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Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
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Related Party Transactions (Schedule of Related Party Transactions) (Detail) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 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	5	4	11	21
Ameren Missouri and Ameren Illinois Rent and Facility Services \| Union Electric Company
Related Party Transaction [Line Items]
Operating Revenues	6	7	17	20
Ameren Missouri and Ameren Illinois Rent and Facility Services \| Ameren Illinois Company
Related Party Transaction [Line Items]
Operating Revenues	1	1	2	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	11	11	28	41
Total Related Party Operating Revenues \| Ameren Illinois Company
Related Party Transaction [Line Items]
Operating Revenues	1	1	2	4
Ameren Illinois Power Supply Agreements with Ameren Missouri \| Ameren Illinois Company
Related Party Transaction [Line Items]
Operating Expenses	5	4	11	21
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	5	4	12	22
Ameren Services Support Services Agreement \| Union Electric Company
Related Party Transaction [Line Items]
Operating Expenses	36	34	101	103
Ameren Services Support Services Agreement \| Ameren Illinois Company
Related Party Transaction [Line Items]
Operating Expenses	33	33	93	99
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