Segment Information (Schedule Of Segment Reporting Information By Segment) (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020
Segment Reporting Information [Line Items]
Revenues	$ 1,566	$ 1,440
Net income (loss) attributable to common shareholders	233	146
Capital expenditures	887	636
Wind generation expenditures	(193)	0
Union Electric Company
Segment Reporting Information [Line Items]
Revenues	704	680
Net income (loss) attributable to common shareholders	47	(10)
Wind generation expenditures	(193)	0
Ameren Illinois Company
Segment Reporting Information [Line Items]
Revenues	823	723
Net income (loss) attributable to common shareholders	149	120
Capital expenditures	337	324
Operating Segments | Union Electric Company
Segment Reporting Information [Line Items]
Revenues	695	670
Net income (loss) attributable to common shareholders	47	(10)
Capital expenditures	534	278
Operating Segments | Ameren Illinois Electric Distribution
Segment Reporting Information [Line Items]
Revenues	411	389
Net income (loss) attributable to common shareholders	46	37
Capital expenditures	157	123
Operating Segments | Ameren Illinois Natural Gas
Segment Reporting Information [Line Items]
Revenues	347	271
Net income (loss) attributable to common shareholders	75	55
Capital expenditures	48	61
Operating Segments | Ameren Transmission
Segment Reporting Information [Line Items]
Revenues	113	110
Net income (loss) attributable to common shareholders	47	47
Capital expenditures	141	170
Operating Segments | Ameren Illinois Company | Ameren Illinois Electric Distribution
Segment Reporting Information [Line Items]
Revenues	411	390
Net income (loss) attributable to common shareholders	46	37
Capital expenditures	157	123
Operating Segments | Ameren Illinois Company | Ameren Illinois Natural Gas
Segment Reporting Information [Line Items]
Revenues	347	271
Net income (loss) attributable to common shareholders	75	55
Capital expenditures	48	61
Operating Segments | Ameren Illinois Company | Ameren Illinois Transmission
Segment Reporting Information [Line Items]
Revenues	65	62
Net income (loss) attributable to common shareholders	28	28
Capital expenditures	132	140
Intersegment Eliminations
Segment Reporting Information [Line Items]
Revenue Not from Contract with Customer	(26)	(24)
Capital expenditures	6	1
Intersegment Eliminations | Union Electric Company
Segment Reporting Information [Line Items]
Revenue Not from Contract with Customer	(9)	(10)
Intersegment Eliminations | Ameren Illinois Electric Distribution
Segment Reporting Information [Line Items]
Revenue Not from Contract with Customer	0	(1)
Intersegment Eliminations | Ameren Illinois Natural Gas
Segment Reporting Information [Line Items]
Revenue Not from Contract with Customer	0	0
Intersegment Eliminations | Ameren Transmission
Segment Reporting Information [Line Items]
Revenue Not from Contract with Customer	(17)	(13)
Intersegment Eliminations | Ameren Illinois Company
Segment Reporting Information [Line Items]
Revenues	(16)	(12)
Intersegment Eliminations | Ameren Illinois Company | Ameren Illinois Electric Distribution
Segment Reporting Information [Line Items]
Revenues	0	0
Intersegment Eliminations | Ameren Illinois Company | Ameren Illinois Natural Gas
Segment Reporting Information [Line Items]
Revenues	0	0
Intersegment Eliminations | Ameren Illinois Company | Ameren Illinois Transmission
Segment Reporting Information [Line Items]
Revenues	(16)	(12)
Other
Segment Reporting Information [Line Items]
Net income (loss) attributable to common shareholders	18	17
Capital expenditures	$ 1	$ 3