1,514	$ 1,434
Net Income Attributable to Ameren Common Shareholders		102	105
Capital expenditures		$ 504	496
Union Electric Company
Segment Reporting Information [Line Items]
Number of Reportable Segments | segment		1
Consolidated
Segment Reporting Information [Line Items]
External revenues		$ 1,514	1,434
Intersegment revenues		0	0
Net Income Attributable to Ameren Common Shareholders		102	105
Capital expenditures		$ 504	496
Ameren Illinois Company
Segment Reporting Information [Line Items]
Number of Reportable Segments | segment		3
Ameren Illinois Company
Segment Reporting Information [Line Items]
External revenues		$ 703	677
Intersegment revenues		0	0
Net Income Available to Common Shareholder		79	59
Capital expenditures		227	211
Intersegment Eliminations
Segment Reporting Information [Line Items]
External revenues		0	0
Intersegment revenues		(25)	(27)
Net Income Available to Common Shareholder		0	0
Capital expenditures		(1)	0
Intersegment Eliminations | Ameren Illinois Company
Segment Reporting Information [Line Items]
External revenues		0	0
Intersegment revenues		(6)	(11)
Net Income Available to Common Shareholder		0	0
Capital expenditures		0	0
Operating Segments [Member] | Union Electric Company
Segment Reporting Information [Line Items]
External revenues		772	726
Intersegment revenues		18	15
Net Income Available to Common Shareholder		5	14
Capital expenditures		196	178
Operating Segments [Member] | Ameren Illinois Electric Distribution [Member]
Segment Reporting Information [Line Items]
External revenues		384	351
Intersegment revenues		1	1
Net Income Available to Common Shareholder		30	11
Capital expenditures		120	117
Operating Segments [Member] | Ameren Illinois Gas [Member]
Segment Reporting Information [Line Items]
External revenues		264	285
Intersegment revenues		0	0
Net Income Available to Common Shareholder		33	35
Capital expenditures		51	35
Operating Segments [Member] | Ameren Transmission [Member]
Segment Reporting Information [Line Items]
External revenues		96	72
Intersegment revenues	[1]	6	11
Net Income Available to Common Shareholder		34	27
Capital expenditures		134	164
Operating Segments [Member] | Other
Segment Reporting Information [Line Items]
External revenues		(2)	0
Intersegment revenues		0	0
Net Income Available to Common Shareholder		0	18
Capital expenditures	[2]	4	2
Operating Segments [Member] | Ameren Illinois Company | Ameren Illinois Electric Distribution [Member]
Segment Reporting Information [Line Items]
External revenues		385	352
Intersegment revenues		0	0
Net Income Available to Common Shareholder		30	11
Capital expenditures		120	117
Operating Segments [Member] | Ameren Illinois Company | Ameren Illinois Gas [Member]
Segment Reporting Information [Line Items]
External revenues		264	285
Intersegment revenues		0	0
Net Income Available to Common Shareholder		33	35
Capital expenditures		51	35
Operating Segments [Member] | Ameren Illinois Company | Ameren Illinois Transmission [Member]
Segment Reporting Information [Line Items]
External revenues		54	40
Intersegment revenues	[3]	6	11
Net Income Available to Common Shareholder		16	13
Capital expenditures		$ 56	$ 59

[1] (a)Ameren Transmission earns revenue from transmission service provided to Ameren Illinois Electric Distribution. See discussion of transactions above.

[2] (b)Includes the elimination of intercompany transfers.

[3] (a)Ameren Illinois Transmission earns revenue from transmission service provided to Ameren Illinois Electric Distribution. See discussion of transactions above.