Summary Of Significant Accounting Policies (Schedule Of Inventories) (Details) - USD ($) $ in Millions		Dec. 31, 2016	Dec. 31, 2015
Accounting Policies [Line Items]
Fuel	[1]	$ 172	$ 173
Gas stored underground		82	97
Other materials and supplies		273	268
Total Inventories		527	538
Union Electric Company
Accounting Policies [Line Items]
Fuel	[1]	172	173
Gas stored underground		9	10
Other materials and supplies		211	204
Total Inventories		392	387
Ameren Illinois Company
Accounting Policies [Line Items]
Fuel	[1]	0	0
Gas stored underground		73	87
Other materials and supplies		62	64
Total Inventories		$ 135	$ 151

[1] (a)Consists of coal, oil, and propane.