Related Party Transactions (Schedule of Affiliate Receivables and Payables) (Details) - USD ($) $ in Millions	Mar. 31, 2021	Dec. 31, 2020
Union Electric Company
Related Party Transaction [Line Items]
Accounts Payable, Related Parties, Current	$ 40	$ 46
Accounts Receivable, Related Parties, Current	69	57
Union Electric Company | Income taxes payable to parent
Related Party Transaction [Line Items]
Accounts Payable, Related Parties, Current	0	0
Union Electric Company | Income taxes receivable from parent
Related Party Transaction [Line Items]
Accounts Receivable, Related Parties, Current	14	9
Ameren Illinois Company
Related Party Transaction [Line Items]
Accounts Payable, Related Parties, Current	43	51
Accounts Receivable, Related Parties, Current	71	64
Ameren Illinois Company | Income taxes payable to parent
Related Party Transaction [Line Items]
Accounts Payable, Related Parties, Current	6	6
Ameren Illinois Company | Income taxes receivable from parent
Related Party Transaction [Line Items]
Accounts Receivable, Related Parties, Current	$ 18	$ 15