Schedule II - Valuation And Qualifying Accounts (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Allowance For Doubtful Accounts
SEC Schedule, 12-09, Valuation and Qualifying Accounts Disclosure [Line Items]
Balance at Beginning of Period	$ 17	$ 18	$ 19
Charged to Costs and Expenses	42	26	27
Charged to Other Accounts	6	4	4
Deductions	15	31	32
Balance at End of Period	50	17	18
Valuation Allowance of Deferred Tax Assets
SEC Schedule, 12-09, Valuation and Qualifying Accounts Disclosure [Line Items]
Balance at Beginning of Period	3	5	11
Charged to Costs and Expenses - Tax Valuation Adjustments	0	(2)	(6)
Charged to Other Accounts	0	0	0
Deductions	0	0	0
Balance at End of Period	3	3	5
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	15	9	9
Charged to Other Accounts	0	0	0
Deductions	6	9	9
Balance at End of Period	16	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	10	11	12
Charged to Costs and Expenses	27	17	18
Charged to Other Accounts	6	4	4
Deductions	9	22	23
Balance at End of Period	$ 34	$ 10	$ 11