Segment Information (Disaggregation of Revenues) (Details) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Disaggregation of Revenue [Line Items]
External revenues	$ 1,316	$ 1,659	$ 1,379	$ 1,556	$ 1,419	$ 1,724	$ 1,563	$ 1,585	$ 5,910	$ 6,291	$ 6,174
Revenues from alternative revenue programs									(70)	(59)	(15)
Other revenues not from contracts with customers									28	42	23
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
Electricity
Disaggregation of Revenue [Line Items]
External revenues									4,981	5,339	5,307
Electricity | Residential
Disaggregation of Revenue [Line Items]
External revenues									2,251	2,427	2,287
Electricity | Commercial
Disaggregation of Revenue [Line Items]
External revenues									1,654	1,782	1,735
Electricity | Industrial
Disaggregation of Revenue [Line Items]
External revenues									405	442	418
Electricity | Other
Disaggregation of Revenue [Line Items]
External revenues									671	688	867
Natural gas
Disaggregation of Revenue [Line Items]
External revenues									929	952	867
Natural gas | Residential
Disaggregation of Revenue [Line Items]
External revenues									651	671	608
Natural gas | Commercial
Disaggregation of Revenue [Line Items]
External revenues									188	196	177
Natural gas | Industrial
Disaggregation of Revenue [Line Items]
External revenues									17	21	16
Natural gas | Other
Disaggregation of Revenue [Line Items]
External revenues									73	64	66
Ameren Illinois Company
Disaggregation of Revenue [Line Items]
External revenues	$ 654	$ 564	$ 547	$ 762	$ 674	$ 564	$ 578	$ 760	2,527	2,576	2,527
Revenues from alternative revenue programs									(107)	(51)	9
Other revenues not from contracts with customers									9	18	8
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									1,418	1,448	1,401
Ameren Illinois Company | Commercial
Disaggregation of Revenue [Line Items]
External revenues									651	670	673
Ameren Illinois Company | Industrial
Disaggregation of Revenue [Line Items]
External revenues									140	147	125
Ameren Illinois Company | Other
Disaggregation of Revenue [Line Items]
External revenues									318	311	328
Ameren Illinois Company | Electricity
Disaggregation of Revenue [Line Items]
External revenues									1,730	1,761	1,784
Ameren Illinois Company | Natural gas
Disaggregation of Revenue [Line Items]
External revenues									797	815	743
Operating Segments | Union Electric Company
Disaggregation of Revenue [Line Items]
External revenues									3,243	3,589	3,537
Revenues from alternative revenue programs									35	(8)	(28)
Other revenues not from contracts with customers									19	24	15
Operating Segments | Ameren Illinois Electric Distribution
Disaggregation of Revenue [Line Items]
External revenues									1,504	1,547	1,568
Revenues from alternative revenue programs									(74)	(3)	(5)
Other revenues not from contracts with customers									7	16	6
Operating Segments | Ameren Illinois Gas
Disaggregation of Revenue [Line Items]
External revenues									797	815	743
Revenues from alternative revenue programs									0	(23)	5
Other revenues not from contracts with customers									2	2	2
Operating Segments | Ameren Transmission
Disaggregation of Revenue [Line Items]
External revenues									464	433	426
Revenues from alternative revenue programs									(31)	(25)	13
Other revenues not from contracts with customers									0	0	0
Operating Segments | Other Segment
Disaggregation of Revenue [Line Items]
External revenues									0	0	(2)
Operating Segments | Electricity | Union Electric Company
Disaggregation of Revenue [Line Items]
External revenues									3,109	3,451	3,411
Operating Segments | Electricity | Union Electric Company | Residential
Disaggregation of Revenue [Line Items]
External revenues									1,403	1,560	1,417
Operating Segments | Electricity | Union Electric Company | Commercial
Disaggregation of Revenue [Line Items]
External revenues									1,157	1,271	1,208
Operating Segments | Electricity | Union Electric Company | Industrial
Disaggregation of Revenue [Line Items]
External revenues									278	312	305
Operating Segments | Electricity | Union Electric Company | Other
Disaggregation of Revenue [Line Items]
External revenues									271	308	481
Operating Segments | Electricity | Ameren Illinois Electric Distribution
Disaggregation of Revenue [Line Items]
External revenues									1,504	1,547	1,568
Operating Segments | Electricity | Ameren Illinois Electric Distribution | Residential
Disaggregation of Revenue [Line Items]
External revenues									848	867	870
Operating Segments | Electricity | Ameren Illinois Electric Distribution | Commercial
Disaggregation of Revenue [Line Items]
External revenues									497	511	527
Operating Segments | Electricity | Ameren Illinois Electric Distribution | Industrial
Disaggregation of Revenue [Line Items]
External revenues									127	130	113
Operating Segments | Electricity | Ameren Illinois Electric Distribution | Other
Disaggregation of Revenue [Line Items]
External revenues									32	39	58
Operating Segments | Electricity | Ameren Illinois Gas
Disaggregation of Revenue [Line Items]
External revenues									0	0	0
Operating Segments | Electricity | Ameren Illinois Gas | Residential
Disaggregation of Revenue [Line Items]
External revenues									0	0	0
Operating Segments | Electricity | Ameren Illinois Gas | Commercial
Disaggregation of Revenue [Line Items]
External revenues									0	0	0
Operating Segments | Electricity | Ameren Illinois Gas | Industrial
Disaggregation of Revenue [Line Items]
External revenues									0	0	0
Operating Segments | Electricity | Ameren Illinois Gas | Other
Disaggregation of Revenue [Line Items]
External revenues									0	0	0
Operating Segments | Electricity | Ameren Transmission
Disaggregation of Revenue [Line Items]
External revenues									464	433	426
Operating Segments | Electricity | Ameren Transmission | Residential
Disaggregation of Revenue [Line Items]
External revenues									0	0	0
Operating Segments | Electricity | Ameren Transmission | Commercial
Disaggregation of Revenue [Line Items]
External revenues									0	0	0
Operating Segments | Electricity | Ameren Transmission | Industrial
Disaggregation of Revenue [Line Items]
External revenues									0	0	0
Operating Segments | Electricity | Ameren Transmission | Other
Disaggregation of Revenue [Line Items]
External revenues									464	433	426
Operating Segments | Electricity | Other Segment
Disaggregation of Revenue [Line Items]
External revenues									0	0	(2)
Operating Segments | Electricity | Other Segment | Residential
Disaggregation of Revenue [Line Items]
External revenues									0	0	0
Operating Segments | Electricity | Other Segment | Commercial
Disaggregation of Revenue [Line Items]
External revenues									0	0	0
Operating Segments | Electricity | Other Segment | Industrial
Disaggregation of Revenue [Line Items]
External revenues									0	0	0
Operating Segments | Electricity | Other Segment | Other
Disaggregation of Revenue [Line Items]
External revenues									0	0	(2)
Operating Segments | Natural gas | Union Electric Company
Disaggregation of Revenue [Line Items]
External revenues									134	138	126
Operating Segments | Natural gas | Union Electric Company | Residential
Disaggregation of Revenue [Line Items]
External revenues									81	90	77
Operating Segments | Natural gas | Union Electric Company | Commercial
Disaggregation of Revenue [Line Items]
External revenues									34	37	31
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									15	7	14
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									797	815	743
Operating Segments | Natural gas | Ameren Illinois Gas | Residential
Disaggregation of Revenue [Line Items]
External revenues									570	581	531
Operating Segments | Natural gas | Ameren Illinois Gas | Commercial
Disaggregation of Revenue [Line Items]
External revenues									154	159	146
Operating Segments | Natural gas | Ameren Illinois Gas | Industrial
Disaggregation of Revenue [Line Items]
External revenues									13	17	12
Operating Segments | Natural gas | Ameren Illinois Gas | Other
Disaggregation of Revenue [Line Items]
External revenues									60	58	54
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 | Natural gas | Other Segment
Disaggregation of Revenue [Line Items]
External revenues									0	0	0
Operating Segments | Natural gas | Other Segment | Residential
Disaggregation of Revenue [Line Items]
External revenues									0	0	0
Operating Segments | Natural gas | Other Segment | Commercial
Disaggregation of Revenue [Line Items]
External revenues									0	0	0
Operating Segments | Natural gas | Other Segment | Industrial
Disaggregation of Revenue [Line Items]
External revenues									0	0	0
Operating Segments | Natural gas | Other Segment | 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									(74)	(3)	(5)
Other revenues not from contracts with customers									7	16	6
Operating Segments | Ameren Illinois Company | Ameren Illinois Gas
Disaggregation of Revenue [Line Items]
Revenues from alternative revenue programs									0	(23)	5
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									(33)	(25)	9
Other revenues not from contracts with customers									0	0	0
Operating Segments | Ameren Illinois Company | Electricity | Ameren Illinois Electric Distribution
Disaggregation of Revenue [Line Items]
External revenues									1,504	1,547	1,568
Operating Segments | Ameren Illinois Company | Electricity | Ameren Illinois Electric Distribution | Residential
Disaggregation of Revenue [Line Items]
External revenues									848	867	870
Operating Segments | Ameren Illinois Company | Electricity | Ameren Illinois Electric Distribution | Commercial
Disaggregation of Revenue [Line Items]
External revenues									497	511	527
Operating Segments | Ameren Illinois Company | Electricity | Ameren Illinois Electric Distribution | Industrial
Disaggregation of Revenue [Line Items]
External revenues									127	130	113
Operating Segments | Ameren Illinois Company | Electricity | Ameren Illinois Electric Distribution | Other
Disaggregation of Revenue [Line Items]
External revenues									32	39	58
Operating Segments | Ameren Illinois Company | Electricity | Ameren Illinois Transmission
Disaggregation of Revenue [Line Items]
External revenues									288	267	258
Operating Segments | Ameren Illinois Company | Electricity | Ameren Illinois Transmission | Residential
Disaggregation of Revenue [Line Items]
External revenues									0	0	0
Operating Segments | Ameren Illinois Company | Electricity | Ameren Illinois Transmission | Commercial
Disaggregation of Revenue [Line Items]
External revenues									0	0	0
Operating Segments | Ameren Illinois Company | Electricity | Ameren Illinois Transmission | Industrial
Disaggregation of Revenue [Line Items]
External revenues									0	0	0
Operating Segments | Ameren Illinois Company | Electricity | Ameren Illinois Transmission | Other
Disaggregation of Revenue [Line Items]
External revenues									288	267	258
Operating Segments | Ameren Illinois Company | Natural gas | Ameren Illinois Gas
Disaggregation of Revenue [Line Items]
External revenues									797	815	743
Operating Segments | Ameren Illinois Company | Natural gas | Ameren Illinois Gas | Residential
Disaggregation of Revenue [Line Items]
External revenues									570	581	531
Operating Segments | Ameren Illinois Company | Natural gas | Ameren Illinois Gas | Commercial
Disaggregation of Revenue [Line Items]
External revenues									154	159	146
Operating Segments | Ameren Illinois Company | Natural gas | Ameren Illinois Gas | Industrial
Disaggregation of Revenue [Line Items]
External revenues									13	17	12
Operating Segments | Ameren Illinois Company | Natural gas | Ameren Illinois Gas | Other
Disaggregation of Revenue [Line Items]
External revenues									60	58	54
Intersegment Elimination
Disaggregation of Revenue [Line Items]
External revenues									(98)	(93)	(98)
Intersegment Elimination | Electricity
Disaggregation of Revenue [Line Items]
External revenues									(96)	(92)	(96)
Intersegment Elimination | Electricity | Residential
Disaggregation of Revenue [Line Items]
External revenues									0	0	0
Intersegment Elimination | Electricity | Commercial
Disaggregation of Revenue [Line Items]
External revenues									0	0	0
Intersegment Elimination | Electricity | Industrial
Disaggregation of Revenue [Line Items]
External revenues									0	0	0
Intersegment Elimination | Electricity | Other
Disaggregation of Revenue [Line Items]
External revenues									(96)	(92)	(96)
Intersegment Elimination | Natural gas
Disaggregation of Revenue [Line Items]
External revenues									(2)	(1)	(2)
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)	(1)	(2)
Intersegment Elimination | Ameren Illinois Company
Disaggregation of Revenue [Line Items]
External revenues									(62)	(53)	(42)
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									$ (62)	$ (53)	$ (42)