Segment Information (Schedule Of Segment Reporting Information By Segment) (Detail) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Segment Reporting Information [Line Items]
External Revenues	$ 1,398	$ 1,379	$ 2,838	$ 2,935
Net income (loss) attributable to common shareholders	243	179	389	370
Capital expenditures	592	581	1,228	1,125
Reportable Subsegments
Segment Reporting Information [Line Items]
External Revenues	1,398	1,379	2,838	2,935
Intersubsegment Eliminations
Segment Reporting Information [Line Items]
Intersegment revenues	0	0	0	0
Ameren Illinois Company
Segment Reporting Information [Line Items]
External Revenues	567	547	1,290	1,309
Net income (loss) attributable to common shareholders	83	62	203	182
Capital expenditures	337	289	661	556
Ameren Illinois Company | Reportable Subsegments
Segment Reporting Information [Line Items]
External Revenues	567	547	1,290	1,309
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	792	798	1,472	1,556
Net income (loss) attributable to common shareholders	152	107	142	146
Capital expenditures	238	255	516	495
Operating Segments | Union Electric Company | Reportable Subsegments
Segment Reporting Information [Line Items]
External Revenues	782	790	1,452	1,541
Operating Segments | Union Electric Company | Intersubsegment Eliminations
Segment Reporting Information [Line Items]
Intersegment revenues	10	8	20	15
Operating Segments | Ameren Illinois Electric Distribution
Segment Reporting Information [Line Items]
External Revenues	352	359	742	746
Net income (loss) attributable to common shareholders	36	37	73	73
Capital expenditures	139	127	262	251
Operating Segments | Ameren Illinois Electric Distribution | Reportable Subsegments
Segment Reporting Information [Line Items]
External Revenues	352	358	741	744
Operating Segments | Ameren Illinois Electric Distribution | Intersubsegment Eliminations
Segment Reporting Information [Line Items]
Intersegment revenues	0	1	1	2
Operating Segments | Ameren Illinois Natural Gas
Segment Reporting Information [Line Items]
External Revenues	140	136	411	456
Net income (loss) attributable to common shareholders	9	1	64	58
Capital expenditures	79	77	140	128
Operating Segments | Ameren Illinois Natural Gas | Reportable Subsegments
Segment Reporting Information [Line Items]
External Revenues	140	136	411	456
Operating Segments | Ameren Illinois Natural Gas | Intersubsegment Eliminations
Segment Reporting Information [Line Items]
Intersegment revenues	0	0	0	0
Operating Segments | Ameren Transmission
Segment Reporting Information [Line Items]
External Revenues	136	109	259	223
Net income (loss) attributable to common shareholders	59	42	106	86
Capital expenditures	135	127	305	248
Operating Segments | Ameren Transmission | Reportable Subsegments
Segment Reporting Information [Line Items]
External Revenues	124	95	234	194
Operating Segments | Ameren Transmission | Intersubsegment Eliminations
Segment Reporting Information [Line Items]
Intersegment revenues	12	14	25	29
Operating Segments | Other
Segment Reporting Information [Line Items]
Net income (loss) attributable to common shareholders	(13)	(8)	4	7
Capital expenditures		0	2	10
Capital expenditures, transfers	(1)
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	36	37	73	73
Capital expenditures	139	127	262	251
Operating Segments | Ameren Illinois Company | Ameren Illinois Electric Distribution | Reportable Subsegments
Segment Reporting Information [Line Items]
External Revenues	352	359	742	746
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	9	1	64	58
Capital expenditures	79	77	140	128
Operating Segments | Ameren Illinois Company | Ameren Illinois Natural Gas | Reportable Subsegments
Segment Reporting Information [Line Items]
External Revenues	140	136	411	456
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	38	24	66	51
Capital expenditures	119	85	259	177
Operating Segments | Ameren Illinois Company | Ameren Illinois Transmission | Reportable Subsegments
Segment Reporting Information [Line Items]
External Revenues	75	52	137	107
Operating Segments | Ameren Illinois Company | Ameren Illinois Transmission | Intersubsegment Eliminations
Segment Reporting Information [Line Items]
Intersegment revenues	12	14	24	29
Intersegment Eliminations
Segment Reporting Information [Line Items]
External Revenues	(22)	(23)	(46)	(46)
Net income (loss) attributable to common shareholders	0	0	0	0
Capital expenditures	2	(5)	3	(7)
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	(22)	(23)	(46)	(46)
Intersegment Eliminations | Ameren Illinois Company
Segment Reporting Information [Line Items]
External Revenues	(12)	(14)	(24)	(29)
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	$ (12)	$ (14)	$ (24)	$ (29)