Segment Information (Disaggregation of Revenues) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Disaggregation of Revenue [Line Items]
External revenues	$ 5,794	$ 5,910	$ 6,291
Revenues from alternative revenue programs	36	(70)	(59)
Other revenues not from contracts with customers	36	28	42
TCJA - Excess Amounts Collected in Rates
Disaggregation of Revenue [Line Items]
TCJA Revenue Reduction			60
Ameren Illinois Electric Distribution | Ameren Missouri Software Licensing with Ameren Illinois
Disaggregation of Revenue [Line Items]
Revenue from Related Parties		14
Ameren Illinois Gas | Ameren Missouri Software Licensing with Ameren Illinois
Disaggregation of Revenue [Line Items]
Revenue from Related Parties		5
Electric
Disaggregation of Revenue [Line Items]
External revenues	4,911	4,981	5,339
Electric | Residential
Disaggregation of Revenue [Line Items]
External revenues	2,240	2,251	2,427
Electric | Commercial
Disaggregation of Revenue [Line Items]
External revenues	1,511	1,654	1,782
Electric | Industrial
Disaggregation of Revenue [Line Items]
External revenues	385	405	442
Electric | Other
Disaggregation of Revenue [Line Items]
External revenues	775	671	688
Natural gas
Disaggregation of Revenue [Line Items]
External revenues	883	929	952
Natural gas | Residential
Disaggregation of Revenue [Line Items]
External revenues	617	651	671
Natural gas | Commercial
Disaggregation of Revenue [Line Items]
External revenues	165	188	196
Natural gas | Industrial
Disaggregation of Revenue [Line Items]
External revenues	18	17	21
Natural gas | Other
Disaggregation of Revenue [Line Items]
External revenues	83	73	64
Ameren Illinois Company
Disaggregation of Revenue [Line Items]
External revenues	2,535	2,527	2,576
Revenues from alternative revenue programs	42	(107)	(51)
Other revenues not from contracts with customers	10	9	18
Ameren Illinois Company | Ameren Missouri Software Licensing with Ameren Illinois
Disaggregation of Revenue [Line Items]
Revenue from Related Parties		19
Ameren Illinois Company | Residential
Disaggregation of Revenue [Line Items]
External revenues	1,408	1,418	1,448
Ameren Illinois Company | Commercial
Disaggregation of Revenue [Line Items]
External revenues	622	651	670
Ameren Illinois Company | Industrial
Disaggregation of Revenue [Line Items]
External revenues	138	140	147
Ameren Illinois Company | Other
Disaggregation of Revenue [Line Items]
External revenues	367	318	311
Ameren Illinois Company | Electric
Disaggregation of Revenue [Line Items]
External revenues	1,775	1,730	1,761
Ameren Illinois Company | Natural gas
Disaggregation of Revenue [Line Items]
External revenues	760	797	815
Operating Segments | Union Electric Company
Disaggregation of Revenue [Line Items]
External revenues	3,109	3,243	3,589
Revenues from alternative revenue programs	(14)	35	(8)
Other revenues not from contracts with customers	25	19	24
Operating Segments | Ameren Illinois Electric Distribution
Disaggregation of Revenue [Line Items]
External revenues	1,498	1,504	1,547
Revenues from alternative revenue programs	(20)	(74)	(3)
Other revenues not from contracts with customers	8	7	16
Operating Segments | Ameren Illinois Gas
Disaggregation of Revenue [Line Items]
External revenues	760	797	815
Revenues from alternative revenue programs	20	0	(23)
Other revenues not from contracts with customers	2	2	2
Operating Segments | Ameren Transmission
Disaggregation of Revenue [Line Items]
External revenues	523	464	433
Revenues from alternative revenue programs	50	(31)	(25)
Other revenues not from contracts with customers	1	0	0
Operating Segments | Electric | Union Electric Company
Disaggregation of Revenue [Line Items]
External revenues	2,984	3,109	3,451
Operating Segments | Electric | Union Electric Company | Residential
Disaggregation of Revenue [Line Items]
External revenues	1,373	1,403	1,560
Operating Segments | Electric | Union Electric Company | Commercial
Disaggregation of Revenue [Line Items]
External revenues	1,025	1,157	1,271
Operating Segments | Electric | Union Electric Company | Industrial
Disaggregation of Revenue [Line Items]
External revenues	261	278	312
Operating Segments | Electric | Union Electric Company | Other
Disaggregation of Revenue [Line Items]
External revenues	325	271	308
Operating Segments | Electric | Ameren Illinois Electric Distribution
Disaggregation of Revenue [Line Items]
External revenues	1,498	1,504	1,547
Operating Segments | Electric | Ameren Illinois Electric Distribution | Residential
Disaggregation of Revenue [Line Items]
External revenues	867	848	867
Operating Segments | Electric | Ameren Illinois Electric Distribution | Commercial
Disaggregation of Revenue [Line Items]
External revenues	486	497	511
Operating Segments | Electric | Ameren Illinois Electric Distribution | Industrial
Disaggregation of Revenue [Line Items]
External revenues	124	127	130
Operating Segments | Electric | Ameren Illinois Electric Distribution | Other
Disaggregation of Revenue [Line Items]
External revenues	21	32	39
Operating Segments | Electric | Ameren Illinois Gas
Disaggregation of Revenue [Line Items]
External revenues	0	0	0
Operating Segments | Electric | Ameren Illinois Gas | Residential
Disaggregation of Revenue [Line Items]
External revenues	0	0	0
Operating Segments | Electric | Ameren Illinois Gas | Commercial
Disaggregation of Revenue [Line Items]
External revenues	0	0	0
Operating Segments | Electric | Ameren Illinois Gas | Industrial
Disaggregation of Revenue [Line Items]
External revenues	0	0	0
Operating Segments | Electric | Ameren Illinois Gas | Other
Disaggregation of Revenue [Line Items]
External revenues	0	0	0
Operating Segments | Electric | Ameren Transmission
Disaggregation of Revenue [Line Items]
External revenues	523	464	433
Operating Segments | Electric | Ameren Transmission | Residential
Disaggregation of Revenue [Line Items]
External revenues	0	0	0
Operating Segments | Electric | Ameren Transmission | Commercial
Disaggregation of Revenue [Line Items]
External revenues	0	0	0
Operating Segments | Electric | Ameren Transmission | Industrial
Disaggregation of Revenue [Line Items]
External revenues	0	0	0
Operating Segments | Electric | Ameren Transmission | Other
Disaggregation of Revenue [Line Items]
External revenues	523	464	433
Operating Segments | Natural gas | Union Electric Company
Disaggregation of Revenue [Line Items]
External revenues	125	134	138
Operating Segments | Natural gas | Union Electric Company | Residential
Disaggregation of Revenue [Line Items]
External revenues	76	81	90
Operating Segments | Natural gas | Union Electric Company | Commercial
Disaggregation of Revenue [Line Items]
External revenues	29	34	37
Operating Segments | Natural gas | Union Electric Company | Industrial
Disaggregation of Revenue [Line Items]
External revenues	4	4	4
Operating Segments | Natural gas | Union Electric Company | Other
Disaggregation of Revenue [Line Items]
External revenues	16	15	7
Operating Segments | Natural gas | Ameren Illinois Electric Distribution
Disaggregation of Revenue [Line Items]
External revenues	0	0	0
Operating Segments | Natural gas | Ameren Illinois Electric Distribution | Residential
Disaggregation of Revenue [Line Items]
External revenues	0	0	0
Operating Segments | Natural gas | Ameren Illinois Electric Distribution | Commercial
Disaggregation of Revenue [Line Items]
External revenues	0	0	0
Operating Segments | Natural gas | Ameren Illinois Electric Distribution | Industrial
Disaggregation of Revenue [Line Items]
External revenues	0	0	0
Operating Segments | Natural gas | Ameren Illinois Electric Distribution | Other
Disaggregation of Revenue [Line Items]
External revenues	0	0	0
Operating Segments | Natural gas | Ameren Illinois Gas
Disaggregation of Revenue [Line Items]
External revenues	760	797	815
Operating Segments | Natural gas | Ameren Illinois Gas | Residential
Disaggregation of Revenue [Line Items]
External revenues	541	570	581
Operating Segments | Natural gas | Ameren Illinois Gas | Commercial
Disaggregation of Revenue [Line Items]
External revenues	136	154	159
Operating Segments | Natural gas | Ameren Illinois Gas | Industrial
Disaggregation of Revenue [Line Items]
External revenues	14	13	17
Operating Segments | Natural gas | Ameren Illinois Gas | Other
Disaggregation of Revenue [Line Items]
External revenues	69	60	58
Operating Segments | Natural gas | Ameren Transmission
Disaggregation of Revenue [Line Items]
External revenues	0	0	0
Operating Segments | Natural gas | Ameren Transmission | Residential
Disaggregation of Revenue [Line Items]
External revenues	0	0	0
Operating Segments | Natural gas | Ameren Transmission | Commercial
Disaggregation of Revenue [Line Items]
External revenues	0	0	0
Operating Segments | Natural gas | Ameren Transmission | Industrial
Disaggregation of Revenue [Line Items]
External revenues	0	0	0
Operating Segments | Natural gas | Ameren Transmission | Other
Disaggregation of Revenue [Line Items]
External revenues	0	0	0
Operating Segments | Ameren Illinois Company | Ameren Illinois Electric Distribution
Disaggregation of Revenue [Line Items]
Revenues from alternative revenue programs	(20)	(74)	(3)
Other revenues not from contracts with customers	8	7	16
Operating Segments | Ameren Illinois Company | Ameren Illinois Gas
Disaggregation of Revenue [Line Items]
Revenues from alternative revenue programs	20	0	(23)
Other revenues not from contracts with customers	2	2	2
Operating Segments | Ameren Illinois Company | Ameren Illinois Transmission
Disaggregation of Revenue [Line Items]
Revenues from alternative revenue programs	42	(33)	(25)
Other revenues not from contracts with customers	0	0	0
Operating Segments | Ameren Illinois Company | Electric | Ameren Illinois Electric Distribution
Disaggregation of Revenue [Line Items]
External revenues	1,498	1,504	1,547
Operating Segments | Ameren Illinois Company | Electric | Ameren Illinois Electric Distribution | Residential
Disaggregation of Revenue [Line Items]
External revenues	867	848	867
Operating Segments | Ameren Illinois Company | Electric | Ameren Illinois Electric Distribution | Commercial
Disaggregation of Revenue [Line Items]
External revenues	486	497	511
Operating Segments | Ameren Illinois Company | Electric | Ameren Illinois Electric Distribution | Industrial
Disaggregation of Revenue [Line Items]
External revenues	124	127	130
Operating Segments | Ameren Illinois Company | Electric | Ameren Illinois Electric Distribution | Other
Disaggregation of Revenue [Line Items]
External revenues	21	32	39
Operating Segments | Ameren Illinois Company | Electric | Ameren Illinois Transmission
Disaggregation of Revenue [Line Items]
External revenues	329	288	267
Operating Segments | Ameren Illinois Company | Electric | Ameren Illinois Transmission | Residential
Disaggregation of Revenue [Line Items]
External revenues	0	0	0
Operating Segments | Ameren Illinois Company | Electric | Ameren Illinois Transmission | Commercial
Disaggregation of Revenue [Line Items]
External revenues	0	0	0
Operating Segments | Ameren Illinois Company | Electric | Ameren Illinois Transmission | Industrial
Disaggregation of Revenue [Line Items]
External revenues	0	0	0
Operating Segments | Ameren Illinois Company | Electric | Ameren Illinois Transmission | Other
Disaggregation of Revenue [Line Items]
External revenues	329	288	267
Operating Segments | Ameren Illinois Company | Natural gas | Ameren Illinois Gas
Disaggregation of Revenue [Line Items]
External revenues	760	797	815
Operating Segments | Ameren Illinois Company | Natural gas | Ameren Illinois Gas | Residential
Disaggregation of Revenue [Line Items]
External revenues	541	570	581
Operating Segments | Ameren Illinois Company | Natural gas | Ameren Illinois Gas | Commercial
Disaggregation of Revenue [Line Items]
External revenues	136	154	159
Operating Segments | Ameren Illinois Company | Natural gas | Ameren Illinois Gas | Industrial
Disaggregation of Revenue [Line Items]
External revenues	14	13	17
Operating Segments | Ameren Illinois Company | Natural gas | Ameren Illinois Gas | Other
Disaggregation of Revenue [Line Items]
External revenues	69	60	58
Intersegment Elimination
Disaggregation of Revenue [Line Items]
External revenues	(96)	(98)	(93)
Intersegment Elimination | Electric
Disaggregation of Revenue [Line Items]
External revenues	(94)	(96)	(92)
Intersegment Elimination | Electric | Residential
Disaggregation of Revenue [Line Items]
External revenues	0	0	0
Intersegment Elimination | Electric | Commercial
Disaggregation of Revenue [Line Items]
External revenues	0	0	0
Intersegment Elimination | Electric | Industrial
Disaggregation of Revenue [Line Items]
External revenues	0	0	0
Intersegment Elimination | Electric | Other
Disaggregation of Revenue [Line Items]
External revenues	(94)	(96)	(92)
Intersegment Elimination | Natural gas
Disaggregation of Revenue [Line Items]
External revenues	(2)	(2)	(1)
Intersegment Elimination | Natural gas | Residential
Disaggregation of Revenue [Line Items]
External revenues	0	0	0
Intersegment Elimination | Natural gas | Commercial
Disaggregation of Revenue [Line Items]
External revenues	0	0	0
Intersegment Elimination | Natural gas | Industrial
Disaggregation of Revenue [Line Items]
External revenues	0	0	0
Intersegment Elimination | Natural gas | Other
Disaggregation of Revenue [Line Items]
External revenues	(2)	(2)	(1)
Intersegment Elimination | Ameren Illinois Company
Disaggregation of Revenue [Line Items]
External revenues	(52)	(62)	(53)
Intersegment Elimination | Ameren Illinois Company | Residential
Disaggregation of Revenue [Line Items]
External revenues	0	0	0
Intersegment Elimination | Ameren Illinois Company | Commercial
Disaggregation of Revenue [Line Items]
External revenues	0	0	0
Intersegment Elimination | Ameren Illinois Company | Industrial
Disaggregation of Revenue [Line Items]
External revenues	0	0	0
Intersegment Elimination | Ameren Illinois Company | Other
Disaggregation of Revenue [Line Items]
External revenues	$ (52)	$ (62)	$ (53)