Related Party Transactions (Schedule of Affiliate Receivables and Payables) (Details) - USD ($) $ in Millions	Sep. 30, 2020	Dec. 31, 2019
Union Electric Company
Related Party Transaction [Line Items]
Accounts Payable, Related Parties, Current	$ 70	$ 52
Accounts Receivable, Related Parties, Current	12	30
Union Electric Company | Income taxes payable to parent
Related Party Transaction [Line Items]
Accounts Payable, Related Parties, Current	38	15
Union Electric Company | Income taxes receivable from parent
Related Party Transaction [Line Items]
Accounts Receivable, Related Parties, Current	0	15
Ameren Illinois Company
Related Party Transaction [Line Items]
Accounts Payable, Related Parties, Current	43	82
Accounts Receivable, Related Parties, Current	15	28
Ameren Illinois Company | Income taxes payable to parent
Related Party Transaction [Line Items]
Accounts Payable, Related Parties, Current	9	43
Ameren Illinois Company | Income taxes receivable from parent
Related Party Transaction [Line Items]
Accounts Receivable, Related Parties, Current	$ 6	$ 17