Related Party Transactions (Schedule of Affiliate Receivables and Payables) (Details) - USD ($) $ in Millions	Mar. 31, 2022	Dec. 31, 2021
Union Electric Company
Related Party Transaction [Line Items]
Accounts Payable, Related Parties, Current	$ 36	$ 46
Accounts Receivable, Related Parties, Current	47	44
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	33	27
Ameren Illinois Company
Related Party Transaction [Line Items]
Accounts Payable, Related Parties, Current	80	64
Accounts Receivable, Related Parties, Current	12	24
Ameren Illinois Company | Income taxes payable to parent
Related Party Transaction [Line Items]
Accounts Payable, Related Parties, Current	42	8
Ameren Illinois Company | Income taxes receivable from parent
Related Party Transaction [Line Items]
Accounts Receivable, Related Parties, Current	$ 0	$ 18