Related Party Transactions (Effects of Related-party Transactions on the Statement of Income) (Details) - USD ($) $ in Millions	3 Months Ended	9 Months Ended
Sep. 30, 2025	Sep. 30, 2024	Sep. 30, 2025	Sep. 30, 2024
Related Party Transaction [Line Items]
Operating Revenues	$ 2,699	$ 2,173	$ 7,017	$ 5,682
Other operations and maintenance	502	520	1,447	1,455
Interest Charges	208	173	570	492
Union Electric Company
Related Party Transaction [Line Items]
Operating Revenues	1,705	1,342	4,002	3,005
Other operations and maintenance	260	288	748	789
Interest Charges	85	62	215	187
Union Electric Company \| Ameren Missouri and Ameren Illinois Rent and Facility Services \| Related Party
Related Party Transaction [Line Items]
Operating Revenues	8	8	23	23
Other operations and maintenance	1	1	1	1
Union Electric Company \| Ameren Missouri and Ameren Illinois Miscellaneous Support Services \| Related Party
Related Party Transaction [Line Items]
Operating Revenues	1	1	1	2
Union Electric Company \| Total Related Party Operating Revenues \| Related Party
Related Party Transaction [Line Items]
Operating Revenues	9	9	24	25
Union Electric Company \| Ameren Missouri and Ameren Illinois Transmission Services from ATXI \| Related Party
Related Party Transaction [Line Items]
Purchased Power	2	3	7	7
Union Electric Company \| Ameren Services Support Services Agreement \| Related Party
Related Party Transaction [Line Items]
Other operations and maintenance	42	41	130	122
Union Electric Company \| Total Related Party Other Operations and Maintenance \| Related Party
Related Party Transaction [Line Items]
Other operations and maintenance	42	41	130	122
Union Electric Company \| Money pool interest \| Related Party
Related Party Transaction [Line Items]
Interest Charges	1	1	1	3
Union Electric Company \| Long-Term Debt \| Related Party
Related Party Transaction [Line Items]
Interest Charges	1	0	2	0
Ameren Illinois Company
Related Party Transaction [Line Items]
Operating Revenues	946	793	2,889	2,559
Purchased Power	284	169	642	474
Other operations and maintenance	241	229	702	663
Interest Charges	67	63	192	178
Ameren Illinois Company \| Related Party
Related Party Transaction [Line Items]
Operating Revenues		1
Ameren Illinois Company \| Ameren Missouri and Ameren Illinois Rent and Facility Services \| Related Party
Related Party Transaction [Line Items]
Operating Revenues	1	1	1	1
Other operations and maintenance		1	3	1
Ameren Illinois Company \| Ameren Missouri and Ameren Illinois Miscellaneous Support Services \| Related Party
Related Party Transaction [Line Items]
Operating Revenues	1	1	5	1
Ameren Illinois Company \| Total Related Party Operating Revenues \| Related Party
Related Party Transaction [Line Items]
Operating Revenues	1		6	1
Ameren Illinois Company \| Ameren Missouri and Ameren Illinois Transmission Services from ATXI \| Related Party
Related Party Transaction [Line Items]
Purchased Power	1	1	2	2
Ameren Illinois Company \| Ameren Services Support Services Agreement \| Related Party
Related Party Transaction [Line Items]
Other operations and maintenance	39	38	119	115
Ameren Illinois Company \| Total Related Party Other Operations and Maintenance \| Related Party
Related Party Transaction [Line Items]
Other operations and maintenance	39	38	122	115
Ameren Illinois Company \| Money pool interest \| Related Party
Related Party Transaction [Line Items]
Interest Charges	1	1	1	2
Ameren Illinois Company \| Long-Term Debt \| Related Party
Related Party Transaction [Line Items]
Interest Charges	$ 1	$ 0	$ 1	$ 0

Amount of interest expense classified as operating and nonoperating. Includes, but is not limited to, cost of borrowing accounted for as interest expense.

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