Segment Information (Schedule Of Segment Reporting Information By Segment) (Detail) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Segment Reporting Information [Line Items]
External Revenues	$ 1,628	$ 1,659	$ 4,466	$ 4,594
Net income (loss) attributable to common shareholders	367	364	756	734
Capital expenditures	656	636	1,884	1,761
Reportable Subsegments
Segment Reporting Information [Line Items]
External Revenues	1,628	1,640	4,466	4,575
Intersubsegment Eliminations
Segment Reporting Information [Line Items]
Intersegment revenues	0	19	0	19
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]
External Revenues	589	564	1,879	1,873
Net income (loss) attributable to common shareholders	77	65	280	247
Capital expenditures	370	344	1,031	900
Ameren Illinois Company | Ameren Missouri Software Licensing with Ameren Illinois
Segment Reporting Information [Line Items]
Intersegment revenues		19		19
Ameren Illinois Company | Reportable Subsegments
Segment Reporting Information [Line Items]
External Revenues	589	564	1,879	1,873
Ameren Illinois Company | Intersubsegment Eliminations
Segment Reporting Information [Line Items]
Intersegment revenues	0	0	0	0
Operating Segments | Union Electric Company
Segment Reporting Information [Line Items]
External Revenues	1,001	1,059	2,473	2,615
Net income (loss) attributable to common shareholders	297	300	439	446
Capital expenditures	262	256	778	751
Operating Segments | Union Electric Company | Reportable Subsegments
Segment Reporting Information [Line Items]
External Revenues	990	1,050	2,442	2,591
Operating Segments | Union Electric Company | Intersubsegment Eliminations
Segment Reporting Information [Line Items]
Intersegment revenues	11	9	31	24
Operating Segments | Ameren Illinois Electric Distribution
Segment Reporting Information [Line Items]
External Revenues	391	389	1,133	1,135
Net income (loss) attributable to common shareholders	34	32	107	105
Capital expenditures	129	139	391	390
Operating Segments | Ameren Illinois Electric Distribution | Reportable Subsegments
Segment Reporting Information [Line Items]
External Revenues	390	374	1,131	1,118
Operating Segments | Ameren Illinois Electric Distribution | Intersubsegment Eliminations
Segment Reporting Information [Line Items]
Intersegment revenues	1	15	2	17
Operating Segments | Ameren Illinois Natural Gas
Segment Reporting Information [Line Items]
External Revenues	122	112	533	568
Net income (loss) attributable to common shareholders	2	(1)	66	57
Capital expenditures	81	113	221	241
Operating Segments | Ameren Illinois Natural Gas | Reportable Subsegments
Segment Reporting Information [Line Items]
External Revenues	122	107	533	563
Operating Segments | Ameren Illinois Natural Gas | Intersubsegment Eliminations
Segment Reporting Information [Line Items]
Intersegment revenues	0	5	0	5
Operating Segments | Ameren Transmission
Segment Reporting Information [Line Items]
External Revenues	141	128	400	351
Net income (loss) attributable to common shareholders	62	53	168	139
Capital expenditures	185	129	490	377
Operating Segments | Ameren Transmission | Reportable Subsegments
Segment Reporting Information [Line Items]
External Revenues	126	109	360	303
Operating Segments | Ameren Transmission | Intersubsegment Eliminations
Segment Reporting Information [Line Items]
Intersegment revenues	15	19	40	48
Operating Segments | Other
Segment Reporting Information [Line Items]
Net income (loss) attributable to common shareholders	(28)	(20)	(24)	(13)
Capital expenditures	2		4	3
Capital expenditures, transfers		(7)
Operating Segments | Other | Reportable Subsegments
Segment Reporting Information [Line Items]
External Revenues	0	0	0	0
Operating Segments | Other | Intersubsegment Eliminations
Segment Reporting Information [Line Items]
Intersegment revenues	0	0	0	0
Operating Segments | Ameren Illinois Company | Ameren Illinois Electric Distribution
Segment Reporting Information [Line Items]
Net income (loss) attributable to common shareholders	34	32	107	105
Capital expenditures	129	139	391	390
Operating Segments | Ameren Illinois Company | Ameren Illinois Electric Distribution | Reportable Subsegments
Segment Reporting Information [Line Items]
External Revenues	391	389	1,133	1,135
Operating Segments | Ameren Illinois Company | Ameren Illinois Electric Distribution | Intersubsegment Eliminations
Segment Reporting Information [Line Items]
Intersegment revenues	0	0	0	0
Operating Segments | Ameren Illinois Company | Ameren Illinois Natural Gas
Segment Reporting Information [Line Items]
Net income (loss) attributable to common shareholders	2	(1)	66	57
Capital expenditures	81	113	221	241
Operating Segments | Ameren Illinois Company | Ameren Illinois Natural Gas | Reportable Subsegments
Segment Reporting Information [Line Items]
External Revenues	122	112	533	568
Operating Segments | Ameren Illinois Company | Ameren Illinois Natural Gas | Intersubsegment Eliminations
Segment Reporting Information [Line Items]
Intersegment revenues	0	0	0	0
Operating Segments | Ameren Illinois Company | Ameren Illinois Transmission
Segment Reporting Information [Line Items]
Net income (loss) attributable to common shareholders	41	34	107	85
Capital expenditures	160	92	419	269
Operating Segments | Ameren Illinois Company | Ameren Illinois Transmission | Reportable Subsegments
Segment Reporting Information [Line Items]
External Revenues	76	63	213	170
Operating Segments | Ameren Illinois Company | Ameren Illinois Transmission | Intersubsegment Eliminations
Segment Reporting Information [Line Items]
Intersegment revenues	15	18	39	47
Intersegment Eliminations
Segment Reporting Information [Line Items]
External Revenues	(27)	(29)	(73)	(75)
Net income (loss) attributable to common shareholders	0	0	0	0
Capital expenditures	(3)	6	0	(1)
Intersegment Eliminations | Reportable Subsegments
Segment Reporting Information [Line Items]
External Revenues	0	0	0	0
Intersegment Eliminations | Intersubsegment Eliminations
Segment Reporting Information [Line Items]
Intersegment revenues	(27)	(29)	(73)	(75)
Intersegment Eliminations | Ameren Illinois Company
Segment Reporting Information [Line Items]
External Revenues	(15)	(18)	(39)	(47)
Net income (loss) attributable to common shareholders	0	0	0	0
Capital expenditures	0	0	0	0
Intersegment Eliminations | Ameren Illinois Company | Reportable Subsegments
Segment Reporting Information [Line Items]
External Revenues	0	0	0	0
Intersegment Eliminations | Ameren Illinois Company | Intersubsegment Eliminations
Segment Reporting Information [Line Items]
Intersegment revenues	$ (15)	$ (18)	$ (39)	$ (47)