Segment Information (Schedule Of Segment Reporting Information By Segment) (Detail) $ in Millions	3 Months Ended
	Mar. 31, 2019 USD ($) segment	Mar. 31, 2018 USD ($)
Segment Reporting Information [Line Items]
Number of Reportable Segments | segment	4
External revenues	$ 1,556	$ 1,585
Intersegment revenues	0	0
Net Income (Loss) Available to Common Stockholders, Basic	191	151
Capital expenditures	$ 544	579
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	$ 762	760
Intersegment revenues	0	0
Net Income Available to Common Shareholder	120	95
Capital expenditures	267	300
Operating Segments | Union Electric Company
Segment Reporting Information [Line Items]
External revenues	751	784
Intersegment revenues	7	8
Net Income Available to Common Shareholder	39	38
Capital expenditures	240	249
Operating Segments | Ameren Illinois Electric Distribution
Segment Reporting Information [Line Items]
External revenues	386	399
Intersegment revenues	1	1
Net Income Available to Common Shareholder	36	33
Capital expenditures	124	122
Operating Segments | Ameren Illinois Gas
Segment Reporting Information [Line Items]
External revenues	320	311
Intersegment revenues	0	0
Net Income Available to Common Shareholder	57	42
Capital expenditures	51	60
Operating Segments | Ameren Transmission
Segment Reporting Information [Line Items]
External revenues	99	91
Intersegment revenues	15	13
Net Income Available to Common Shareholder	44	37
Capital expenditures	121	145
Operating Segments | Other
Segment Reporting Information [Line Items]
External revenues	0	0
Intersegment revenues	0	0
Net Income Available to Common Shareholder	15	1
Capital expenditures	10	7
Operating Segments | Ameren Illinois Company | Ameren Illinois Electric Distribution
Segment Reporting Information [Line Items]
External revenues	387	400
Intersegment revenues	0	0
Net Income Available to Common Shareholder	36	33
Capital expenditures	124	122
Operating Segments | Ameren Illinois Company | Ameren Illinois Gas
Segment Reporting Information [Line Items]
External revenues	320	311
Intersegment revenues	0	0
Net Income Available to Common Shareholder	57	42
Capital expenditures	51	60
Operating Segments | Ameren Illinois Company | Ameren Illinois Transmission
Segment Reporting Information [Line Items]
External revenues	55	49
Intersegment revenues	15	13
Net Income Available to Common Shareholder	27	20
Capital expenditures	92	118
Intersegment Eliminations
Segment Reporting Information [Line Items]
External revenues	0	0
Intersegment revenues	(23)	(22)
Net Income Available to Common Shareholder	0	0
Capital expenditures	(2)	(4)
Intersegment Eliminations | Ameren Illinois Company
Segment Reporting Information [Line Items]
External revenues	0	0
Intersegment revenues	(15)	(13)
Net Income Available to Common Shareholder	0	0
Capital expenditures	$ 0	$ 0