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, 2024	Jun. 30, 2023	Jun. 30, 2024	Jun. 30, 2023
Related Party Transaction [Line Items]
Operating Revenues	$ 1,693	$ 1,760	$ 3,509	$ 3,822
Other operations and maintenance	465	450	935	898
(Interest Charges)/Other Income, Net	(165)	(134)	(319)	(261)
Union Electric Company
Related Party Transaction [Line Items]
Operating Revenues	888	941	1,663	1,864
Other operations and maintenance	247	237	501	476
(Interest Charges)/Other Income, Net	(63)	(52)	(125)	(103)
Union Electric Company \| Ameren Missouri Power Supply Agreements with Ameren Illinois \| Related Party
Related Party Transaction [Line Items]
Operating Revenues	1	1	1	1
Union Electric Company \| Ameren Missouri and Ameren Illinois Rent and Facility Services \| Related Party
Related Party Transaction [Line Items]
Operating Revenues	8	8	15	18
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	9	8	16	18
Union Electric Company \| Ameren Missouri and Ameren Illinois Transmission Services from ATXI \| Related Party
Related Party Transaction [Line Items]
Purchased Power	2	1	4	1
Union Electric Company \| Purchased Power \| Related Party
Related Party Transaction [Line Items]
Purchased Power	2	1	4	1
Union Electric Company \| Ameren Services Support Services Agreement \| Related Party
Related Party Transaction [Line Items]
Other operations and maintenance	43	35	81	70
Union Electric Company \| Total Related Party Other Operations and Maintenance \| Related Party
Related Party Transaction [Line Items]
Other operations and maintenance	43	35	81	70
Union Electric Company \| Money pool interest \| Related Party
Related Party Transaction [Line Items]
(Interest Charges)/Other Income, Net	(1)	(1)	(3)	(1)
Ameren Illinois Company
Related Party Transaction [Line Items]
Operating Revenues	766	779	1,766	1,880
Purchased Power	141	192	305	479
Other operations and maintenance	224	201	434	403
(Interest Charges)/Other Income, Net	(60)	(50)	(115)	(97)
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	1	1	2
Ameren Illinois Company \| Ameren Missouri and Ameren Illinois Miscellaneous Support Services \| Related Party
Related Party Transaction [Line Items]
Operating Revenues	1	1	1	1
Ameren Illinois Company \| Total Related Party Operating Revenues \| Related Party
Related Party Transaction [Line Items]
Operating Revenues	1	1	1	1
Ameren Illinois Company \| Ameren Illinois Power Supply Agreements with Ameren Missouri \| Related Party
Related Party Transaction [Line Items]
Purchased Power	1	1	1	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 \| Purchased Power \| 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	33	77	68
Ameren Illinois Company \| Total Related Party Other Operations and Maintenance \| Related Party
Related Party Transaction [Line Items]
Other operations and maintenance	40	33	77	70
Ameren Illinois Company \| Money pool interest \| Related Party
Related Party Transaction [Line Items]
(Interest Charges)/Other Income, Net	$ (1)	$ (1)	$ (2)	$ (1)

Amount of the cost of borrowed funds accounted for as interest expense.

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