Segment Information (Disaggregation of Revenues) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	$ 6,291	$ 6,174	$ 6,076
Revenues from alternative revenue programs	(59)	(15)	(52)
Other revenues not from contracts with customers	42	23	24
TCJA - Excess Amounts Collected in Rates
Disaggregation of Revenue [Line Items]
TCJA Revenue Reduction	60
Intersegment Elimination
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	(93)	(98)	(106)
Electricity
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	5,339	5,307	5,196
Electricity | Residential
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	2,427	2,287	2,317
Electricity | Commercial
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	1,782	1,735	1,741
Electricity | Industrial
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	442	418	411
Electricity | Other
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	688	867	727
Electricity | Intersegment Elimination
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	(92)	(96)	(104)
Electricity | Intersegment Elimination | Residential
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	0	0	0
Electricity | Intersegment Elimination | Commercial
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	0	0	0
Electricity | Intersegment Elimination | Industrial
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	0	0	0
Electricity | Intersegment Elimination | Other
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	(92)	(96)	(104)
Natural gas
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	952	867	880
Natural gas | Residential
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	671	608	607
Natural gas | Commercial
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	196	177	183
Natural gas | Industrial
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	21	16	14
Natural gas | Other
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	64	66	76
Natural gas | Intersegment Elimination
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	(1)	(2)	(2)
Natural gas | Intersegment Elimination | Residential
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	0	0	0
Natural gas | Intersegment Elimination | Commercial
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	0	0	0
Natural gas | Intersegment Elimination | Industrial
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	0	0	0
Natural gas | Intersegment Elimination | Other
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	(1)	(2)	(2)
Ameren Illinois Company
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	2,576	2,527	2,489
Ameren Illinois Company | Intersegment Elimination
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	(53)	(42)	(45)
Ameren Illinois Company | Intersegment Elimination | Residential
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	0	0	0
Ameren Illinois Company | Intersegment Elimination | Commercial
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	0	0	0
Ameren Illinois Company | Intersegment Elimination | Industrial
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	0	0	0
Ameren Illinois Company | Intersegment Elimination | Other
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	(53)	(42)	(45)
Ameren Illinois Company | Ameren Illinois Company
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	2,576	2,527	2,489
Revenues from alternative revenue programs	(51)	9	(57)
Other revenues not from contracts with customers	18	8	8
Ameren Illinois Company | Ameren Illinois Company | Residential
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	1,448	1,401	1,425
Ameren Illinois Company | Ameren Illinois Company | Commercial
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	670	673	670
Ameren Illinois Company | Ameren Illinois Company | Industrial
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	147	125	106
Ameren Illinois Company | Ameren Illinois Company | Other
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	311	328	288
Ameren Illinois Company | Electricity
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	1,761	1,784	1,735
Ameren Illinois Company | Natural gas
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	815	743	754
Operating Segments | Union Electric Company
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	3,589	3,537	3,524
Revenues from alternative revenue programs	(8)	(28)	8
Other revenues not from contracts with customers	24	15	16
Operating Segments | Ameren Illinois Electric Distribution [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	1,547	1,568	1,548
Revenues from alternative revenue programs	(3)	(5)	(70)
Other revenues not from contracts with customers	16	6	6
Operating Segments | Ameren Illinois Gas [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	815	743	754
Revenues from alternative revenue programs	(23)	5	11
Other revenues not from contracts with customers	2	2	2
Operating Segments | Ameren Transmission [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	433	426	355
Revenues from alternative revenue programs	(25)	13	(1)
Other revenues not from contracts with customers	0	0	0
Operating Segments | Other Segment
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	0	(2)	1
Operating Segments | Electricity | Union Electric Company
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	3,451	3,411	3,396
Operating Segments | Electricity | Union Electric Company | Residential
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	1,560	1,417	1,422
Operating Segments | Electricity | Union Electric Company | Commercial
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	1,271	1,208	1,224
Operating Segments | Electricity | Union Electric Company | Industrial
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	312	305	315
Operating Segments | Electricity | Union Electric Company | Other
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	308	481	435
Operating Segments | Electricity | Ameren Illinois Electric Distribution [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	1,547	1,568	1,548
Operating Segments | Electricity | Ameren Illinois Electric Distribution [Member] | Residential
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	867	870	895
Operating Segments | Electricity | Ameren Illinois Electric Distribution [Member] | Commercial
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	511	527	517
Operating Segments | Electricity | Ameren Illinois Electric Distribution [Member] | Industrial
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	130	113	96
Operating Segments | Electricity | Ameren Illinois Electric Distribution [Member] | Other
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	39	58	40
Operating Segments | Electricity | Ameren Illinois Gas [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	0	0	0
Operating Segments | Electricity | Ameren Illinois Gas [Member] | Residential
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	0	0	0
Operating Segments | Electricity | Ameren Illinois Gas [Member] | Commercial
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	0	0	0
Operating Segments | Electricity | Ameren Illinois Gas [Member] | Industrial
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	0	0	0
Operating Segments | Electricity | Ameren Illinois Gas [Member] | Other
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	0	0	0
Operating Segments | Electricity | Ameren Transmission [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	433	426	355
Operating Segments | Electricity | Ameren Transmission [Member] | Residential
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	0	0	0
Operating Segments | Electricity | Ameren Transmission [Member] | Commercial
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	0	0	0
Operating Segments | Electricity | Ameren Transmission [Member] | Industrial
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	0	0	0
Operating Segments | Electricity | Ameren Transmission [Member] | Other
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	433	426	355
Operating Segments | Electricity | Other Segment
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	0	(2)	1
Operating Segments | Electricity | Other Segment | Residential
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	0	0	0
Operating Segments | Electricity | Other Segment | Commercial
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	0	0	0
Operating Segments | Electricity | Other Segment | Industrial
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	0	0	0
Operating Segments | Electricity | Other Segment | Other
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	0	(2)	1
Operating Segments | Natural gas | Union Electric Company
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	138	126	128
Operating Segments | Natural gas | Union Electric Company | Residential
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	90	77	77
Operating Segments | Natural gas | Union Electric Company | Commercial
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	37	31	30
Operating Segments | Natural gas | Union Electric Company | Industrial
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	4	4	4
Operating Segments | Natural gas | Union Electric Company | Other
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	7	14	17
Operating Segments | Natural gas | Ameren Illinois Electric Distribution [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	0	0	0
Operating Segments | Natural gas | Ameren Illinois Electric Distribution [Member] | Residential
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	0	0	0
Operating Segments | Natural gas | Ameren Illinois Electric Distribution [Member] | Commercial
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	0	0	0
Operating Segments | Natural gas | Ameren Illinois Electric Distribution [Member] | Industrial
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	0	0	0
Operating Segments | Natural gas | Ameren Illinois Electric Distribution [Member] | Other
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	0	0	0
Operating Segments | Natural gas | Ameren Illinois Gas [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	815	743	754
Operating Segments | Natural gas | Ameren Illinois Gas [Member] | Residential
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	581	531	530
Operating Segments | Natural gas | Ameren Illinois Gas [Member] | Commercial
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	159	146	153
Operating Segments | Natural gas | Ameren Illinois Gas [Member] | Industrial
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	17	12	10
Operating Segments | Natural gas | Ameren Illinois Gas [Member] | Other
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	58	54	61
Operating Segments | Natural gas | Ameren Transmission [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	0	0	0
Operating Segments | Natural gas | Ameren Transmission [Member] | Residential
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	0	0	0
Operating Segments | Natural gas | Ameren Transmission [Member] | Commercial
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	0	0	0
Operating Segments | Natural gas | Ameren Transmission [Member] | Industrial
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	0	0	0
Operating Segments | Natural gas | Ameren Transmission [Member] | Other
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	0	0	0
Operating Segments | Natural gas | Other Segment
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	0	0	0
Operating Segments | Natural gas | Other Segment | Residential
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	0	0	0
Operating Segments | Natural gas | Other Segment | Commercial
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	0	0	0
Operating Segments | Natural gas | Other Segment | Industrial
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	0	0	0
Operating Segments | Natural gas | Other Segment | Other
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	0	0	0
Operating Segments | Ameren Illinois Company | Ameren Illinois Electric Distribution [Member]
Disaggregation of Revenue [Line Items]
Revenues from alternative revenue programs	(3)	(5)	(70)
Other revenues not from contracts with customers	16	6	6
Operating Segments | Ameren Illinois Company | Ameren Illinois Gas [Member]
Disaggregation of Revenue [Line Items]
Revenues from alternative revenue programs	(23)	5	11
Other revenues not from contracts with customers	2	2	2
Operating Segments | Ameren Illinois Company | Ameren Illinois Transmission [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	267	258	232
Revenues from alternative revenue programs	(25)	9	2
Other revenues not from contracts with customers	0	0	0
Operating Segments | Ameren Illinois Company | Ameren Illinois Transmission [Member] | Residential
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	0	0	0
Operating Segments | Ameren Illinois Company | Ameren Illinois Transmission [Member] | Commercial
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	0	0	0
Operating Segments | Ameren Illinois Company | Ameren Illinois Transmission [Member] | Industrial
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	0	0	0
Operating Segments | Ameren Illinois Company | Ameren Illinois Transmission [Member] | Other
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	267	258	232
Operating Segments | Ameren Illinois Company | Electricity | Ameren Illinois Electric Distribution [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	1,547	1,568	1,548
Operating Segments | Ameren Illinois Company | Electricity | Ameren Illinois Electric Distribution [Member] | Residential
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	867	870	895
Operating Segments | Ameren Illinois Company | Electricity | Ameren Illinois Electric Distribution [Member] | Commercial
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	511	527	517
Operating Segments | Ameren Illinois Company | Electricity | Ameren Illinois Electric Distribution [Member] | Industrial
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	130	113	96
Operating Segments | Ameren Illinois Company | Electricity | Ameren Illinois Electric Distribution [Member] | Other
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	39	58	40
Operating Segments | Ameren Illinois Company | Natural gas | Ameren Illinois Gas [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	815	743	754
Operating Segments | Ameren Illinois Company | Natural gas | Ameren Illinois Gas [Member] | Residential
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	581	531	530
Operating Segments | Ameren Illinois Company | Natural gas | Ameren Illinois Gas [Member] | Commercial
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	159	146	153
Operating Segments | Ameren Illinois Company | Natural gas | Ameren Illinois Gas [Member] | Industrial
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	17	12	10
Operating Segments | Ameren Illinois Company | Natural gas | Ameren Illinois Gas [Member] | Other
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	$ 58	$ 54	$ 61