Segment Information (Schedule Of Segment Reporting Information By Segment) (Detail) - USD ($) $ in Millions	3 Months Ended
Mar. 31, 2020	Mar. 31, 2019
Segment Reporting Information [Line Items]
External Revenues	$ 1,440	$ 1,556
Net income (loss) attributable to common shareholders	146	191
Capital expenditures	636	544
Reportable Subsegments
Segment Reporting Information [Line Items]
External Revenues	1,440	1,556
Intersubsegment Eliminations
Segment Reporting Information [Line Items]
Intersegment revenues	0	0
Ameren Illinois Company
Segment Reporting Information [Line Items]
External Revenues	723	762
Net income (loss) attributable to common shareholders	120	120
Capital expenditures	324	267
Ameren Illinois Company \| Reportable Subsegments
Segment Reporting Information [Line Items]
External Revenues	723	762
Ameren Illinois Company \| Intersubsegment Eliminations
Segment Reporting Information [Line Items]
Intersegment revenues	0	0
Operating Segments \| Union Electric Company
Segment Reporting Information [Line Items]
External Revenues	680	758
Net income (loss) attributable to common shareholders	(10)	39
Capital expenditures	278	240
Operating Segments \| Union Electric Company \| Reportable Subsegments
Segment Reporting Information [Line Items]
External Revenues	670	751
Operating Segments \| Union Electric Company \| Intersubsegment Eliminations
Segment Reporting Information [Line Items]
Intersegment revenues	10	7
Operating Segments \| Ameren Illinois Electric Distribution
Segment Reporting Information [Line Items]
External Revenues	390	387
Net income (loss) attributable to common shareholders	37	36
Capital expenditures	123	124
Operating Segments \| Ameren Illinois Electric Distribution \| Reportable Subsegments
Segment Reporting Information [Line Items]
External Revenues	389	386
Operating Segments \| Ameren Illinois Electric Distribution \| Intersubsegment Eliminations
Segment Reporting Information [Line Items]
Intersegment revenues	1	1
Operating Segments \| Ameren Illinois Natural Gas
Segment Reporting Information [Line Items]
External Revenues	271	320
Net income (loss) attributable to common shareholders	55	57
Capital expenditures	61	51
Operating Segments \| Ameren Illinois Natural Gas \| Reportable Subsegments
Segment Reporting Information [Line Items]
External Revenues	271	320
Operating Segments \| Ameren Illinois Natural Gas \| Intersubsegment Eliminations
Segment Reporting Information [Line Items]
Intersegment revenues	0	0
Operating Segments \| Ameren Transmission
Segment Reporting Information [Line Items]
External Revenues	123	114
Net income (loss) attributable to common shareholders	47	44
Capital expenditures	170	121
Operating Segments \| Ameren Transmission \| Reportable Subsegments
Segment Reporting Information [Line Items]
External Revenues	110	99
Operating Segments \| Ameren Transmission \| Intersubsegment Eliminations
Segment Reporting Information [Line Items]
Intersegment revenues	13	15
Operating Segments \| Other
Segment Reporting Information [Line Items]
Net income (loss) attributable to common shareholders	17	15
Capital expenditures	3	10
Operating Segments \| Other \| Reportable Subsegments
Segment Reporting Information [Line Items]
External Revenues	0	0
Operating Segments \| Other \| Intersubsegment Eliminations
Segment Reporting Information [Line Items]
Intersegment revenues	0	0
Operating Segments \| Ameren Illinois Company \| Ameren Illinois Electric Distribution
Segment Reporting Information [Line Items]
Net income (loss) attributable to common shareholders	37	36
Capital expenditures	123	124
Operating Segments \| Ameren Illinois Company \| Ameren Illinois Electric Distribution \| Reportable Subsegments
Segment Reporting Information [Line Items]
External Revenues	390	387
Operating Segments \| Ameren Illinois Company \| Ameren Illinois Electric Distribution \| Intersubsegment Eliminations
Segment Reporting Information [Line Items]
Intersegment revenues	0	0
Operating Segments \| Ameren Illinois Company \| Ameren Illinois Natural Gas
Segment Reporting Information [Line Items]
Net income (loss) attributable to common shareholders	55	57
Capital expenditures	61	51
Operating Segments \| Ameren Illinois Company \| Ameren Illinois Natural Gas \| Reportable Subsegments
Segment Reporting Information [Line Items]
External Revenues	271	320
Operating Segments \| Ameren Illinois Company \| Ameren Illinois Natural Gas \| Intersubsegment Eliminations
Segment Reporting Information [Line Items]
Intersegment revenues	0	0
Operating Segments \| Ameren Illinois Company \| Ameren Illinois Transmission
Segment Reporting Information [Line Items]
Net income (loss) attributable to common shareholders	28	27
Capital expenditures	140	92
Operating Segments \| Ameren Illinois Company \| Ameren Illinois Transmission \| Reportable Subsegments
Segment Reporting Information [Line Items]
External Revenues	62	55
Operating Segments \| Ameren Illinois Company \| Ameren Illinois Transmission \| Intersubsegment Eliminations
Segment Reporting Information [Line Items]
Intersegment revenues	12	15
Intersegment Eliminations
Segment Reporting Information [Line Items]
External Revenues	(24)	(23)
Net income (loss) attributable to common shareholders	0	0
Capital expenditures	1	(2)
Intersegment Eliminations \| Reportable Subsegments
Segment Reporting Information [Line Items]
External Revenues	0	0
Intersegment Eliminations \| Intersubsegment Eliminations
Segment Reporting Information [Line Items]
Intersegment revenues	(24)	(23)
Intersegment Eliminations \| Ameren Illinois Company
Segment Reporting Information [Line Items]
External Revenues	(12)	(15)
Net income (loss) attributable to common shareholders	0	0
Capital expenditures	0	0
Intersegment Eliminations \| Ameren Illinois Company \| Reportable Subsegments
Segment Reporting Information [Line Items]
External Revenues	0	0
Intersegment Eliminations \| Ameren Illinois Company \| Intersubsegment Eliminations
Segment Reporting Information [Line Items]
Intersegment revenues	$ (12)	$ (15)

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

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