Related Party Transactions (Schedule of Affiliate Receivables and Payables) (Details) - USD ($) $ in Millions	Jun. 30, 2021	Dec. 31, 2020
Union Electric Company
Related Party Transaction [Line Items]
Accounts Payable, Related Parties, Current	$ 85	$ 46
Accounts Receivable, Related Parties, Current	49	57
Union Electric Company | Income taxes payable to parent
Related Party Transaction [Line Items]
Accounts Payable, Related Parties, Current	47	0
Union Electric Company | Income taxes receivable from parent
Related Party Transaction [Line Items]
Accounts Receivable, Related Parties, Current	0	9
Ameren Illinois Company
Related Party Transaction [Line Items]
Accounts Payable, Related Parties, Current	62	51
Accounts Receivable, Related Parties, Current	57	64
Ameren Illinois Company | Income taxes payable to parent
Related Party Transaction [Line Items]
Accounts Payable, Related Parties, Current	26	6
Ameren Illinois Company | Income taxes receivable from parent
Related Party Transaction [Line Items]
Accounts Receivable, Related Parties, Current	$ 0	$ 15