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, 2023	Jun. 30, 2022	Jun. 30, 2023	Jun. 30, 2022
Related Party Transaction [Line Items]
Operating Revenues	$ 1,760	$ 1,726	$ 3,822	$ 3,605
Purchased Power	328	318	823	495
Other operations and maintenance	450	491	898	952
(Interest Charges)/Other Income, Net	134	126	261	230
Union Electric Company
Related Party Transaction [Line Items]
Operating Revenues	941	919	1,864	1,737
Purchased Power	137	161	345	211
Other operations and maintenance	237	260	476	492
(Interest Charges)/Other Income, Net	52	60	103	99
Union Electric Company \| Ameren Missouri Power Supply Agreements with Ameren Illinois \| Related Party
Related Party Transaction [Line Items]
Operating Revenues	1	1	1	5
Union Electric Company \| Ameren Missouri and Ameren Illinois Rent and Facility Services \| Related Party
Related Party Transaction [Line Items]
Operating Revenues	8	6	18	12
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	8	7	18	17
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	35	33	70	71
Union Electric Company \| Total Related Party Other Operations and Maintenance \| Related Party
Related Party Transaction [Line Items]
Other operations and maintenance	35	33	70	71
Union Electric Company \| Money pool borrowings (advances) \| Related Party
Related Party Transaction [Line Items]
(Interest Charges)/Other Income, Net	1	1	1	1
Ameren Illinois Company
Related Party Transaction [Line Items]
Operating Revenues	779	769	1,880	1,793
Purchased Power	192	158	479	289
Other operations and maintenance	201	225	403	448
(Interest Charges)/Other Income, Net	50	41	97	83
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	1
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	5
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	5
Ameren Illinois Company \| Ameren Services Support Services Agreement \| Related Party
Related Party Transaction [Line Items]
Other operations and maintenance	33	32	68	67
Ameren Illinois Company \| Total Related Party Other Operations and Maintenance \| Related Party
Related Party Transaction [Line Items]
Other operations and maintenance	33	32	70	68
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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