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Related Party Transactions (Schedule of Related Party Transactions) (Detail) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
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	3	6	6	17
Ameren Missouri and Ameren Illinois Rent and Facility Services \| Union Electric Company
Related Party Transaction [Line Items]
Operating Revenues	6	6	11	13
Ameren Missouri and Ameren Illinois Rent and Facility Services \| Ameren Illinois Company
Related Party Transaction [Line Items]
Operating Revenues	1	1	2	2
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 [Member] \| Union Electric Company
Related Party Transaction [Line Items]
Operating Revenues	9	12	17	30
Total Related Party Operating Revenues [Member] \| Ameren Illinois Company
Related Party Transaction [Line Items]
Operating Revenues	1	2	2	3
Ameren Illinois Power Supply Agreements with Ameren Missouri \| Ameren Illinois Company
Related Party Transaction [Line Items]
Operating Expenses	3	6	6	17
Ameren Illinois Transmission Agreements 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	4	7	7	18
Ameren Services Support Services Agreement \| Union Electric Company
Related Party Transaction [Line Items]
Operating Expenses	32	34	65	69
Ameren Services Support Services Agreement \| Ameren Illinois Company
Related Party Transaction [Line Items]
Operating Expenses	30	34	60	66
Related Party Money Pool Interest \| Union Electric Company
Related Party Transaction [Line Items]
Interest Charges	1	1	1	1
Related Party Money Pool Interest \| Ameren Illinois Company
Related Party Transaction [Line Items]
Interest Charges	$ 1	$ 1	$ 1	$ 1