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, 2024	Sep. 30, 2023	Sep. 30, 2024	Sep. 30, 2023
Related Party Transaction [Line Items]
Operating Revenues	$ 2,173	$ 2,060	$ 5,682	$ 5,882
Other operations and maintenance	520	470	1,455	1,368
(Interest Charges)/Other Income, Net	(173)	(152)	(492)	(413)
Union Electric Company
Related Party Transaction [Line Items]
Operating Revenues	1,342	1,237	3,005	3,101
Other operations and maintenance	288	256	789	732
(Interest Charges)/Other Income, Net	(62)	(63)	(187)	(166)
Union Electric Company \| Ameren Missouri Power Supply Agreements with Ameren Illinois \| Related Party
Related Party Transaction [Line Items]
Operating Revenues	1		1
Union Electric Company \| Ameren Missouri Power Supply Agreements with Ameren Illinois \| Related Party Transaction [Domain]
Related Party Transaction [Line Items]
Operating Revenues		2		2
Union Electric Company \| Ameren Missouri and Ameren Illinois Rent and Facility Services \| Related Party
Related Party Transaction [Line Items]
Operating Revenues	8	7	23	25
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	2	1
Union Electric Company \| Total Related Party Operating Revenues \| Related Party
Related Party Transaction [Line Items]
Operating Revenues	9	9	25	27
Union Electric Company \| Ameren Missouri and Ameren Illinois Transmission Services from ATXI \| Related Party
Related Party Transaction [Line Items]
Purchased Power	3	1	7	1
Union Electric Company \| Purchased Power \| Related Party
Related Party Transaction [Line Items]
Purchased Power	3	1	7	1
Union Electric Company \| Ameren Services Support Services Agreement \| Related Party
Related Party Transaction [Line Items]
Other operations and maintenance	41	36	122	106
Union Electric Company \| Total Related Party Other Operations and Maintenance \| Related Party
Related Party Transaction [Line Items]
Other operations and maintenance	41	36	122	106
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	793	783	2,559	2,663
Purchased Power	169	200	474	679
Other operations and maintenance	229	200	663	603
(Interest Charges)/Other Income, Net	(63)	(54)	(178)	(151)
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	2	1	2
Ameren Illinois Company \| Total Related Party Operating Revenues \| Related Party
Related Party Transaction [Line Items]
Operating Revenues	1	2	1	2
Ameren Illinois Company \| Ameren Illinois Power Supply Agreements with Ameren Missouri \| Related Party
Related Party Transaction [Line Items]
Purchased Power		2	1	2
Ameren Illinois Company \| Ameren Missouri and Ameren Illinois Transmission Services from ATXI \| Related Party
Related Party Transaction [Line Items]
Purchased Power	1	1	2	1
Ameren Illinois Company \| Purchased Power \| Related Party
Related Party Transaction [Line Items]
Purchased Power	1	3	2	3
Ameren Illinois Company \| Ameren Services Support Services Agreement \| Related Party
Related Party Transaction [Line Items]
Other operations and maintenance	38	33	115	101
Ameren Illinois Company \| Total Related Party Other Operations and Maintenance \| Related Party
Related Party Transaction [Line Items]
Other operations and maintenance	38	33	115	103
Ameren Illinois Company \| Money pool interest \| Related Party
Related Party Transaction [Line Items]
(Interest Charges)/Other Income, Net	$ (1)	$ (1)	$ (2)	$ (1)

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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