Segment Information (Schedule Of Segment Reporting Information By Segment) (Detail) $ in Millions	3 Months Ended	9 Months Ended
Sep. 30, 2019 USD ($)	Sep. 30, 2018 USD ($)	Sep. 30, 2019 USD ($) segment	Sep. 30, 2018 USD ($)
Segment Reporting Information [Line Items]
Number of Reportable Segments \| segment			4
External revenues	$ 1,640	$ 1,724	$ 4,575	$ 4,872
Intersegment revenues	19	0	19	0
Net income (loss) attributable to common shareholders	364	357	734	747
Capital expenditures	636	577	$ 1,761	1,689
Union Electric Company
Segment Reporting Information [Line Items]
Number of Reportable Segments \| segment			1
Ameren Illinois Electric Distribution \| Ameren Missouri Software Licensing with Ameren Illinois
Segment Reporting Information [Line Items]
Intersegment revenues	14		$ 14
Ameren Illinois Natural Gas \| Ameren Missouri Software Licensing with Ameren Illinois
Segment Reporting Information [Line Items]
Intersegment revenues	5		$ 5
Ameren Illinois Company
Segment Reporting Information [Line Items]
Number of Reportable Segments \| segment			3
Ameren Illinois Company
Segment Reporting Information [Line Items]
External revenues	564	564	$ 1,873	1,902
Intersegment revenues	0	0	0	0
Net income (loss) attributable to common shareholders	65	63	247	220
Capital expenditures	344	345	900	947
Ameren Illinois Company \| Ameren Missouri Software Licensing with Ameren Illinois
Segment Reporting Information [Line Items]
Intersegment revenues	19		19
Operating Segments \| Union Electric Company
Segment Reporting Information [Line Items]
External revenues	1,050	1,118	2,591	2,848
Intersegment revenues	9	11	24	28
Net income (loss) attributable to common shareholders	300	294	446	500
Capital expenditures	256	210	751	664
Operating Segments \| Ameren Illinois Electric Distribution
Segment Reporting Information [Line Items]
External revenues	374	392	1,118	1,177
Intersegment revenues	15	0	17	2
Net income (loss) attributable to common shareholders	32	35	105	101
Capital expenditures	139	135	390	389
Operating Segments \| Ameren Illinois Natural Gas
Segment Reporting Information [Line Items]
External revenues	107	116	563	569
Intersegment revenues	5	0	5	0
Net income (loss) attributable to common shareholders	(1)	0	57	49
Capital expenditures	113	111	241	237
Operating Segments \| Ameren Transmission
Segment Reporting Information [Line Items]
External revenues	109	98	303	278
Intersegment revenues	19	15	48	42
Net income (loss) attributable to common shareholders	53	48	139	121
Capital expenditures	129	124	377	399
Operating Segments \| Other
Segment Reporting Information [Line Items]
External revenues	0	0	0	0
Intersegment revenues	0	0	0	0
Net income (loss) attributable to common shareholders	(20)	(20)	(13)	(24)
Capital expenditures	(7)	1	3	6
Operating Segments \| Ameren Illinois Company \| Ameren Illinois Electric Distribution
Segment Reporting Information [Line Items]
External revenues	389	392	1,135	1,179
Intersegment revenues	0	0	0	0
Net income (loss) attributable to common shareholders	32	35	105	101
Capital expenditures	139	135	390	389
Operating Segments \| Ameren Illinois Company \| Ameren Illinois Natural Gas
Segment Reporting Information [Line Items]
External revenues	112	116	568	569
Intersegment revenues	0	0	0	0
Net income (loss) attributable to common shareholders	(1)	0	57	49
Capital expenditures	113	111	241	237
Operating Segments \| Ameren Illinois Company \| Ameren Illinois Transmission
Segment Reporting Information [Line Items]
External revenues	63	56	170	154
Intersegment revenues	18	15	47	41
Net income (loss) attributable to common shareholders	34	28	85	70
Capital expenditures	92	99	269	321
Intersegment Eliminations
Segment Reporting Information [Line Items]
External revenues	0	0	0	0
Intersegment revenues	(29)	(26)	(75)	(72)
Net income (loss) attributable to common shareholders	0	0	0	0
Capital expenditures	6	(4)	(1)	(6)
Intersegment Eliminations \| Ameren Illinois Company
Segment Reporting Information [Line Items]
External revenues	0	0	0	0
Intersegment revenues	(18)	(15)	(47)	(41)
Net income (loss) attributable to common shareholders	0	0	0	0
Capital expenditures	$ 0	$ 0	$ 0	$ 0

The portion of profit or loss for the period, net of income taxes, which is attributable to the parent.

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