Related Party Transactions (Effects of Related-party Transactions on the Statement of Income (Details) - USD ($) $ in Millions	12 Months Ended
Dec. 31, 2022	Dec. 31, 2021	Dec. 31, 2020
Union Electric Company \| Ameren Missouri Power Supply Agreements with Ameren Illinois
Related Party Transaction [Line Items]
Operating Revenues	$ 9	$ 16	$ 11
Union Electric Company \| Ameren Missouri and Ameren Illinois Rent and Facility Services
Related Party Transaction [Line Items]
Operating Revenues	25	26	26
Operating Expenses	1	1	1
Union Electric Company \| Ameren Missouri and Ameren Illinois miscellaneous support services and services provided to ATXI
Related Party Transaction [Line Items]
Operating Revenues	1	1	3
Union Electric Company \| Total Operating Revenues
Related Party Transaction [Line Items]
Operating Revenues	34	42	40
Union Electric Company \| Ameren Illinois Transmission Services From ATXI
Related Party Transaction [Line Items]
Operating Expenses	1	4
Union Electric Company \| Total Purchased Power
Related Party Transaction [Line Items]
Operating Expenses	1	4
Union Electric Company \| Ameren Services Support Services Agreement
Related Party Transaction [Line Items]
Operating Expenses	150	147	140
Union Electric Company \| Total Related Party Other Operations and Maintenance
Related Party Transaction [Line Items]
Operating Expenses	150	148	140
Union Electric Company \| Money Pool Borrowings (Advances)
Related Party Transaction [Line Items]
Interest (Charges) Income	1	1	1
Ameren Illinois Company \| Ameren Missouri and Ameren Illinois Rent and Facility Services
Related Party Transaction [Line Items]
Operating Revenues	1	1	1
Operating Expenses	3	4	4
Ameren Illinois Company \| Ameren Missouri and Ameren Illinois miscellaneous support services and services provided to ATXI
Related Party Transaction [Line Items]
Operating Revenues	2	5	1
Ameren Illinois Company \| Total Operating Revenues
Related Party Transaction [Line Items]
Operating Revenues	2	6	2
Ameren Illinois Company \| Ameren Illinois Power Supply Agreements with Ameren Missouri
Related Party Transaction [Line Items]
Operating Expenses	9	16	11
Ameren Illinois Company \| Ameren Illinois Transmission Services From ATXI
Related Party Transaction [Line Items]
Operating Expenses	1	1	2
Ameren Illinois Company \| Total Purchased Power
Related Party Transaction [Line Items]
Operating Expenses	9	17	13
Ameren Illinois Company \| Ameren Services Support Services Agreement
Related Party Transaction [Line Items]
Operating Expenses	141	137	133
Ameren Illinois Company \| Total Related Party Other Operations and Maintenance
Related Party Transaction [Line Items]
Operating Expenses	144	141	137
Ameren Illinois Company \| Money Pool Borrowings (Advances)
Related Party Transaction [Line Items]
Interest (Charges) Income	$ 1	$ 1	$ 1

Amount of interest expense incurred on a debt or other obligation to related party.

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+ References

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Amount of revenue, fees and commissions earned from transactions between (a) a parent company and its subsidiaries; (b) subsidiaries of a common parent; (c) an entity and trusts for the benefit of employees, for example, but not limited to, pension and profit-sharing trusts that are managed by or under the trusteeship of the entity's management; (d) an entity and its principal, owners, management, or members of their immediate families; and (e) affiliates.

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