Segment Information (Disaggregation of Revenue) (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2022	Jun. 30, 2021	Jun. 30, 2022	Jun. 30, 2021
Disaggregation of Revenue [Line Items]
Revenues	$ 1,726	$ 1,472	$ 3,605	$ 3,038
Revenues	1,726	1,472	3,605	3,038
Revenues from alternative revenue programs	40	36	85	89
Other revenues not from contracts with customers	(34)	67	(31)	69
Union Electric Company
Disaggregation of Revenue [Line Items]
Revenues from alternative revenue programs	0	(5)	(6)	(15)
Other revenues not from contracts with customers	(36)	66	(36)	64
Ameren Illinois Electric Distribution
Disaggregation of Revenue [Line Items]
Revenues from alternative revenue programs	41	34	96	95
Other revenues not from contracts with customers	1	0	3	3
Ameren Illinois Natural Gas
Disaggregation of Revenue [Line Items]
Revenues from alternative revenue programs	3	2	(2)	5
Other revenues not from contracts with customers	1	1	2	2
Ameren Transmission
Disaggregation of Revenue [Line Items]
Revenues from alternative revenue programs	(4)	5	(3)	4
Other revenues not from contracts with customers	0	0	0	0
Electric
Disaggregation of Revenue [Line Items]
Revenues	1,513	1,284	2,831	2,440
Revenues	1,513	1,284	2,831	2,440
Electric | Residential
Disaggregation of Revenue [Line Items]
Revenues	655	546	1,250	1,087
Electric | Commercial
Disaggregation of Revenue [Line Items]
Revenues	478	398	876	746
Electric | Industrial
Disaggregation of Revenue [Line Items]
Revenues	126	105	228	191
Electric | Other
Disaggregation of Revenue [Line Items]
Revenues	254	235	477	416
Natural gas
Disaggregation of Revenue [Line Items]
Revenues	213	188	774	598
Revenues	213	188	774	598
Natural gas | Residential
Disaggregation of Revenue [Line Items]
Revenues	133	123	553	408
Natural gas | Commercial
Disaggregation of Revenue [Line Items]
Revenues	38	33	157	112
Natural gas | Industrial
Disaggregation of Revenue [Line Items]
Revenues	12	4	31	19
Natural gas | Other
Disaggregation of Revenue [Line Items]
Revenues	30	28	33	59
Ameren Illinois Company
Disaggregation of Revenue [Line Items]
Revenues	769	629	1,793	1,452
Revenues	769	629	1,793	1,452
Revenues from alternative revenue programs	41	38	92	101
Other revenues not from contracts with customers	2	1	5	5
Ameren Illinois Company | Residential
Disaggregation of Revenue [Line Items]
Revenues	401	330	1,033	810
Ameren Illinois Company | Commercial
Disaggregation of Revenue [Line Items]
Revenues	210	156	465	352
Ameren Illinois Company | Industrial
Disaggregation of Revenue [Line Items]
Revenues	64	37	126	85
Ameren Illinois Company | Other
Disaggregation of Revenue [Line Items]
Revenues	94	106	169	205
Ameren Illinois Company | Ameren Illinois Electric Distribution
Disaggregation of Revenue [Line Items]
Revenues from alternative revenue programs	41	34	96	95
Other revenues not from contracts with customers	1	0	3	3
Ameren Illinois Company | Ameren Illinois Natural Gas
Disaggregation of Revenue [Line Items]
Revenues from alternative revenue programs	3	2	(2)	5
Other revenues not from contracts with customers	1	1	2	2
Ameren Illinois Company | Ameren Illinois Transmission
Disaggregation of Revenue [Line Items]
Revenues from alternative revenue programs	(3)	2	(2)	1
Other revenues not from contracts with customers	0	0	0	0
Ameren Illinois Company | Electric
Disaggregation of Revenue [Line Items]
Revenues	585	461	1,128	937
Ameren Illinois Company | Natural gas
Disaggregation of Revenue [Line Items]
Revenues	184	168	665	515
Operating Segments | Union Electric Company
Disaggregation of Revenue [Line Items]
Revenues	912	799	1,720	1,494
Revenues	919	809	1,737	1,513
Operating Segments | Ameren Illinois Electric Distribution
Disaggregation of Revenue [Line Items]
Revenues	504	386	968	797
Revenues	504	388	969	799
Operating Segments | Ameren Illinois Natural Gas
Disaggregation of Revenue [Line Items]
Revenues	184	168	665	515
Revenues	184	168	665	515
Operating Segments | Ameren Transmission
Disaggregation of Revenue [Line Items]
Revenues	126	119	252	232
Revenues	150	136	296	266
Operating Segments | Electric | Union Electric Company
Disaggregation of Revenue [Line Items]
Revenues	890	789	1,628	1,430
Operating Segments | Electric | Union Electric Company | Residential
Disaggregation of Revenue [Line Items]
Revenues	371	328	703	640
Operating Segments | Electric | Union Electric Company | Commercial
Disaggregation of Revenue [Line Items]
Revenues	298	271	538	487
Operating Segments | Electric | Union Electric Company | Industrial
Disaggregation of Revenue [Line Items]
Revenues	73	71	130	123
Operating Segments | Electric | Union Electric Company | Other
Disaggregation of Revenue [Line Items]
Revenues	148	119	257	180
Operating Segments | Electric | Ameren Illinois Electric Distribution
Disaggregation of Revenue [Line Items]
Revenues	504	388	969	799
Operating Segments | Electric | Ameren Illinois Electric Distribution | Residential
Disaggregation of Revenue [Line Items]
Revenues	284	218	547	447
Operating Segments | Electric | Ameren Illinois Electric Distribution | Commercial
Disaggregation of Revenue [Line Items]
Revenues	180	127	338	259
Operating Segments | Electric | Ameren Illinois Electric Distribution | Industrial
Disaggregation of Revenue [Line Items]
Revenues	53	34	98	68
Operating Segments | Electric | Ameren Illinois Electric Distribution | Other
Disaggregation of Revenue [Line Items]
Revenues	(13)	9	(14)	25
Operating Segments | Electric | Ameren Illinois Natural Gas
Disaggregation of Revenue [Line Items]
Revenues	0	0	0	0
Operating Segments | Electric | Ameren Illinois Natural Gas | Residential
Disaggregation of Revenue [Line Items]
Revenues	0	0	0	0
Operating Segments | Electric | Ameren Illinois Natural Gas | Commercial
Disaggregation of Revenue [Line Items]
Revenues	0	0	0	0
Operating Segments | Electric | Ameren Illinois Natural Gas | Industrial
Disaggregation of Revenue [Line Items]
Revenues	0	0	0	0
Operating Segments | Electric | Ameren Illinois Natural Gas | Other
Disaggregation of Revenue [Line Items]
Revenues	0	0	0	0
Operating Segments | Electric | Ameren Transmission
Disaggregation of Revenue [Line Items]
Revenues	150	136	296	266
Operating Segments | Electric | Ameren Transmission | Residential
Disaggregation of Revenue [Line Items]
Revenues	0	0	0	0
Operating Segments | Electric | Ameren Transmission | Commercial
Disaggregation of Revenue [Line Items]
Revenues	0	0	0	0
Operating Segments | Electric | Ameren Transmission | Industrial
Disaggregation of Revenue [Line Items]
Revenues	0	0	0	0
Operating Segments | Electric | Ameren Transmission | Other
Disaggregation of Revenue [Line Items]
Revenues	150	136	296	266
Operating Segments | Natural gas | Union Electric Company
Disaggregation of Revenue [Line Items]
Revenues	29	20	109	83
Operating Segments | Natural gas | Union Electric Company | Residential
Disaggregation of Revenue [Line Items]
Revenues	16	11	67	45
Operating Segments | Natural gas | Union Electric Company | Commercial
Disaggregation of Revenue [Line Items]
Revenues	8	4	30	19
Operating Segments | Natural gas | Union Electric Company | Industrial
Disaggregation of Revenue [Line Items]
Revenues	1	1	3	2
Operating Segments | Natural gas | Union Electric Company | Other
Disaggregation of Revenue [Line Items]
Revenues	4	4	9	17
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	184	168	665	515
Operating Segments | Natural gas | Ameren Illinois Natural Gas | Residential
Disaggregation of Revenue [Line Items]
Revenues	117	112	486	363
Operating Segments | Natural gas | Ameren Illinois Natural Gas | Commercial
Disaggregation of Revenue [Line Items]
Revenues	30	29	127	93
Operating Segments | Natural gas | Ameren Illinois Natural Gas | Industrial
Disaggregation of Revenue [Line Items]
Revenues	11	3	28	17
Operating Segments | Natural gas | Ameren Illinois Natural Gas | Other
Disaggregation of Revenue [Line Items]
Revenues	26	24	24	42
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	504	388	969	799
Operating Segments | Ameren Illinois Company | Ameren Illinois Natural Gas
Disaggregation of Revenue [Line Items]
Revenues	184	168	665	515
Operating Segments | Ameren Illinois Company | Ameren Illinois Transmission
Disaggregation of Revenue [Line Items]
Revenues	81	73	159	138
Operating Segments | Ameren Illinois Company | Electric | Ameren Illinois Electric Distribution
Disaggregation of Revenue [Line Items]
Revenues	504	388	969	799
Operating Segments | Ameren Illinois Company | Electric | Ameren Illinois Electric Distribution | Residential
Disaggregation of Revenue [Line Items]
Revenues	284	218	547	447
Operating Segments | Ameren Illinois Company | Electric | Ameren Illinois Electric Distribution | Commercial
Disaggregation of Revenue [Line Items]
Revenues	180	127	338	259
Operating Segments | Ameren Illinois Company | Electric | Ameren Illinois Electric Distribution | Industrial
Disaggregation of Revenue [Line Items]
Revenues	53	34	98	68
Operating Segments | Ameren Illinois Company | Electric | Ameren Illinois Electric Distribution | Other
Disaggregation of Revenue [Line Items]
Revenues	(13)	9	(14)	25
Operating Segments | Ameren Illinois Company | Electric | Ameren Illinois Transmission
Disaggregation of Revenue [Line Items]
Revenues	105	88	203	169
Operating Segments | Ameren Illinois Company | Electric | Ameren Illinois Transmission | Residential
Disaggregation of Revenue [Line Items]
Revenues	0	0	0	0
Operating Segments | Ameren Illinois Company | Electric | Ameren Illinois Transmission | Commercial
Disaggregation of Revenue [Line Items]
Revenues	0	0	0	0
Operating Segments | Ameren Illinois Company | Electric | Ameren Illinois Transmission | Industrial
Disaggregation of Revenue [Line Items]
Revenues	0	0	0	0
Operating Segments | Ameren Illinois Company | Electric | Ameren Illinois Transmission | Other
Disaggregation of Revenue [Line Items]
Revenues	105	88	203	169
Operating Segments | Ameren Illinois Company | Natural gas | Ameren Illinois Natural Gas
Disaggregation of Revenue [Line Items]
Revenues	184	168	665	515
Operating Segments | Ameren Illinois Company | Natural gas | Ameren Illinois Natural Gas | Residential
Disaggregation of Revenue [Line Items]
Revenues	117	112	486	363
Operating Segments | Ameren Illinois Company | Natural gas | Ameren Illinois Natural Gas | Commercial
Disaggregation of Revenue [Line Items]
Revenues	30	29	127	93
Operating Segments | Ameren Illinois Company | Natural gas | Ameren Illinois Natural Gas | Industrial
Disaggregation of Revenue [Line Items]
Revenues	11	3	28	17
Operating Segments | Ameren Illinois Company | Natural gas | Ameren Illinois Natural Gas | Other
Disaggregation of Revenue [Line Items]
Revenues	26	24	24	42
Intersegment Eliminations
Disaggregation of Revenue [Line Items]
Revenues	(31)	(29)	(62)	(55)
Revenues	(31)	(29)	(62)	(55)
Intersegment Eliminations | Union Electric Company
Disaggregation of Revenue [Line Items]
Revenues	(7)	(10)	(17)	(19)
Intersegment Eliminations | Ameren Illinois Electric Distribution
Disaggregation of Revenue [Line Items]
Revenues	0	(2)	(1)	(2)
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	(24)	(17)	(44)	(34)
Intersegment Eliminations | Electric
Disaggregation of Revenue [Line Items]
Revenues	(31)	(29)	(62)	(55)
Intersegment Eliminations | Electric | Residential
Disaggregation of Revenue [Line Items]
Revenues	0	0	0	0
Intersegment Eliminations | Electric | Commercial
Disaggregation of Revenue [Line Items]
Revenues	0	0	0	0
Intersegment Eliminations | Electric | Industrial
Disaggregation of Revenue [Line Items]
Revenues	0	0	0	0
Intersegment Eliminations | Electric | Other
Disaggregation of Revenue [Line Items]
Revenues	(31)	(29)	(62)	(55)
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	(24)	(15)	(44)	(31)
Revenues	(24)	(15)	(44)	(31)
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	(24)	(15)	(44)	(31)
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	$ (24)	$ (15)	$ (44)	$ (31)