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Related Party Transactions (Schedule of Related Party Transactions) (Detail) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Related Party Transaction [Line Items]
Operating Revenues	$ 0	$ 0	$ 0	$ 0
Ameren Illinois Company
Related Party Transaction [Line Items]
Operating Revenues	0	0	0	0
Ameren Missouri Power Supply Agreements with Ameren Illinois \| Union Electric Company
Related Party Transaction [Line Items]
Operating Revenues	2	3	2	6
Ameren Missouri and Ameren Illinois Rent and Facility Services \| Union Electric Company
Related Party Transaction [Line Items]
Operating Revenues	6	6	13	11
Operating Expenses	1	1	1	1
Ameren Missouri and Ameren Illinois Rent and Facility Services \| Ameren Illinois Company
Related Party Transaction [Line Items]
Operating Revenues	1	1	1	2
Operating Expenses	2	1	3	3
Ameren Missouri and Ameren Illinois Miscellaneous Support Services \| Union Electric Company
Related Party Transaction [Line Items]
Operating Revenues	1	1	1	1
Ameren Missouri and Ameren Illinois Miscellaneous Support Services \| Ameren Illinois Company
Related Party Transaction [Line Items]
Operating Revenues	1	1	1	1
Total Related Party Operating Revenues \| Union Electric Company
Related Party Transaction [Line Items]
Operating Revenues	8	9	15	17
Total Related Party Operating Revenues \| Ameren Illinois Company
Related Party Transaction [Line Items]
Operating Revenues	1	1	2	2
Ameren Illinois Power Supply Agreements with Ameren Missouri \| Ameren Illinois Company
Related Party Transaction [Line Items]
Operating Expenses	2	3	2	6
Ameren Illinois Transmission Services With ATXI \| Ameren Illinois Company
Related Party Transaction [Line Items]
Operating Expenses	1	1	1	1
Purchased Power \| Ameren Illinois Company
Related Party Transaction [Line Items]
Operating Expenses	2	4	2	7
Ameren Services Support Services Agreement \| Union Electric Company
Related Party Transaction [Line Items]
Operating Expenses	32	32	64	65
Ameren Services Support Services Agreement \| Ameren Illinois Company
Related Party Transaction [Line Items]
Operating Expenses	31	30	61	60
Total Related Party Other Operations and Maintenance \| Union Electric Company
Related Party Transaction [Line Items]
Operating Expenses	33	32	65	66
Total Related Party Other Operations and Maintenance \| Ameren Illinois Company
Related Party Transaction [Line Items]
Operating Expenses	33	31	64	63
Money Pool \| Union Electric Company
Related Party Transaction [Line Items]
Interest Charges	1	1	1	1
Money Pool \| Ameren Illinois Company
Related Party Transaction [Line Items]
Interest Charges	$ 1	$ 1	$ 1	$ 1