Schedule II - Valuation And Qualifying Accounts (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Allowance For Doubtful Accounts
SEC Schedule, 12-09, Valuation and Qualifying Accounts Disclosure [Line Items]
Balance at Beginning of Period	$ 18	$ 19	$ 19
Charged to Costs and Expenses	26	27	26
Charged to Other Accounts	4	4	7
Deductions	31	32	33
Balance at End of Period	17	18	19
Valuation Allowance of Deferred Tax Assets
SEC Schedule, 12-09, Valuation and Qualifying Accounts Disclosure [Line Items]
Balance at Beginning of Period	5	5	11
Charged to Costs and Expenses	(2)	0	(6)
Charged to Other Accounts	0	0	0
Deductions	0	0	0
Balance at End of Period	3	5	5
Parent Company | Valuation Allowance of Deferred Tax Assets
SEC Schedule, 12-09, Valuation and Qualifying Accounts Disclosure [Line Items]
Charged to Costs and Expenses	3
Union Electric Company | Allowance For Doubtful Accounts
SEC Schedule, 12-09, Valuation and Qualifying Accounts Disclosure [Line Items]
Balance at Beginning of Period	7	7	7
Charged to Costs and Expenses	9	9	9
Charged to Other Accounts	0	0	0
Deductions	9	9	9
Balance at End of Period	7	7	7
Ameren Illinois Company | Allowance For Doubtful Accounts
SEC Schedule, 12-09, Valuation and Qualifying Accounts Disclosure [Line Items]
Balance at Beginning of Period	11	12	12
Charged to Costs and Expenses	17	18	17
Charged to Other Accounts	4	4	7
Deductions	22	23	24
Balance at End of Period	$ 10	$ 11	$ 12