Segment Information (Schedule Of Segment Reporting Information By Segment) (Detail) $ in Millions	3 Months Ended
	Mar. 31, 2018 USD ($) segment	Mar. 31, 2017 USD ($)
Segment Reporting Information [Line Items]
Number of Reportable Segments | segment	4
External revenues	$ 1,585	$ 1,515
Intersegment revenues	0	0
Net Income Attributable to Ameren Common Shareholders	151	102
Capital expenditures	$ 579	504
Union Electric Company
Segment Reporting Information [Line Items]
Number of Reportable Segments | segment	1
Ameren Illinois Company
Segment Reporting Information [Line Items]
Number of Reportable Segments | segment	3
Ameren Illinois Company
Segment Reporting Information [Line Items]
External revenues	$ 760	703
Intersegment revenues	0	0
Net Income Available to Common Shareholder	95	79
Capital expenditures	300	227
Intersegment Eliminations
Segment Reporting Information [Line Items]
External revenues	0	0
Intersegment revenues	(22)	(25)
Net Income Available to Common Shareholder	0	0
Capital expenditures	(4)	(1)
Intersegment Eliminations | Ameren Illinois Company
Segment Reporting Information [Line Items]
External revenues	0	0
Intersegment revenues	(13)	(6)
Net Income Available to Common Shareholder	0	0
Capital expenditures	0	0
Operating Segments | Union Electric Company
Segment Reporting Information [Line Items]
External revenues	784	773
Intersegment revenues	8	18
Net Income Available to Common Shareholder	38	5
Capital expenditures	249	196
Operating Segments | Ameren Illinois Electric Distribution
Segment Reporting Information [Line Items]
External revenues	399	384
Intersegment revenues	1	1
Net Income Available to Common Shareholder	33	30
Capital expenditures	122	120
Operating Segments | Ameren Illinois Gas
Segment Reporting Information [Line Items]
External revenues	311	264
Intersegment revenues	0	0
Net Income Available to Common Shareholder	42	33
Capital expenditures	60	51
Operating Segments | Ameren Transmission
Segment Reporting Information [Line Items]
External revenues	91	96
Intersegment revenues	13	6
Net Income Available to Common Shareholder	37	34
Capital expenditures	145	134
Operating Segments | Other
Segment Reporting Information [Line Items]
External revenues	0	(2)
Intersegment revenues	0	0
Net Income Available to Common Shareholder	1	0
Capital expenditures	7	4
Operating Segments | Ameren Illinois Company | Ameren Illinois Electric Distribution
Segment Reporting Information [Line Items]
External revenues	400	385
Intersegment revenues	0	0
Net Income Available to Common Shareholder	33	30
Capital expenditures	122	120
Operating Segments | Ameren Illinois Company | Ameren Illinois Gas
Segment Reporting Information [Line Items]
External revenues	311	264
Intersegment revenues	0	0
Net Income Available to Common Shareholder	42	33
Capital expenditures	60	51
Operating Segments | Ameren Illinois Company | Ameren Illinois Transmission
Segment Reporting Information [Line Items]
External revenues	49	54
Intersegment revenues	13	6
Net Income Available to Common Shareholder	20	16
Capital expenditures	$ 118	$ 56