Supplemental Information (Inventories) (Details) - USD ($) $ in Millions	Dec. 31, 2020	Dec. 31, 2019
Public Utilities, Inventory [Line Items]
Fuel	$ 115	$ 126
Gas stored underground	57	63
Other materials and supplies	349	305
Total Inventories	521	494
Union Electric Company
Public Utilities, Inventory [Line Items]
Fuel	115	126
Gas stored underground	5	6
Other materials and supplies	266	241
Total Inventories	386	373
Ameren Illinois Company
Public Utilities, Inventory [Line Items]
Fuel	0	0
Gas stored underground	52	57
Other materials and supplies	83	64
Total Inventories	$ 135	$ 121