Segment Information (Schedule Of Segment Reporting Information By Segment) (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020
Segment Reporting Information [Line Items]
Revenues	$ 1,472	$ 1,398	$ 3,038	$ 2,838
Net income (loss) attributable to common shareholders	207	243	440	389
Capital expenditures	876	592	1,763	1,228
Wind generation expenditures	224		417	0
Union Electric Company
Segment Reporting Information [Line Items]
Revenues	809	792	1,513	1,472
Net income (loss) attributable to common shareholders	111	152	158	142
Wind generation expenditures	224		417	0
Ameren Illinois Company
Segment Reporting Information [Line Items]
Revenues	629	567	1,452	1,290
Net income (loss) attributable to common shareholders	86	83	235	203
Capital expenditures	309	337	646	661
Operating Segments | Union Electric Company
Segment Reporting Information [Line Items]
Revenues	799	782	1,494	1,452
Net income (loss) attributable to common shareholders	111	152	158	142
Capital expenditures	567	238	1,101	516
Operating Segments | Ameren Illinois Electric Distribution
Segment Reporting Information [Line Items]
Revenues	386	352	797	741
Net income (loss) attributable to common shareholders	41	36	87	73
Capital expenditures	129	139	286	262
Operating Segments | Ameren Illinois Natural Gas
Segment Reporting Information [Line Items]
Revenues	168	140	515	411
Net income (loss) attributable to common shareholders	8	9	83	64
Capital expenditures	61	79	109	140
Operating Segments | Ameren Transmission
Segment Reporting Information [Line Items]
Revenues	119	124	232	234
Net income (loss) attributable to common shareholders	55	59	102	106
Capital expenditures	131	135	272	305
Operating Segments | Ameren Illinois Company | Ameren Illinois Electric Distribution
Segment Reporting Information [Line Items]
Revenues	388	352	799	742
Net income (loss) attributable to common shareholders	41	36	87	73
Capital expenditures	129	139	286	262
Operating Segments | Ameren Illinois Company | Ameren Illinois Natural Gas
Segment Reporting Information [Line Items]
Revenues	168	140	515	411
Net income (loss) attributable to common shareholders	8	9	83	64
Capital expenditures	61	79	109	140
Operating Segments | Ameren Illinois Company | Ameren Illinois Transmission
Segment Reporting Information [Line Items]
Revenues	73	75	138	137
Net income (loss) attributable to common shareholders	37	38	65	66
Capital expenditures	119	119	251	259
Intersegment Eliminations
Segment Reporting Information [Line Items]
Intersegment revenues	(29)	(22)	(55)	(46)
Capital expenditures	(12)	2	(6)	3
Intersegment Eliminations | Union Electric Company
Segment Reporting Information [Line Items]
Intersegment revenues	(10)	(10)	(19)	(20)
Intersegment Eliminations | Ameren Illinois Electric Distribution
Segment Reporting Information [Line Items]
Intersegment revenues	(2)	0	(2)	(1)
Intersegment Eliminations | Ameren Illinois Natural Gas
Segment Reporting Information [Line Items]
Intersegment revenues	0	0	0	0
Intersegment Eliminations | Ameren Transmission
Segment Reporting Information [Line Items]
Intersegment revenues	(17)	(12)	(34)	(25)
Intersegment Eliminations | Ameren Illinois Company
Segment Reporting Information [Line Items]
Revenues	(15)	(12)	(31)	(24)
Intersegment Eliminations | Ameren Illinois Company | Ameren Illinois Electric Distribution
Segment Reporting Information [Line Items]
Revenues	0	0	0	0
Intersegment Eliminations | Ameren Illinois Company | Ameren Illinois Natural Gas
Segment Reporting Information [Line Items]
Revenues	0	0	0	0
Intersegment Eliminations | Ameren Illinois Company | Ameren Illinois Transmission
Segment Reporting Information [Line Items]
Revenues	(15)	(12)	(31)	(24)
Other
Segment Reporting Information [Line Items]
Net income (loss) attributable to common shareholders	(8)	(13)	10	4
Capital expenditures	$ 0	$ (1)	$ 1	$ 2