Segment Information (Disaggregation of Revenue) (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Disaggregation of Revenue [Line Items]
External Revenues	$ 1,628	$ 1,659	$ 4,466	$ 4,594
Revenues from alternative revenue programs	(102)	(130)	(16)	(93)
Other revenues not from contracts with customers	11	7	28	21
Ameren Illinois Electric Distribution | Ameren Missouri Software Licensing with Ameren Illinois
Disaggregation of Revenue [Line Items]
Intersegment revenues		14		14
Ameren Illinois Natural Gas | Ameren Missouri Software Licensing with Ameren Illinois
Disaggregation of Revenue [Line Items]
Intersegment revenues		5		5
Electricity
Disaggregation of Revenue [Line Items]
External Revenues	1,489	1,528	3,846	3,928
Electricity | Residential
Disaggregation of Revenue [Line Items]
External Revenues	689	713	1,775	1,774
Electricity | Commercial
Disaggregation of Revenue [Line Items]
External Revenues	470	517	1,193	1,313
Electricity | Industrial
Disaggregation of Revenue [Line Items]
External Revenues	113	121	298	320
Electricity | Other
Disaggregation of Revenue [Line Items]
External Revenues	217	177	580	521
Natural Gas
Disaggregation of Revenue [Line Items]
External Revenues	139	131	620	666
Natural Gas | Residential
Disaggregation of Revenue [Line Items]
External Revenues	76	73	427	455
Natural Gas | Commercial
Disaggregation of Revenue [Line Items]
External Revenues	21	21	114	129
Natural Gas | Industrial
Disaggregation of Revenue [Line Items]
External Revenues	6	3	14	12
Natural Gas | Other
Disaggregation of Revenue [Line Items]
External Revenues	36	34	65	70
Ameren Illinois Company
Disaggregation of Revenue [Line Items]
External Revenues	589	564	1,879	1,873
Revenues from alternative revenue programs	(102)	(156)	(13)	(135)
Other revenues not from contracts with customers	5	2	7	7
Ameren Illinois Company | Ameren Missouri Software Licensing with Ameren Illinois
Disaggregation of Revenue [Line Items]
Intersegment revenues		19		19
Ameren Illinois Company | Residential
Disaggregation of Revenue [Line Items]
External Revenues	302	289	1,039	1,039
Ameren Illinois Company | Commercial
Disaggregation of Revenue [Line Items]
External Revenues	145	140	459	475
Ameren Illinois Company | Industrial
Disaggregation of Revenue [Line Items]
External Revenues	31	29	102	103
Ameren Illinois Company | Other
Disaggregation of Revenue [Line Items]
External Revenues	111	106	279	256
Ameren Illinois Company | Electricity
Disaggregation of Revenue [Line Items]
External Revenues	467	452	1,346	1,305
Ameren Illinois Company | Natural Gas
Disaggregation of Revenue [Line Items]
External Revenues	122	112	533	568
Operating Segments | Union Electric Company
Disaggregation of Revenue [Line Items]
External Revenues	1,001	1,059	2,473	2,615
Revenues from alternative revenue programs	1	26	(8)	41
Other revenues not from contracts with customers	6	5	21	14
Operating Segments | Ameren Illinois Electric Distribution
Disaggregation of Revenue [Line Items]
External Revenues	391	389	1,133	1,135
Revenues from alternative revenue programs	(110)	(145)	(59)	(111)
Other revenues not from contracts with customers	5	1	6	5
Operating Segments | Ameren Illinois Natural Gas
Disaggregation of Revenue [Line Items]
External Revenues	122	112	533	568
Revenues from alternative revenue programs	3	1	17	2
Other revenues not from contracts with customers	0	1	1	2
Operating Segments | Ameren Transmission
Disaggregation of Revenue [Line Items]
External Revenues	141	128	400	351
Revenues from alternative revenue programs	4	(12)	34	(25)
Other revenues not from contracts with customers	0	0	0	0
Operating Segments | Electricity | Union Electric Company
Disaggregation of Revenue [Line Items]
External Revenues	984	1,040	2,386	2,517
Operating Segments | Electricity | Union Electric Company | Residential
Disaggregation of Revenue [Line Items]
External Revenues	455	489	1,111	1,134
Operating Segments | Electricity | Union Electric Company | Commercial
Disaggregation of Revenue [Line Items]
External Revenues	343	394	828	943
Operating Segments | Electricity | Union Electric Company | Industrial
Disaggregation of Revenue [Line Items]
External Revenues	87	94	207	226
Operating Segments | Electricity | Union Electric Company | Other
Disaggregation of Revenue [Line Items]
External Revenues	99	63	240	214
Operating Segments | Electricity | Ameren Illinois Electric Distribution
Disaggregation of Revenue [Line Items]
External Revenues	391	389	1,133	1,135
Operating Segments | Electricity | Ameren Illinois Electric Distribution | Residential
Disaggregation of Revenue [Line Items]
External Revenues	234	224	664	640
Operating Segments | Electricity | Ameren Illinois Electric Distribution | Commercial
Disaggregation of Revenue [Line Items]
External Revenues	127	123	365	370
Operating Segments | Electricity | Ameren Illinois Electric Distribution | Industrial
Disaggregation of Revenue [Line Items]
External Revenues	26	27	91	94
Operating Segments | Electricity | Ameren Illinois Electric Distribution | Other
Disaggregation of Revenue [Line Items]
External Revenues	4	15	13	31
Operating Segments | Electricity | Ameren Illinois Natural Gas
Disaggregation of Revenue [Line Items]
External Revenues	0	0	0	0
Operating Segments | Electricity | Ameren Illinois Natural Gas | Residential
Disaggregation of Revenue [Line Items]
External Revenues	0	0	0	0
Operating Segments | Electricity | Ameren Illinois Natural Gas | Commercial
Disaggregation of Revenue [Line Items]
External Revenues	0	0	0	0
Operating Segments | Electricity | Ameren Illinois Natural Gas | Industrial
Disaggregation of Revenue [Line Items]
External Revenues	0	0	0	0
Operating Segments | Electricity | Ameren Illinois Natural Gas | Other
Disaggregation of Revenue [Line Items]
External Revenues	0	0	0	0
Operating Segments | Electricity | Ameren Transmission
Disaggregation of Revenue [Line Items]
External Revenues	141	128	400	351
Operating Segments | Electricity | Ameren Transmission | Residential
Disaggregation of Revenue [Line Items]
External Revenues	0	0	0	0
Operating Segments | Electricity | Ameren Transmission | Commercial
Disaggregation of Revenue [Line Items]
External Revenues	0	0	0	0
Operating Segments | Electricity | Ameren Transmission | Industrial
Disaggregation of Revenue [Line Items]
External Revenues	0	0	0	0
Operating Segments | Electricity | Ameren Transmission | Other
Disaggregation of Revenue [Line Items]
External Revenues	141	128	400	351
Operating Segments | Natural Gas | Union Electric Company
Disaggregation of Revenue [Line Items]
External Revenues	17	19	87	98
Operating Segments | Natural Gas | Union Electric Company | Residential
Disaggregation of Revenue [Line Items]
External Revenues	8	8	52	56
Operating Segments | Natural Gas | Union Electric Company | Commercial
Disaggregation of Revenue [Line Items]
External Revenues	3	4	20	24
Operating Segments | Natural Gas | Union Electric Company | Industrial
Disaggregation of Revenue [Line Items]
External Revenues	1	1	3	3
Operating Segments | Natural Gas | Union Electric Company | Other
Disaggregation of Revenue [Line Items]
External Revenues	5	6	12	15
Operating Segments | Natural Gas | Ameren Illinois Electric Distribution
Disaggregation of Revenue [Line Items]
External Revenues	0	0	0	0
Operating Segments | Natural Gas | Ameren Illinois Electric Distribution | Residential
Disaggregation of Revenue [Line Items]
External Revenues	0	0	0	0
Operating Segments | Natural Gas | Ameren Illinois Electric Distribution | Commercial
Disaggregation of Revenue [Line Items]
External Revenues	0	0	0	0
Operating Segments | Natural Gas | Ameren Illinois Electric Distribution | Industrial
Disaggregation of Revenue [Line Items]
External Revenues	0	0	0	0
Operating Segments | Natural Gas | Ameren Illinois Electric Distribution | Other
Disaggregation of Revenue [Line Items]
External Revenues	0	0	0	0
Operating Segments | Natural Gas | Ameren Illinois Natural Gas
Disaggregation of Revenue [Line Items]
External Revenues	122	112	533	568
Operating Segments | Natural Gas | Ameren Illinois Natural Gas | Residential
Disaggregation of Revenue [Line Items]
External Revenues	68	65	375	399
Operating Segments | Natural Gas | Ameren Illinois Natural Gas | Commercial
Disaggregation of Revenue [Line Items]
External Revenues	18	17	94	105
Operating Segments | Natural Gas | Ameren Illinois Natural Gas | Industrial
Disaggregation of Revenue [Line Items]
External Revenues	5	2	11	9
Operating Segments | Natural Gas | Ameren Illinois Natural Gas | Other
Disaggregation of Revenue [Line Items]
External Revenues	31	28	53	55
Operating Segments | Natural Gas | Ameren Transmission
Disaggregation of Revenue [Line Items]
External Revenues	0	0	0	0
Operating Segments | Natural Gas | Ameren Transmission | Residential
Disaggregation of Revenue [Line Items]
External Revenues	0	0	0	0
Operating Segments | Natural Gas | Ameren Transmission | Commercial
Disaggregation of Revenue [Line Items]
External Revenues	0	0	0	0
Operating Segments | Natural Gas | Ameren Transmission | Industrial
Disaggregation of Revenue [Line Items]
External Revenues	0	0	0	0
Operating Segments | Natural Gas | Ameren Transmission | Other
Disaggregation of Revenue [Line Items]
External Revenues	0	0	0	0
Operating Segments | Ameren Illinois Company | Ameren Illinois Electric Distribution
Disaggregation of Revenue [Line Items]
Revenues from alternative revenue programs	(110)	(145)	(59)	(111)
Other revenues not from contracts with customers	5	1	6	5
Operating Segments | Ameren Illinois Company | Ameren Illinois Natural Gas
Disaggregation of Revenue [Line Items]
Revenues from alternative revenue programs	3	1	17	2
Other revenues not from contracts with customers	0	1	1	2
Operating Segments | Ameren Illinois Company | Ameren Illinois Transmission
Disaggregation of Revenue [Line Items]
Revenues from alternative revenue programs	5	(12)	29	(26)
Other revenues not from contracts with customers	0	0	0	0
Operating Segments | Ameren Illinois Company | Electricity | Ameren Illinois Electric Distribution
Disaggregation of Revenue [Line Items]
External Revenues	391	389	1,133	1,135
Operating Segments | Ameren Illinois Company | Electricity | Ameren Illinois Electric Distribution | Residential
Disaggregation of Revenue [Line Items]
External Revenues	234	224	664	640
Operating Segments | Ameren Illinois Company | Electricity | Ameren Illinois Electric Distribution | Commercial
Disaggregation of Revenue [Line Items]
External Revenues	127	123	365	370
Operating Segments | Ameren Illinois Company | Electricity | Ameren Illinois Electric Distribution | Industrial
Disaggregation of Revenue [Line Items]
External Revenues	26	27	91	94
Operating Segments | Ameren Illinois Company | Electricity | Ameren Illinois Electric Distribution | Other
Disaggregation of Revenue [Line Items]
External Revenues	4	15	13	31
Operating Segments | Ameren Illinois Company | Electricity | Ameren Illinois Transmission
Disaggregation of Revenue [Line Items]
External Revenues	91	81	252	217
Operating Segments | Ameren Illinois Company | Electricity | Ameren Illinois Transmission | Residential
Disaggregation of Revenue [Line Items]
External Revenues	0	0	0	0
Operating Segments | Ameren Illinois Company | Electricity | Ameren Illinois Transmission | Commercial
Disaggregation of Revenue [Line Items]
External Revenues	0	0	0	0
Operating Segments | Ameren Illinois Company | Electricity | Ameren Illinois Transmission | Industrial
Disaggregation of Revenue [Line Items]
External Revenues	0	0	0	0
Operating Segments | Ameren Illinois Company | Electricity | Ameren Illinois Transmission | Other
Disaggregation of Revenue [Line Items]
External Revenues	91	81	252	217
Operating Segments | Ameren Illinois Company | Natural Gas | Ameren Illinois Natural Gas
Disaggregation of Revenue [Line Items]
External Revenues	122	112	533	568
Operating Segments | Ameren Illinois Company | Natural Gas | Ameren Illinois Natural Gas | Residential
Disaggregation of Revenue [Line Items]
External Revenues	68	65	375	399
Operating Segments | Ameren Illinois Company | Natural Gas | Ameren Illinois Natural Gas | Commercial
Disaggregation of Revenue [Line Items]
External Revenues	18	17	94	105
Operating Segments | Ameren Illinois Company | Natural Gas | Ameren Illinois Natural Gas | Industrial
Disaggregation of Revenue [Line Items]
External Revenues	5	2	11	9
Operating Segments | Ameren Illinois Company | Natural Gas | Ameren Illinois Natural Gas | Other
Disaggregation of Revenue [Line Items]
External Revenues	31	28	53	55
Intersegment Eliminations
Disaggregation of Revenue [Line Items]
External Revenues	(27)	(29)	(73)	(75)
Intersegment Eliminations | Electricity
Disaggregation of Revenue [Line Items]
External Revenues	(27)	(29)	(73)	(75)
Intersegment Eliminations | Electricity | Residential
Disaggregation of Revenue [Line Items]
External Revenues	0	0	0	0
Intersegment Eliminations | Electricity | Commercial
Disaggregation of Revenue [Line Items]
External Revenues	0	0	0	0
Intersegment Eliminations | Electricity | Industrial
Disaggregation of Revenue [Line Items]
External Revenues	0	0	0	0
Intersegment Eliminations | Electricity | Other
Disaggregation of Revenue [Line Items]
External Revenues	(27)	(29)	(73)	(75)
Intersegment Eliminations | Natural Gas
Disaggregation of Revenue [Line Items]
External Revenues	0	0	0	0
Intersegment Eliminations | Natural Gas | Residential
Disaggregation of Revenue [Line Items]
External Revenues	0	0	0	0
Intersegment Eliminations | Natural Gas | Commercial
Disaggregation of Revenue [Line Items]
External Revenues	0	0	0	0
Intersegment Eliminations | Natural Gas | Industrial
Disaggregation of Revenue [Line Items]
External Revenues	0	0	0	0
Intersegment Eliminations | Natural Gas | Other
Disaggregation of Revenue [Line Items]
External Revenues	0	0	0	0
Intersegment Eliminations | Ameren Illinois Company
Disaggregation of Revenue [Line Items]
External Revenues	(15)	(18)	(39)	(47)
Intersegment Eliminations | Ameren Illinois Company | Residential
Disaggregation of Revenue [Line Items]
External Revenues	0	0	0	0
Intersegment Eliminations | Ameren Illinois Company | Commercial
Disaggregation of Revenue [Line Items]
External Revenues	0	0	0	0
Intersegment Eliminations | Ameren Illinois Company | Industrial
Disaggregation of Revenue [Line Items]
External Revenues	0	0	0	0
Intersegment Eliminations | Ameren Illinois Company | Other
Disaggregation of Revenue [Line Items]
External Revenues	$ (15)	$ (18)	$ (39)	$ (47)