Related Party Transactions (Schedule of Affiliate Receivables and Payables) (Details) - USD ($) $ in Millions	Dec. 31, 2020	Dec. 31, 2019
Union Electric Company
Related Party Transaction [Line Items]
Accounts payable – affiliates	$ 46	$ 52
Accounts Receivable, Related Parties, Current	57	30
Union Electric Company | Income taxes payable to parent
Related Party Transaction [Line Items]
Accounts payable – affiliates	0	15
Union Electric Company | Income taxes receivable from parent
Related Party Transaction [Line Items]
Accounts Receivable, Related Parties, Current	9	15
Ameren Illinois Company
Related Party Transaction [Line Items]
Accounts payable – affiliates	51	82
Accounts Receivable, Related Parties, Current	64	28
Ameren Illinois Company | Income taxes payable to parent
Related Party Transaction [Line Items]
Accounts payable – affiliates	6	43
Ameren Illinois Company | Income taxes receivable from parent
Related Party Transaction [Line Items]
Accounts Receivable, Related Parties, Current	$ 15	$ 17