Schedule II - Valuation And Qualifying Accounts (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Allowance For Doubtful Accounts
SEC Schedule, 12-09, Valuation and Qualifying Accounts Disclosure [Line Items]
Balance at Beginning of Period	$ 19	$ 19	$ 19
Charged to Costs and Expenses	27	26	32
Charged to Other Accounts	4	7	3
Deductions	32	33	35
Balance at End of Period	18	19	19
Valuation Allowance of Deferred Tax Assets
SEC Schedule, 12-09, Valuation and Qualifying Accounts Disclosure [Line Items]
Balance at Beginning of Period	5	11	6
Charged to Costs and Expenses		(6)	7
Charged to Other Accounts	0	0	(2)
Deductions	0	0	0
Balance at End of Period	5	5	11
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	10
Charged to Other Accounts	0	0	0
Deductions	9	9	10
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	12	12	12
Charged to Costs and Expenses	18	17	22
Charged to Other Accounts	4	7	3
Deductions	23	24	25
Balance at End of Period	$ 11	$ 12	$ 12