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, 2023	Sep. 30, 2022	Sep. 30, 2023	Sep. 30, 2022
Related Party Transaction [Line Items]
Operating Revenues	$ 2,060	$ 2,306	$ 5,882	$ 5,911
Purchased Power	272	563	1,095	1,058
Other operations and maintenance	470	475	1,368	1,427
(Interest Charges)/Other Income, Net	152	126	413	356
Union Electric Company
Related Party Transaction [Line Items]
Operating Revenues	1,237	1,359	3,101	3,096
Purchased Power	75	247	420	458
Other operations and maintenance	256	252	732	744
(Interest Charges)/Other Income, Net	63	58	166	157
Union Electric Company \| Ameren Missouri Power Supply Agreements with Ameren Illinois \| Related Party
Related Party Transaction [Line Items]
Operating Revenues	2	2	2	7
Union Electric Company \| Ameren Missouri and Ameren Illinois Rent and Facility Services \| Related Party
Related Party Transaction [Line Items]
Operating Revenues	7	6	25	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	27	25
Union Electric Company \| Ameren Missouri and Ameren Illinois Transmission Services from ATXI \| Related Party
Related Party Transaction [Line Items]
Purchased Power	1	1	1	1
Union Electric Company \| Purchased Power \| Related Party
Related Party Transaction [Line Items]
Purchased Power	1	1	1	1
Union Electric Company \| Ameren Services Support Services Agreement \| Related Party
Related Party Transaction [Line Items]
Other operations and maintenance	36	38	106	109
Union Electric Company \| Total Related Party Other Operations and Maintenance \| Related Party
Related Party Transaction [Line Items]
Other operations and maintenance	36	38	106	109
Union Electric Company \| Money pool borrowings (advances) \| Related Party
Related Party Transaction [Line Items]
(Interest Charges)/Other Income, Net	1	1		1
Ameren Illinois Company
Related Party Transaction [Line Items]
Operating Revenues	783	904	2,663	2,697
Purchased Power	200	319	679	608
Other operations and maintenance	200	215	603	663
(Interest Charges)/Other Income, Net	54	42	151	125
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	2	2
Ameren Illinois Company \| Ameren Missouri and Ameren Illinois Miscellaneous Support Services \| Related Party
Related Party Transaction [Line Items]
Operating Revenues	2	1	2	1
Ameren Illinois Company \| Total Related Party Operating Revenues \| Related Party
Related Party Transaction [Line Items]
Operating Revenues	2	1	2	1
Ameren Illinois Company \| Ameren Illinois Power Supply Agreements with Ameren Missouri \| Related Party
Related Party Transaction [Line Items]
Purchased Power	2	2	2	7
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	3	2	3	7
Ameren Illinois Company \| Ameren Services Support Services Agreement \| Related Party
Related Party Transaction [Line Items]
Other operations and maintenance	33	36	101	103
Ameren Illinois Company \| Total Related Party Other Operations and Maintenance \| Related Party
Related Party Transaction [Line Items]
Other operations and maintenance	33	37	103	105
Ameren Illinois Company \| Money pool borrowings (advances) \| Related Party
Related Party Transaction [Line Items]
(Interest Charges)/Other Income, Net	$ 1	$ 1	$ 1	$ 1

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

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