Related Party Transactions (Effects of Related-party Transactions on the Statement of Income (Details) - USD ($) $ in Millions	12 Months Ended
Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Related Party Transaction [Line Items]
Operating Revenues	$ 7,500	$ 7,957	$ 6,394
Purchased power	1,298	1,547	606
Other operations and maintenance	1,866	1,937	1,774
Interest Charges	566	486	383
Ameren Missouri
Related Party Transaction [Line Items]
Operating Revenues	3,859	4,046	3,353
Purchased power	483	677	227
Other operations and maintenance	1,003	1,028	948
Interest Charges	227	213	137
Ameren Missouri \| Ameren Missouri Power Supply Agreements with Ameren Illinois \| Related Party
Related Party Transaction [Line Items]
Operating Revenues	2	9	16
Ameren Missouri \| Ameren Missouri and Ameren Illinois Rent and Facility Services \| Related Party
Related Party Transaction [Line Items]
Operating Revenues	32	25	26
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	1	1
Ameren Missouri \| Total Operating Revenues \| Related Party
Related Party Transaction [Line Items]
Operating Revenues	34	34	42
Ameren Missouri \| Ameren Illinois Transmission Services From ATXI \| Related Party
Related Party Transaction [Line Items]
Purchased power	2	1	4
Ameren Missouri \| Total Purchased Power \| Related Party
Related Party Transaction [Line Items]
Purchased power	2	1	4
Ameren Missouri \| Ameren Services Support Services Agreement \| Related Party
Related Party Transaction [Line Items]
Other operations and maintenance	148	150	147
Ameren Missouri \| Total Related Party Other Operations and Maintenance \| Related Party
Related Party Transaction [Line Items]
Other operations and maintenance	148	150	148
Ameren Missouri \| Money Pool Borrowings (Advances) \| Related Party
Related Party Transaction [Line Items]
Interest Charges	1	1	1
Ameren Illinois
Related Party Transaction [Line Items]
Operating Revenues	3,482	3,756	2,895
Purchased power	820	880	400
Other operations and maintenance	818	882	820
Interest Charges	204	168	164
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	3	4
Ameren Illinois \| Ameren Missouri and Ameren Illinois miscellaneous support services and services provided to ATXI \| Related Party
Related Party Transaction [Line Items]
Operating Revenues	2	2	5
Ameren Illinois \| Total Operating Revenues \| Related Party
Related Party Transaction [Line Items]
Operating Revenues	2	2	6
Ameren Illinois \| Ameren Illinois Power Supply Agreements with Ameren Missouri \| Related Party
Related Party Transaction [Line Items]
Purchased power	2	9	16
Ameren Illinois \| Ameren Illinois Transmission Services From ATXI \| Related Party
Related Party Transaction [Line Items]
Purchased power	1	1	1
Ameren Illinois \| Total Purchased Power \| Related Party
Related Party Transaction [Line Items]
Purchased power	3	9	17
Ameren Illinois \| Ameren Services Support Services Agreement \| Related Party
Related Party Transaction [Line Items]
Other operations and maintenance	138	141	137
Ameren Illinois \| Total Related Party Other Operations and Maintenance \| Related Party
Related Party Transaction [Line Items]
Other operations and maintenance	141	144	141
Ameren Illinois \| Money Pool Borrowings (Advances) \| Related Party
Related Party Transaction [Line Items]
Interest Charges	$ 1	$ 1	$ 1

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

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