Summary Of Significant Accounting Policies (Schedule Of Inventories) (Details) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
Accounting Policies [Line Items]
Fuel	$ 154	$ 172
Gas stored underground	82	82
Other materials and supplies	286	273
Total Inventories	522	527
Union Electric Company
Accounting Policies [Line Items]
Fuel	154	172
Gas stored underground	8	9
Other materials and supplies	226	211
Total Inventories	388	392
Ameren Illinois Company
Accounting Policies [Line Items]
Fuel	0	0
Gas stored underground	74	73
Other materials and supplies	60	62
Total Inventories	$ 134	$ 135