Related Party Transactions (Effects of Related-party Transactions on the Statement of Income) (Details) - USD ($) $ in Millions	3 Months Ended	6 Months Ended
Jun. 30, 2025	Jun. 30, 2024	Jun. 30, 2025	Jun. 30, 2024
Related Party Transaction [Line Items]
Operating Revenues	$ 2,221	$ 1,693	$ 4,318	$ 3,509
Other operations and maintenance	460	465	945	935
Interest Charges	187	165	362	319
Union Electric Company
Related Party Transaction [Line Items]
Operating Revenues	1,340	888	2,297	1,663
Other operations and maintenance	238	247	488	501
Interest Charges	70	63	130	125
Union Electric Company \| Ameren Missouri and Ameren Illinois Rent and Facility Services \| Related Party
Related Party Transaction [Line Items]
Operating Revenues	8	8	15	15
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	1
Union Electric Company \| Total Related Party Operating Revenues \| Related Party
Related Party Transaction [Line Items]
Operating Revenues	8	9	15	16
Union Electric Company \| Ameren Missouri and Ameren Illinois Transmission Services from ATXI \| Related Party
Related Party Transaction [Line Items]
Purchased Power	2	2	5	4
Union Electric Company \| Ameren Services Support Services Agreement \| Related Party
Related Party Transaction [Line Items]
Other operations and maintenance	44	43	88	81
Union Electric Company \| Total Related Party Other Operations and Maintenance \| Related Party
Related Party Transaction [Line Items]
Other operations and maintenance	44	43	88	81
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	1	0
Ameren Illinois Company
Related Party Transaction [Line Items]
Operating Revenues	843	766	1,943	1,766
Purchased Power	183	141	358	305
Other operations and maintenance	225	224	461	434
Interest Charges	63	60	125	115
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	2	1	3	1
Ameren Illinois Company \| Ameren Missouri and Ameren Illinois Miscellaneous Support Services \| Related Party
Related Party Transaction [Line Items]
Operating Revenues	3	1	5	1
Ameren Illinois Company \| Total Related Party Operating Revenues \| Related Party
Related Party Transaction [Line Items]
Operating Revenues	4		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	1	1
Ameren Illinois Company \| Ameren Services Support Services Agreement \| Related Party
Related Party Transaction [Line Items]
Other operations and maintenance	40	40	80	77
Ameren Illinois Company \| Total Related Party Other Operations and Maintenance \| Related Party
Related Party Transaction [Line Items]
Other operations and maintenance	42	40	83	77
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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