Related Party Transactions (Effects of Related-party Transactions on the Statement of Income) (Details) - USD ($) $ in Millions	3 Months Ended
Mar. 31, 2025	Mar. 31, 2024
Related Party Transaction [Line Items]
Operating Revenues	$ 2,097	$ 1,816
Other operations and maintenance	485	470
Interest Charges	175	154
Union Electric Company
Related Party Transaction [Line Items]
Operating Revenues	957	775
Other operations and maintenance	250	254
Interest Charges	60	62
Union Electric Company \| Ameren Missouri Power Supply Agreements with Ameren Illinois \| Related Party
Related Party Transaction [Line Items]
Operating Revenues		1
Union Electric Company \| Ameren Missouri and Ameren Illinois Rent and Facility Services \| Related Party
Related Party Transaction [Line Items]
Operating Revenues	7	7
Other operations and maintenance	1	1
Union Electric Company \| Ameren Missouri and Ameren Illinois Miscellaneous Support Services \| Related Party
Related Party Transaction [Line Items]
Operating Revenues	1	1
Union Electric Company \| Total Related Party Operating Revenues \| Related Party
Related Party Transaction [Line Items]
Operating Revenues	7	7
Union Electric Company \| Ameren Missouri and Ameren Illinois Transmission Services from ATXI \| Related Party
Related Party Transaction [Line Items]
Purchased Power	3	2
Union Electric Company \| Purchased Power \| Related Party
Related Party Transaction [Line Items]
Purchased Power	3	2
Union Electric Company \| Ameren Services Support Services Agreement \| Related Party
Related Party Transaction [Line Items]
Other operations and maintenance	44	38
Union Electric Company \| Total Related Party Other Operations and Maintenance \| Related Party
Related Party Transaction [Line Items]
Other operations and maintenance	44	38
Union Electric Company \| Money pool interest \| Related Party
Related Party Transaction [Line Items]
Interest Charges	1	3
Union Electric Company \| Long-Term Debt \| Related Party
Related Party Transaction [Line Items]
Interest Charges	1	0
Ameren Illinois Company
Related Party Transaction [Line Items]
Operating Revenues	1,100	1,000
Purchased Power	175	164
Other operations and maintenance	236	210
Interest Charges	62	55
Ameren Illinois Company \| Ameren Missouri and Ameren Illinois Rent and Facility Services \| Related Party
Related Party Transaction [Line Items]
Operating Revenues	1	1
Other operations and maintenance	1	1
Ameren Illinois Company \| Ameren Missouri and Ameren Illinois Miscellaneous Support Services \| Related Party
Related Party Transaction [Line Items]
Operating Revenues	2	1
Ameren Illinois Company \| Total Related Party Operating Revenues \| Related Party
Related Party Transaction [Line Items]
Operating Revenues	2	1
Ameren Illinois Company \| Ameren Illinois Power Supply Agreements with Ameren Missouri \| Related Party
Related Party Transaction [Line Items]
Purchased Power		1
Ameren Illinois Company \| Ameren Missouri and Ameren Illinois Transmission Services from ATXI \| Related Party
Related Party Transaction [Line Items]
Purchased Power	1	1
Ameren Illinois Company \| Purchased Power \| Related Party
Related Party Transaction [Line Items]
Purchased Power	1	1
Ameren Illinois Company \| Ameren Services Support Services Agreement \| Related Party
Related Party Transaction [Line Items]
Other operations and maintenance	40	37
Ameren Illinois Company \| Total Related Party Other Operations and Maintenance \| Related Party
Related Party Transaction [Line Items]
Other operations and maintenance	41	37
Ameren Illinois Company \| Money pool interest \| Related Party
Related Party Transaction [Line Items]
Interest Charges	1	2
Ameren Illinois Company \| Long-Term Debt \| Related Party
Related Party Transaction [Line Items]
Interest Charges	$ 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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