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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, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Union Electric Company \| Ameren Missouri Power Supply Agreements with Ameren Illinois
Related Party Transaction [Line Items]
Operating Revenues	$ 3	$ 2	$ 6	$ 2
Union Electric Company \| Ameren Missouri and Ameren Illinois Rent and Facility Services
Related Party Transaction [Line Items]
Operating Revenues	6	6	13	13
Operating Expenses	1	1	1	1
Union Electric Company \| Ameren Missouri and Ameren Illinois Miscellaneous Support Services
Related Party Transaction [Line Items]
Operating Revenues	1	1	1	1
Union Electric Company \| Total Related Party Operating Revenues
Related Party Transaction [Line Items]
Operating Revenues	10	8	20	15
Union Electric Company \| Ameren Services Support Services Agreement
Related Party Transaction [Line Items]
Operating Expenses	32	32	67	64
Union Electric Company \| Total Related Party Other Operations and Maintenance
Related Party Transaction [Line Items]
Operating Expenses	32	33	67	65
Union Electric Company \| Money Pool Borrowings (Advances)
Related Party Transaction [Line Items]
Interest Charges (Income)	1	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	1
Operating Expenses	1	2	2	3
Ameren Illinois Company \| Ameren Missouri and Ameren Illinois Miscellaneous Support Services
Related Party Transaction [Line Items]
Operating Revenues	1	1	1	1
Ameren Illinois Company \| Total Related Party Operating Revenues
Related Party Transaction [Line Items]
Operating Revenues	1	1	1	2
Ameren Illinois Company \| Ameren Illinois Power Supply Agreements with Ameren Missouri
Related Party Transaction [Line Items]
Operating Expenses	3	2	6	2
Ameren Illinois Company \| Ameren Illinois Transmission Services With ATXI
Related Party Transaction [Line Items]
Operating Expenses	1	1	1	1
Ameren Illinois Company \| Purchased Power
Related Party Transaction [Line Items]
Operating Expenses	4	2	7	2
Ameren Illinois Company \| Ameren Services Support Services Agreement
Related Party Transaction [Line Items]
Operating Expenses	31	31	64	61
Ameren Illinois Company \| Total Related Party Other Operations and Maintenance
Related Party Transaction [Line Items]
Operating Expenses	32	33	66	64
Ameren Illinois Company \| Money Pool Borrowings (Advances)
Related Party Transaction [Line Items]
Interest Charges (Income)	$ 1	$ 1	$ 1	$ 1