Summary Of Significant Accounting Policies (Schedule Of Excise Taxes) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Accounting Policies [Line Items]
Excise tax expense	$ 210	$ 208	$ 213
Union Electric Company
Accounting Policies [Line Items]
Excise tax expense	153	151	156
Ameren Illinois Company
Accounting Policies [Line Items]
Excise tax expense	$ 57	$ 57	$ 57