Segment Information (Disaggregation of Revenues) (Details) - USD ($) $ in Millions	12 Months Ended
Dec. 31, 2025	Dec. 31, 2024	Dec. 31, 2023
Disaggregation of Revenue [Line Items]
External revenues	$ 8,799	$ 7,623	$ 7,500
Revenues	8,799	7,623	7,500
Revenues from alternative revenue programs	(200)	(9)	179
Revenue Not from Contract with Customer, Other	18	19	0
Ameren Missouri
Disaggregation of Revenue [Line Items]
Revenues from alternative revenue programs	(5)	4	(5)
Ameren Illinois Electric Distribution
Disaggregation of Revenue [Line Items]
Revenues from alternative revenue programs	(135)	(43)	116
Ameren Illinois Gas
Disaggregation of Revenue [Line Items]
Revenues from alternative revenue programs	(34)	(3)	49
Ameren Transmission
Disaggregation of Revenue [Line Items]
Revenues from alternative revenue programs	(26)	33	19
Electric
Disaggregation of Revenue [Line Items]
External revenues	7,668	6,540	6,439
Revenues	7,668	6,540	6,439
Electric \| Residential
Disaggregation of Revenue [Line Items]
External revenues	3,322	2,892	2,921
Electric \| Commercial
Disaggregation of Revenue [Line Items]
External revenues	2,235	1,993	2,027
Electric \| Industrial
Disaggregation of Revenue [Line Items]
External revenues	541	489	492
Electric \| Other
Disaggregation of Revenue [Line Items]
Revenue Not from Contract with Customer	1,570	1,166	999
Natural gas
Disaggregation of Revenue [Line Items]
External revenues	1,131	1,083	1,061
Revenues	1,131	1,083	1,061
Natural gas \| Residential
Disaggregation of Revenue [Line Items]
External revenues	781	751	757
Natural gas \| Commercial
Disaggregation of Revenue [Line Items]
External revenues	229	203	210
Natural gas \| Industrial
Disaggregation of Revenue [Line Items]
External revenues	17	14	19
Natural gas \| Other
Disaggregation of Revenue [Line Items]
Revenue Not from Contract with Customer	104	115	75
Ameren Illinois
Disaggregation of Revenue [Line Items]
External revenues	3,844	3,472	3,482
Revenues	3,844	3,472	3,482
Revenues from alternative revenue programs	(188)	(17)	177
Revenue Not from Contract with Customer, Other	16	12	9
Ameren Illinois \| Residential
Disaggregation of Revenue [Line Items]
External revenues	2,163	1,915	2,001
Ameren Illinois \| Commercial
Disaggregation of Revenue [Line Items]
External revenues	970	846	911
Ameren Illinois \| Industrial
Disaggregation of Revenue [Line Items]
External revenues	211	188	200
Ameren Illinois \| Other
Disaggregation of Revenue [Line Items]
Revenue Not from Contract with Customer	500	523	370
Ameren Illinois \| Ameren Illinois Electric Distribution
Disaggregation of Revenue [Line Items]
Revenues from alternative revenue programs	(135)	(43)	116
Revenue Not from Contract with Customer, Other	13	10	7
Ameren Illinois \| Ameren Illinois Gas
Disaggregation of Revenue [Line Items]
Revenues from alternative revenue programs	(34)	(3)	49
Revenue Not from Contract with Customer, Other	3	2	2
Ameren Illinois \| Ameren Illinois Transmission
Disaggregation of Revenue [Line Items]
Revenues from alternative revenue programs	(19)	29	12
Revenue Not from Contract with Customer, Other	0	0	0
Ameren Illinois \| Electric
Disaggregation of Revenue [Line Items]
External revenues	2,876	2,534	2,585
Ameren Illinois \| Natural gas
Disaggregation of Revenue [Line Items]
External revenues	968	938	897
Operating Segments \| Ameren Missouri
Disaggregation of Revenue [Line Items]
External revenues	4,763	3,960	3,825
Revenues	4,795	3,993	3,859
Revenue Not from Contract with Customer, Other	6	7	(9)
Operating Segments \| Ameren Illinois Electric Distribution
Disaggregation of Revenue [Line Items]
External revenues	2,393	2,088	2,217
Revenues	2,399	2,089	2,218
Revenue Not from Contract with Customer, Other	13	10	7
Operating Segments \| Ameren Illinois Gas
Disaggregation of Revenue [Line Items]
External revenues	968	938	897
Revenues	968	938	897
Revenue Not from Contract with Customer, Other	3	2	2
Operating Segments \| Ameren Transmission
Disaggregation of Revenue [Line Items]
External revenues	675	637	561
Revenues	862	781	677
Revenue Not from Contract with Customer, Other	0	0	0
Operating Segments \| Electric \| Ameren Missouri
Disaggregation of Revenue [Line Items]
Revenues	4,631	3,847	3,694
Operating Segments \| Electric \| Ameren Missouri \| Residential
Disaggregation of Revenue [Line Items]
External revenues	1,839	1,638	1,577
Operating Segments \| Electric \| Ameren Missouri \| Commercial
Disaggregation of Revenue [Line Items]
External revenues	1,450	1,313	1,280
Operating Segments \| Electric \| Ameren Missouri \| Industrial
Disaggregation of Revenue [Line Items]
External revenues	342	311	306
Operating Segments \| Electric \| Ameren Missouri \| Other
Disaggregation of Revenue [Line Items]
Revenue Not from Contract with Customer	1,000	585	531
Operating Segments \| Electric \| Ameren Illinois Electric Distribution
Disaggregation of Revenue [Line Items]
Revenues	2,399	2,089	2,218
Operating Segments \| Electric \| Ameren Illinois Electric Distribution \| Residential
Disaggregation of Revenue [Line Items]
External revenues	1,483	1,254	1,344
Operating Segments \| Electric \| Ameren Illinois Electric Distribution \| Commercial
Disaggregation of Revenue [Line Items]
External revenues	785	680	747
Operating Segments \| Electric \| Ameren Illinois Electric Distribution \| Industrial
Disaggregation of Revenue [Line Items]
External revenues	199	178	186
Operating Segments \| Electric \| Ameren Illinois Electric Distribution \| Other
Disaggregation of Revenue [Line Items]
Revenue Not from Contract with Customer	(68)	(23)	(59)
Operating Segments \| Electric \| Ameren Illinois Gas
Disaggregation of Revenue [Line Items]
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]
Revenue Not from Contract with Customer	0	0	0
Operating Segments \| Electric \| Ameren Transmission
Disaggregation of Revenue [Line Items]
Revenues	862	781	677
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]
Revenue Not from Contract with Customer	862	781	677
Operating Segments \| Natural gas \| Ameren Missouri
Disaggregation of Revenue [Line Items]
Revenues	164	146	165
Operating Segments \| Natural gas \| Ameren Missouri \| Residential
Disaggregation of Revenue [Line Items]
External revenues	101	90	100
Operating Segments \| Natural gas \| Ameren Missouri \| Commercial
Disaggregation of Revenue [Line Items]
External revenues	44	37	46
Operating Segments \| Natural gas \| Ameren Missouri \| Industrial
Disaggregation of Revenue [Line Items]
External revenues	5	4	5
Operating Segments \| Natural gas \| Ameren Missouri \| Other
Disaggregation of Revenue [Line Items]
Revenue Not from Contract with Customer	14	15	14
Operating Segments \| Natural gas \| Ameren Illinois Electric Distribution
Disaggregation of Revenue [Line Items]
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]
Revenue Not from Contract with Customer	0	0	0
Operating Segments \| Natural gas \| Ameren Illinois Gas
Disaggregation of Revenue [Line Items]
Revenues	968	938	897
Operating Segments \| Natural gas \| Ameren Illinois Gas \| Residential
Disaggregation of Revenue [Line Items]
External revenues	680	661	657
Operating Segments \| Natural gas \| Ameren Illinois Gas \| Commercial
Disaggregation of Revenue [Line Items]
External revenues	185	166	164
Operating Segments \| Natural gas \| Ameren Illinois Gas \| Industrial
Disaggregation of Revenue [Line Items]
External revenues	12	10	14
Operating Segments \| Natural gas \| Ameren Illinois Gas \| Other
Disaggregation of Revenue [Line Items]
Revenue Not from Contract with Customer	91	101	62
Operating Segments \| Natural gas \| Ameren Transmission
Disaggregation of Revenue [Line Items]
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]
Revenue Not from Contract with Customer	0	0	0
Operating Segments \| Ameren Illinois \| Ameren Illinois Electric Distribution
Disaggregation of Revenue [Line Items]
External revenues	2,399	2,089	2,218
Revenues	2,399	2,089	2,218
Operating Segments \| Ameren Illinois \| Ameren Illinois Gas
Disaggregation of Revenue [Line Items]
External revenues	968	938	897
Revenues	968	938	897
Operating Segments \| Ameren Illinois \| Ameren Illinois Transmission
Disaggregation of Revenue [Line Items]
External revenues	477	445	367
Revenues	637	564	480
Operating Segments \| Ameren Illinois \| Electric \| Ameren Illinois Electric Distribution
Disaggregation of Revenue [Line Items]
Revenues	2,399	2,089	2,218
Operating Segments \| Ameren Illinois \| Electric \| Ameren Illinois Electric Distribution \| Residential
Disaggregation of Revenue [Line Items]
External revenues	1,483	1,254	1,344
Operating Segments \| Ameren Illinois \| Electric \| Ameren Illinois Electric Distribution \| Commercial
Disaggregation of Revenue [Line Items]
External revenues	785	680	747
Operating Segments \| Ameren Illinois \| Electric \| Ameren Illinois Electric Distribution \| Industrial
Disaggregation of Revenue [Line Items]
External revenues	199	178	186
Operating Segments \| Ameren Illinois \| Electric \| Ameren Illinois Electric Distribution \| Other
Disaggregation of Revenue [Line Items]
Revenue Not from Contract with Customer	(68)	(23)	(59)
Operating Segments \| Ameren Illinois \| Electric \| Ameren Illinois Transmission
Disaggregation of Revenue [Line Items]
Revenues	637	564	480
Operating Segments \| Ameren Illinois \| Electric \| Ameren Illinois Transmission \| Residential
Disaggregation of Revenue [Line Items]
External revenues	0	0	0
Operating Segments \| Ameren Illinois \| Electric \| Ameren Illinois Transmission \| Commercial
Disaggregation of Revenue [Line Items]
External revenues	0	0	0
Operating Segments \| Ameren Illinois \| Electric \| Ameren Illinois Transmission \| Industrial
Disaggregation of Revenue [Line Items]
External revenues	0	0	0
Operating Segments \| Ameren Illinois \| Electric \| Ameren Illinois Transmission \| Other
Disaggregation of Revenue [Line Items]
Revenue Not from Contract with Customer	637	564	480
Operating Segments \| Ameren Illinois \| Natural gas \| Ameren Illinois Gas
Disaggregation of Revenue [Line Items]
Revenues	968	938	897
Operating Segments \| Ameren Illinois \| Natural gas \| Ameren Illinois Gas \| Residential
Disaggregation of Revenue [Line Items]
External revenues	680	661	657
Operating Segments \| Ameren Illinois \| Natural gas \| Ameren Illinois Gas \| Commercial
Disaggregation of Revenue [Line Items]
External revenues	185	166	164
Operating Segments \| Ameren Illinois \| Natural gas \| Ameren Illinois Gas \| Industrial
Disaggregation of Revenue [Line Items]
External revenues	12	10	14
Operating Segments \| Ameren Illinois \| Natural gas \| Ameren Illinois Gas \| Other
Disaggregation of Revenue [Line Items]
Revenue Not from Contract with Customer	91	101	62
Intersegment Elimination
Disaggregation of Revenue [Line Items]
Revenue Not from Contract with Customer	(225)	(178)	(151)
Revenues	(225)	(178)	(151)
Revenue Not from Contract with Customer, Other	(4)	0	0
Intersegment Elimination \| Ameren Missouri
Disaggregation of Revenue [Line Items]
Revenue Not from Contract with Customer	(32)	(33)	(34)
Intersegment Elimination \| Ameren Illinois Electric Distribution
Disaggregation of Revenue [Line Items]
Revenue Not from Contract with Customer	(6)	(1)	(1)
Intersegment Elimination \| Ameren Illinois Gas
Disaggregation of Revenue [Line Items]
Revenue Not from Contract with Customer	0	0	0
Intersegment Elimination \| Ameren Transmission
Disaggregation of Revenue [Line Items]
Revenue Not from Contract with Customer	(187)	(144)	(116)
Intersegment Elimination \| Electric
Disaggregation of Revenue [Line Items]
Revenues	(224)	(177)	(150)
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]
Revenue Not from Contract with Customer	(224)	(177)	(150)
Intersegment Elimination \| Natural gas
Disaggregation of Revenue [Line Items]
Revenues	(1)	(1)	(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]
Revenue Not from Contract with Customer	(1)	(1)	(1)
Intersegment Elimination \| Ameren Illinois
Disaggregation of Revenue [Line Items]
Revenue Not from Contract with Customer	(160)	(119)	(113)
Revenues	(160)	(119)	(113)
Intersegment Elimination \| Ameren Illinois \| Residential
Disaggregation of Revenue [Line Items]
External revenues	0	0	0
Intersegment Elimination \| Ameren Illinois \| Commercial
Disaggregation of Revenue [Line Items]
External revenues	0	0	0
Intersegment Elimination \| Ameren Illinois \| Industrial
Disaggregation of Revenue [Line Items]
External revenues	0	0	0
Intersegment Elimination \| Ameren Illinois \| Other
Disaggregation of Revenue [Line Items]
Revenue Not from Contract with Customer	(160)	(119)	(113)
Intersegment Elimination \| Ameren Illinois \| Ameren Illinois Electric Distribution
Disaggregation of Revenue [Line Items]
Revenue Not from Contract with Customer	0	0	0
Intersegment Elimination \| Ameren Illinois \| Ameren Illinois Gas
Disaggregation of Revenue [Line Items]
Revenue Not from Contract with Customer	0	0	0
Intersegment Elimination \| Ameren Illinois \| Ameren Illinois Transmission
Disaggregation of Revenue [Line Items]
Revenue Not from Contract with Customer	$ (160)	$ (119)	$ (113)

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