Segment Information (Disaggregation of Revenue) (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020
Disaggregation of Revenue [Line Items]
Revenues	$ 1,472	$ 1,398	$ 3,038	$ 2,838
Revenues	1,472	1,398	3,038	2,838
Revenues from alternative revenue programs	36	20	89	86
Other revenues not from contracts with customers	67	7	69	17
Union Electric Company
Disaggregation of Revenue [Line Items]
Revenues from alternative revenue programs	(5)	(6)	(15)	(9)
Other revenues not from contracts with customers	66	7	64	15
Ameren Illinois Electric Distribution
Disaggregation of Revenue [Line Items]
Revenues from alternative revenue programs	34	5	95	51
Other revenues not from contracts with customers	0	0	3	1
Ameren Illinois Natural Gas
Disaggregation of Revenue [Line Items]
Revenues from alternative revenue programs	2	3	5	14
Other revenues not from contracts with customers	1	0	2	1
Ameren Transmission
Disaggregation of Revenue [Line Items]
Revenues from alternative revenue programs	5	18	4	30
Other revenues not from contracts with customers	0	0	0	0
Electricity
Disaggregation of Revenue [Line Items]
Revenues	1,284	1,237	2,440	2,357
Revenues	1,284	1,237	2,440	2,357
Electricity | Residential
Disaggregation of Revenue [Line Items]
Revenues	546	569	1,087	1,086
Electricity | Commercial
Disaggregation of Revenue [Line Items]
Revenues	398	376	746	723
Electricity | Industrial
Disaggregation of Revenue [Line Items]
Revenues	105	97	191	185
Electricity | Other
Disaggregation of Revenue [Line Items]
Revenues	235	195	416	363
Natural Gas
Disaggregation of Revenue [Line Items]
Revenues	188	161	598	481
Revenues	188	161	598	481
Natural Gas | Residential
Disaggregation of Revenue [Line Items]
Revenues	123	105	408	351
Natural Gas | Commercial
Disaggregation of Revenue [Line Items]
Revenues	33	26	112	93
Natural Gas | Industrial
Disaggregation of Revenue [Line Items]
Revenues	4	4	19	8
Natural Gas | Other
Disaggregation of Revenue [Line Items]
Revenues	28	26	59	29
Ameren Illinois Company
Disaggregation of Revenue [Line Items]
Revenues	629	567	1,452	1,290
Revenues	629	567	1,452	1,290
Revenues from alternative revenue programs	38	22	101	89
Other revenues not from contracts with customers	1	0	5	2
Ameren Illinois Company | Residential
Disaggregation of Revenue [Line Items]
Revenues	330	304	810	737
Ameren Illinois Company | Commercial
Disaggregation of Revenue [Line Items]
Revenues	156	134	352	314
Ameren Illinois Company | Industrial
Disaggregation of Revenue [Line Items]
Revenues	37	33	85	71
Ameren Illinois Company | Other
Disaggregation of Revenue [Line Items]
Revenues	106	96	205	168
Ameren Illinois Company | Ameren Illinois Electric Distribution
Disaggregation of Revenue [Line Items]
Revenues from alternative revenue programs	34	5	95	51
Other revenues not from contracts with customers	0	0	3	1
Ameren Illinois Company | Ameren Illinois Natural Gas
Disaggregation of Revenue [Line Items]
Revenues from alternative revenue programs	2	3	5	14
Other revenues not from contracts with customers	1	0	2	1
Ameren Illinois Company | Ameren Illinois Transmission
Disaggregation of Revenue [Line Items]
Revenues from alternative revenue programs	2	14	1	24
Other revenues not from contracts with customers	0	0	0	0
Ameren Illinois Company | Electricity
Disaggregation of Revenue [Line Items]
Revenues	461	427	937	879
Ameren Illinois Company | Natural Gas
Disaggregation of Revenue [Line Items]
Revenues	168	140	515	411
Operating Segments | Union Electric Company
Disaggregation of Revenue [Line Items]
Revenues	799	782	1,494	1,452
Revenues	809	792	1,513	1,472
Operating Segments | Ameren Illinois Electric Distribution
Disaggregation of Revenue [Line Items]
Revenues	386	352	797	741
Revenues	388	352	799	742
Operating Segments | Ameren Illinois Natural Gas
Disaggregation of Revenue [Line Items]
Revenues	168	140	515	411
Revenues	168	140	515	411
Operating Segments | Ameren Transmission
Disaggregation of Revenue [Line Items]
Revenues	119	124	232	234
Revenues	136	136	266	259
Operating Segments | Electricity | Union Electric Company
Disaggregation of Revenue [Line Items]
Revenues	789	771	1,430	1,402
Operating Segments | Electricity | Union Electric Company | Residential
Disaggregation of Revenue [Line Items]
Revenues	328	359	640	656
Operating Segments | Electricity | Union Electric Company | Commercial
Disaggregation of Revenue [Line Items]
Revenues	271	264	487	485
Operating Segments | Electricity | Union Electric Company | Industrial
Disaggregation of Revenue [Line Items]
Revenues	71	67	123	120
Operating Segments | Electricity | Union Electric Company | Other
Disaggregation of Revenue [Line Items]
Revenues	119	81	180	141
Operating Segments | Electricity | Ameren Illinois Electric Distribution
Disaggregation of Revenue [Line Items]
Revenues	388	352	799	742
Operating Segments | Electricity | Ameren Illinois Electric Distribution | Residential
Disaggregation of Revenue [Line Items]
Revenues	218	210	447	430
Operating Segments | Electricity | Ameren Illinois Electric Distribution | Commercial
Disaggregation of Revenue [Line Items]
Revenues	127	112	259	238
Operating Segments | Electricity | Ameren Illinois Electric Distribution | Industrial
Disaggregation of Revenue [Line Items]
Revenues	34	30	68	65
Operating Segments | Electricity | Ameren Illinois Electric Distribution | Other
Disaggregation of Revenue [Line Items]
Revenues	9	0	25	9
Operating Segments | Electricity | Ameren Illinois Natural Gas
Disaggregation of Revenue [Line Items]
Revenues	0	0	0	0
Operating Segments | Electricity | Ameren Illinois Natural Gas | Residential
Disaggregation of Revenue [Line Items]
Revenues	0	0	0	0
Operating Segments | Electricity | Ameren Illinois Natural Gas | Commercial
Disaggregation of Revenue [Line Items]
Revenues	0	0	0	0
Operating Segments | Electricity | Ameren Illinois Natural Gas | Industrial
Disaggregation of Revenue [Line Items]
Revenues	0	0	0	0
Operating Segments | Electricity | Ameren Illinois Natural Gas | Other
Disaggregation of Revenue [Line Items]
Revenues	0	0	0	0
Operating Segments | Electricity | Ameren Transmission
Disaggregation of Revenue [Line Items]
Revenues	136	136	266	259
Operating Segments | Electricity | Ameren Transmission | Residential
Disaggregation of Revenue [Line Items]
Revenues	0	0	0	0
Operating Segments | Electricity | Ameren Transmission | Commercial
Disaggregation of Revenue [Line Items]
Revenues	0	0	0	0
Operating Segments | Electricity | Ameren Transmission | Industrial
Disaggregation of Revenue [Line Items]
Revenues	0	0	0	0
Operating Segments | Electricity | Ameren Transmission | Other
Disaggregation of Revenue [Line Items]
Revenues	136	136	266	259
Operating Segments | Natural Gas | Union Electric Company
Disaggregation of Revenue [Line Items]
Revenues	20	21	83	70
Operating Segments | Natural Gas | Union Electric Company | Residential
Disaggregation of Revenue [Line Items]
Revenues	11	11	45	44
Operating Segments | Natural Gas | Union Electric Company | Commercial
Disaggregation of Revenue [Line Items]
Revenues	4	4	19	17
Operating Segments | Natural Gas | Union Electric Company | Industrial
Disaggregation of Revenue [Line Items]
Revenues	1	1	2	2
Operating Segments | Natural Gas | Union Electric Company | Other
Disaggregation of Revenue [Line Items]
Revenues	4	5	17	7
Operating Segments | Natural Gas | Ameren Illinois Electric Distribution
Disaggregation of Revenue [Line Items]
Revenues	0	0	0	0
Operating Segments | Natural Gas | Ameren Illinois Electric Distribution | Residential
Disaggregation of Revenue [Line Items]
Revenues	0	0	0	0
Operating Segments | Natural Gas | Ameren Illinois Electric Distribution | Commercial
Disaggregation of Revenue [Line Items]
Revenues	0	0	0	0
Operating Segments | Natural Gas | Ameren Illinois Electric Distribution | Industrial
Disaggregation of Revenue [Line Items]
Revenues	0	0	0	0
Operating Segments | Natural Gas | Ameren Illinois Electric Distribution | Other
Disaggregation of Revenue [Line Items]
Revenues	0	0	0	0
Operating Segments | Natural Gas | Ameren Illinois Natural Gas
Disaggregation of Revenue [Line Items]
Revenues	168	140	515	411
Operating Segments | Natural Gas | Ameren Illinois Natural Gas | Residential
Disaggregation of Revenue [Line Items]
Revenues	112	94	363	307
Operating Segments | Natural Gas | Ameren Illinois Natural Gas | Commercial
Disaggregation of Revenue [Line Items]
Revenues	29	22	93	76
Operating Segments | Natural Gas | Ameren Illinois Natural Gas | Industrial
Disaggregation of Revenue [Line Items]
Revenues	3	3	17	6
Operating Segments | Natural Gas | Ameren Illinois Natural Gas | Other
Disaggregation of Revenue [Line Items]
Revenues	24	21	42	22
Operating Segments | Natural Gas | Ameren Transmission
Disaggregation of Revenue [Line Items]
Revenues	0	0	0	0
Operating Segments | Natural Gas | Ameren Transmission | Residential
Disaggregation of Revenue [Line Items]
Revenues	0	0	0	0
Operating Segments | Natural Gas | Ameren Transmission | Commercial
Disaggregation of Revenue [Line Items]
Revenues	0	0	0	0
Operating Segments | Natural Gas | Ameren Transmission | Industrial
Disaggregation of Revenue [Line Items]
Revenues	0	0	0	0
Operating Segments | Natural Gas | Ameren Transmission | Other
Disaggregation of Revenue [Line Items]
Revenues	0	0	0	0
Operating Segments | Ameren Illinois Company | Ameren Illinois Electric Distribution
Disaggregation of Revenue [Line Items]
Revenues	388	352	799	742
Operating Segments | Ameren Illinois Company | Ameren Illinois Natural Gas
Disaggregation of Revenue [Line Items]
Revenues	168	140	515	411
Operating Segments | Ameren Illinois Company | Ameren Illinois Transmission
Disaggregation of Revenue [Line Items]
Revenues	73	75	138	137
Operating Segments | Ameren Illinois Company | Electricity | Ameren Illinois Electric Distribution
Disaggregation of Revenue [Line Items]
Revenues	388	352	799	742
Operating Segments | Ameren Illinois Company | Electricity | Ameren Illinois Electric Distribution | Residential
Disaggregation of Revenue [Line Items]
Revenues	218	210	447	430
Operating Segments | Ameren Illinois Company | Electricity | Ameren Illinois Electric Distribution | Commercial
Disaggregation of Revenue [Line Items]
Revenues	127	112	259	238
Operating Segments | Ameren Illinois Company | Electricity | Ameren Illinois Electric Distribution | Industrial
Disaggregation of Revenue [Line Items]
Revenues	34	30	68	65
Operating Segments | Ameren Illinois Company | Electricity | Ameren Illinois Electric Distribution | Other
Disaggregation of Revenue [Line Items]
Revenues	9	0	25	9
Operating Segments | Ameren Illinois Company | Electricity | Ameren Illinois Transmission
Disaggregation of Revenue [Line Items]
Revenues	88	87	169	161
Operating Segments | Ameren Illinois Company | Electricity | Ameren Illinois Transmission | Residential
Disaggregation of Revenue [Line Items]
Revenues	0	0	0	0
Operating Segments | Ameren Illinois Company | Electricity | Ameren Illinois Transmission | Commercial
Disaggregation of Revenue [Line Items]
Revenues	0	0	0	0
Operating Segments | Ameren Illinois Company | Electricity | Ameren Illinois Transmission | Industrial
Disaggregation of Revenue [Line Items]
Revenues	0	0	0	0
Operating Segments | Ameren Illinois Company | Electricity | Ameren Illinois Transmission | Other
Disaggregation of Revenue [Line Items]
Revenues	88	87	169	161
Operating Segments | Ameren Illinois Company | Natural Gas | Ameren Illinois Natural Gas
Disaggregation of Revenue [Line Items]
Revenues	168	140	515	411
Operating Segments | Ameren Illinois Company | Natural Gas | Ameren Illinois Natural Gas | Residential
Disaggregation of Revenue [Line Items]
Revenues	112	94	363	307
Operating Segments | Ameren Illinois Company | Natural Gas | Ameren Illinois Natural Gas | Commercial
Disaggregation of Revenue [Line Items]
Revenues	29	22	93	76
Operating Segments | Ameren Illinois Company | Natural Gas | Ameren Illinois Natural Gas | Industrial
Disaggregation of Revenue [Line Items]
Revenues	3	3	17	6
Operating Segments | Ameren Illinois Company | Natural Gas | Ameren Illinois Natural Gas | Other
Disaggregation of Revenue [Line Items]
Revenues	24	21	42	22
Intersegment Eliminations
Disaggregation of Revenue [Line Items]
Revenues	(29)	(22)	(55)	(46)
Revenues	(29)	(22)	(55)	(46)
Intersegment Eliminations | Union Electric Company
Disaggregation of Revenue [Line Items]
Revenues	(10)	(10)	(19)	(20)
Intersegment Eliminations | Ameren Illinois Electric Distribution
Disaggregation of Revenue [Line Items]
Revenues	(2)	0	(2)	(1)
Intersegment Eliminations | Ameren Illinois Natural Gas
Disaggregation of Revenue [Line Items]
Revenues	0	0	0	0
Intersegment Eliminations | Ameren Transmission
Disaggregation of Revenue [Line Items]
Revenues	(17)	(12)	(34)	(25)
Intersegment Eliminations | Electricity
Disaggregation of Revenue [Line Items]
Revenues	(29)	(22)	(55)	(46)
Intersegment Eliminations | Electricity | Residential
Disaggregation of Revenue [Line Items]
Revenues	0	0	0	0
Intersegment Eliminations | Electricity | Commercial
Disaggregation of Revenue [Line Items]
Revenues	0	0	0	0
Intersegment Eliminations | Electricity | Industrial
Disaggregation of Revenue [Line Items]
Revenues	0	0	0	0
Intersegment Eliminations | Electricity | Other
Disaggregation of Revenue [Line Items]
Revenues	(29)	(22)	(55)	(46)
Intersegment Eliminations | Natural Gas
Disaggregation of Revenue [Line Items]
Revenues	0	0	0	0
Intersegment Eliminations | Natural Gas | Residential
Disaggregation of Revenue [Line Items]
Revenues	0	0	0	0
Intersegment Eliminations | Natural Gas | Commercial
Disaggregation of Revenue [Line Items]
Revenues	0	0	0	0
Intersegment Eliminations | Natural Gas | Industrial
Disaggregation of Revenue [Line Items]
Revenues	0	0	0	0
Intersegment Eliminations | Natural Gas | Other
Disaggregation of Revenue [Line Items]
Revenues	0	0	0	0
Intersegment Eliminations | Ameren Illinois Company
Disaggregation of Revenue [Line Items]
Revenues	(15)	(12)	(31)	(24)
Revenues	(15)	(12)	(31)	(24)
Intersegment Eliminations | Ameren Illinois Company | Residential
Disaggregation of Revenue [Line Items]
Revenues	0	0	0	0
Intersegment Eliminations | Ameren Illinois Company | Commercial
Disaggregation of Revenue [Line Items]
Revenues	0	0	0	0
Intersegment Eliminations | Ameren Illinois Company | Industrial
Disaggregation of Revenue [Line Items]
Revenues	0	0	0	0
Intersegment Eliminations | Ameren Illinois Company | Other
Disaggregation of Revenue [Line Items]
Revenues	(15)	(12)	(31)	(24)
Intersegment Eliminations | Ameren Illinois Company | Ameren Illinois Electric Distribution
Disaggregation of Revenue [Line Items]
Revenues	0	0	0	0
Intersegment Eliminations | Ameren Illinois Company | Ameren Illinois Natural Gas
Disaggregation of Revenue [Line Items]
Revenues	0	0	0	0
Intersegment Eliminations | Ameren Illinois Company | Ameren Illinois Transmission
Disaggregation of Revenue [Line Items]
Revenues	$ (15)	$ (12)	$ (31)	$ (24)