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-Publisher FASB
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Related Party Transactions (Schedule of Related Party Transactions) (Detail) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Related Party Transaction [Line Items]
Operating Revenues	$ 19	$ 0	$ 19	$ 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	1	5	3	11
Ameren Missouri and Ameren Illinois Rent and Facility Services \| Union Electric Company
Related Party Transaction [Line Items]
Operating Revenues	7	6	20	17
Operating Expenses	1	1	1	2
Ameren Missouri and Ameren Illinois Rent and Facility Services \| Ameren Illinois Company
Related Party Transaction [Line Items]
Operating Revenues	1	1	2	2
Operating Expenses	1	1	4	4
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
Ameren Missouri Software Licensing with Ameren Illinois \| Ameren Illinois Natural Gas
Related Party Transaction [Line Items]
Operating Revenues	5
Ameren Missouri Software Licensing with Ameren Illinois \| Ameren Illinois Company
Related Party Transaction [Line Items]
Operating Revenues	19		19
Total Related Party Operating Revenues \| Union Electric Company
Related Party Transaction [Line Items]
Operating Revenues	9	11	24	28
Total Related Party Operating Revenues \| Ameren Illinois Company
Related Party Transaction [Line Items]
Operating Revenues	20	1	22	2
Ameren Illinois Power Supply Agreements with Ameren Missouri \| Ameren Illinois Company
Related Party Transaction [Line Items]
Operating Expenses	1	5	3	11
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	5	4	12
Ameren Services Support Services Agreement \| Union Electric Company
Related Party Transaction [Line Items]
Operating Expenses	34	36	98	101
Ameren Services Support Services Agreement \| Ameren Illinois Company
Related Party Transaction [Line Items]
Operating Expenses	30	33	91	93
Total Related Party Other Operations and Maintenance \| Union Electric Company
Related Party Transaction [Line Items]
Operating Expenses	34	37	99	103
Total Related Party Other Operations and Maintenance \| Ameren Illinois Company
Related Party Transaction [Line Items]
Operating Expenses	31	34	95	97
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