Supplemental Information (Schedule of Excise Taxes) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Accounting Policies [Line Items]
Excise Tax Expense	$ 254	$ 264	$ 282
Union Electric Company
Accounting Policies [Line Items]
Excise Tax Expense	139	147	164
Ameren Illinois Company
Accounting Policies [Line Items]
Excise Tax Expense	$ 115	$ 117	$ 118