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Related Party Transactions (Effects of Related-party Transactions on the Statement of Income (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2025	Dec. 31, 2024	Dec. 31, 2023
Related Party Transaction [Line Items]
Operating Revenues	$ 8,799	$ 7,623	$ 7,500
Other operations and maintenance	1,974	1,969	1,866
Interest Charges	776	663	566
Ameren Missouri
Related Party Transaction [Line Items]
Operating Revenues	4,795	3,993	3,859
Other operations and maintenance	1,029	1,050	1,003
Interest Charges	297	244	227
Ameren Missouri | Ameren Missouri Power Supply Agreements with Ameren Illinois | Related Party
Related Party Transaction [Line Items]
Operating Revenues	0	0	2
Ameren Missouri | Ameren Missouri and Ameren Illinois Rent and Facility Services | Related Party
Related Party Transaction [Line Items]
Operating Revenues	31	31	32
Other operations and maintenance	1	1	1
Ameren Missouri | Ameren Missouri and Ameren Illinois miscellaneous support services and services provided to ATXI | Related Party
Related Party Transaction [Line Items]
Operating Revenues	1	2	1
Ameren Missouri | Total Operating Revenues | Related Party
Related Party Transaction [Line Items]
Operating Revenues	32	33	34
Ameren Missouri | Ameren Missouri Interconnection Agreement With Ameren Illinois | Related Party
Related Party Transaction [Line Items]
Purchased power	1	1	0
Ameren Missouri | Ameren Illinois Transmission Services From ATXI | Related Party
Related Party Transaction [Line Items]
Purchased power	10	9	2
Ameren Missouri | Total Purchased Power | Related Party
Related Party Transaction [Line Items]
Purchased power	11	9	2
Ameren Missouri | Ameren Services Support Services Agreement | Related Party
Related Party Transaction [Line Items]
Other operations and maintenance	177	169	148
Ameren Missouri | Total Related Party Other Operations and Maintenance | Related Party
Related Party Transaction [Line Items]
Other operations and maintenance	177	170	148
Ameren Missouri | Money Pool Borrowings (Advances) | Related Party
Related Party Transaction [Line Items]
Interest Charges	(1)	(4)	1
Ameren Missouri | Long-Term Debt | Related Party
Related Party Transaction [Line Items]
Interest Charges	(3)	1
Ameren Illinois
Related Party Transaction [Line Items]
Operating Revenues	3,844	3,472	3,482
Purchased power	781	621	820
Other operations and maintenance	945	906	818
Interest Charges	260	241	204
Ameren Illinois | Ameren Missouri and Ameren Illinois Rent and Facility Services | Related Party
Related Party Transaction [Line Items]
Operating Revenues	1	1	1
Other operations and maintenance	3	1	3
Ameren Illinois | Ameren Missouri and Ameren Illinois miscellaneous support services and services provided to ATXI | Related Party
Related Party Transaction [Line Items]
Operating Revenues	5	2	2
Ameren Illinois | Total Operating Revenues | Related Party
Related Party Transaction [Line Items]
Operating Revenues	7	3	2
Ameren Illinois | Ameren Illinois Power Supply Agreements with Ameren Missouri | Related Party
Related Party Transaction [Line Items]
Purchased power	0	0	2
Ameren Illinois | Ameren Illinois Interconnection Agreement With Ameren Missouri | Related Party
Related Party Transaction [Line Items]
Operating Revenues	1	1	0
Ameren Illinois | Ameren Illinois Transmission Services From ATXI | Related Party
Related Party Transaction [Line Items]
Purchased power	2	2	1
Ameren Illinois | Total Purchased Power | Related Party
Related Party Transaction [Line Items]
Purchased power	2	2	3
Ameren Illinois | Ameren Services Support Services Agreement | Related Party
Related Party Transaction [Line Items]
Other operations and maintenance	164	158	138
Ameren Illinois | Total Related Party Other Operations and Maintenance | Related Party
Related Party Transaction [Line Items]
Other operations and maintenance	167	159	141
Ameren Illinois | Money Pool Borrowings (Advances) | Related Party
Related Party Transaction [Line Items]
Interest Charges	1	1	$ 1
Ameren Illinois | Long-Term Debt | Related Party
Related Party Transaction [Line Items]
Interest Charges	$ 1	$ 1

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